342 IEEE
TRANSACTIONS
ON UIXRASONICS. FERROELECTRICS. A K D FREQUENCY CONTROL VOL 35. N O
3.
MAY I9XX
Precision Surface-Acoustic-Wave (SAW) Oscillators Abstract-Since theirintroduction in 1969, surface-acoustic-wave
(SAW) oscillators have found increased usage in a wide variety of military and commercialapplications.However,typicalmilitarysystem
requirementshavecontinuallydictatedthatimprovements
in overall
oscillatorperformance he realized, e.g., betterlong-termfrequency
stability; lower phase noise levels, both near to and far from the carSAW oscilrier; improved vibration sensitivity; etc. The evolution of
latortechnologyoverthe
past 17 years is first described andthena
review of the current state of the art for high-performance SAW oscillators is presented. This review draws heavily upon the authors' own
experience and efforts, which have focused upon the development of a
wide variety of SAW oscillators in response to numerous high-performance military system requirements.
4) The inherent ruggedness of the device, since its frequency is established by the periodicity of the interdigitaltransducerand/orreflectorpatternsonthe
surface of the substrate, and not the physical dimensions of the device.
5) Finally,theconsiderableeffortthathasgoneinto
the development of BAW oscillators over the last 60
years,especiallyimprovements in quartzsubstrate
material, substrate cleaning and processing technology, and devicepackagingtechniques,hasleveraged the effort to develop precision SAW oscillators
~71.
I. I N I R O D U C T I O N
INCE the first reported use of a surface-acoustic-wave
(SAW) delay-line oscillator asa means to characterize
delay-timevariationwithtemperaturefor
LiNb03 [ l ] ,
there has beenconsiderable interestevidenced in both
SAW delay-line and resonator-based oscillators. Much of
this attention has focused on applications involving radar,
[2]-[6].
avionic,andsatellitecommunicationsystems
Closelyassociated with thisinteresthasbeen
acorresponding drive to improve oscillator performance, especially in the critical areas of long-term frequency stability, phase noise, and vibration sensitivity.
Although there are many reasons behind this high level
of interest, the five principal motivating factors are as follows.
Fig. 1 shows a simple block diagram for a generalized
SAW stabilized feedback-loop oscillator design of the type
described by Leeson [S]. Althoughindividualoscillators
will differ in their specifics, they all share the following
common features: 1) one or moreloop amplifiers ( G , , G,)
of sufficient gain to overcome total feedback loop losses:
2 ) some means for gain limiting (compression) within the
feedbacklooptoinsurestableoscillation
(in many instances this gain-limiting action may actually occur in the
3)
secondstage or highlevelfeedback-loopamplifier);
provisions for gain and phase adjustment within the feed3 dB (2-4 dB) of
back loop toestablishapproximately
(nominally
3 dB of gain
excess
small-signal
gain
compressionwhenequilibrium is reached)and 27r.W net
phaseshift, where N is an integer; 4) feedback-loop signal
sampling, which may be either capacitive, resistive, etc.;
Fundamentalfrequencyoscillatoroperationspanand S ) a buffer amplifier (G,) to isolate the feedback loop
ning the range of 100 M H z to 2000 MHz. This wide
fromloadvariations.Theneedforelectronicphaseshift
range of operationaffordsreducedcost,size,and
in the feedback loop to provide frequency modulation and
powerconsumptionwhencomparedtoalternative
a low-pass filter at the oscillator's output to suppress sputechniques,suchasfrequencymultiplicationifilterrious harmonic signals may or may not be necessary deing/amplification,usingamuchlowerfrequency
pending upon specific performance requirements.
bulk-acoustic-wave (BAW) oscillator.
The SAW devicemay be either a two-port delay line or
SAW delay-line or resonator-based oscillators offer
resonator, as illustrated in Figs. 2(a) and 2(b).respecfrequency stability approaching that of quartz-crystively, or a one-port resonatoras shown in Fig. 2(c). When
tal BAW oscillators. The choice between delay lines
used in a "pseudo two-port'' configuration, as shown in
and resonators offers the designer considerable flexFig. 3(b), the transmission characteristic of the one-port
ibility in selecting modulation range and phase noise
resonatorisdominated
by thestatictransducercapacilevel.
tance C;,. This is illustrated in Fig. 4(a). A parallel inducThe relative easeofSAWdevicefabrication
and
tor may be used to tune-out C;,, and the result is shown in
compatibilitywithmoststandardmicroelectronic
Fig. 4(b). Theresponse near the acoustic resonance is now
processing techniques.
very clean, but the low insertion loss at both high and low
freauencies Dresents Droblems. Even when used in a conManuscript recelved April IO. 1987: revised June 20. 1987.
ventionalimpedance-controlled(negativeresistance)
osThe authors arc with thc Raytheon Research Divi\ion, 131 Sprin? Street.
Lexington. M A 02 173.
shown
thatas
cillator
circuit,
such
in Fig. S , probthese
IEEE Log Number 8719303.
persist,
lems
along
with a marked
sensitivity
to circuit
S
0885-3010/8810500-0342$01.OO O 1988 IEEE PARKER A N D MONTRESS:P RECISION
313
SAW OSCILLATORS
I
l
I Electronic Frequency
L-
Control Fig. 1 . Block diagram of feedback loop oscillator.
r-
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l
6 **a
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Fig. 2 . Two-port and one-port SAW devices, (a) Two-port delay line. (h) Two-port resonator. ( c ) One-port resonator.
344
IEEE TRANSACTIONS ON ULTRASONICS,F ERROELECTRICS.A NDFREQUENCYCONTROL.
VOL. 35. NO. S. MAY 1988
6 6
(b)
Fig. 3. One-portSAWresonator.
(a) Electrical equivalent circuit. (b)
“Pseudo two-port” electrical connection for one-port resonator, including external inductor L , to tune out static transducer capacitance C,:.
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Fig. 4 . Insertion loss and phase responses for “pseudo two-port” resonator connection shown in Fig. 3(b). (a) Without inductor
L,. (b) With inductor L,.
345
PARKER ANDMONTRESS:PRECISIONSAWOSCILLATORS
a d
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0
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20
40
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TEMPERATURE(“C)
Fig. 5 . Schematic diagram of Pierce oscillator circuit, including one-port
SAW resonator.
Fig. 6 . Fractionalfrequencychangeversustemperaturefor
ST-X cuts of quartz.
A T , BT, and
other material properties-for example, propagation loss
[ 181, [ 191 and the influence of metalization thickness upon
the
frequency-versus-temperature
characteristic
[20]mustalso be takenintoaccount in designing adevice,
especially for frequencies above 500 MHz.
The primary advantage accruing from the use of SAW
oscillators in the 100 MHz to 2 GHz frequency range is
that the SAW oscillator provides frequency stability similar to that obtained from a BAW oscillator while simultaneously eliminating the complex frequency multiplication/filtering/amplification chainsrequiredwhenusing
low-frequency BAW oscillators (10-50 MHz) to realize
stable VHF, UHF, and microwave sources. Over certain
portions of the single-sideband phase noise spectrum the
fundamental frequency SAW oscillator provides superior
1 ) The availabilityofatemperature-stablefamilyof
performance in comparison to a multiplied BAW oscillasubstrateorientations(ST-cut,Xpropagating,and
tor.However,thetemperatureandlong-termstabilities
relatedsinglyrotated Y-X cuts) [ 121. Fig. 6 com- that have been demonstrated to date for SAW oscillators
pares the frequency-versus-temperature behavior for are not quite as good as those obtained with high-quality
thesurface-acoustic-wave ST-X cut [l21 andthe
lower-frequency BAW oscillators.
bulk-acoustic-wave BT (6 mode) [l31 and AT (c
The planar construction and considerable performance
mode) [ 141 cuts. The ST-X cut turnover temperature flexibility afforded by SAW resonators and delay lines add
is denoted by To in the figure.
new dimensions
to
high-frequency
oscillator
design.
2) The exceptionally goodstabilityandagingassociHowever, it was our experience that until a reproducible
ated with quartz duetoitsextensivedevelopment
long-term stability of f1 PPM/year was achieved, milfor BAW oscillators (71.
itary system designers were not willing to consider SAW
Table I summarizes several important material properoscillators (resonators primarily) as a viable alternative to
ties for auseful range of quartz substrate orientations[ 151, the BAW oscillators that were routinely finding applica[16]. The rotation angle 8 is defined in Fig. 7 relative to
tion in most radar and avionic systems.
Our own efforts
crystal. It should be
the x - , y - , and z-axes of the quartz
to achieve this objective led directly to the use
of SAW
emphasized, however, that the entrees contained in Table resonator-basedoscillators in several systems. Now the
I are only nominal design values. Accurate device design most important areas for fruitful improvements
to oscilto a specific frequency and turnover temperature can only lator performance appear to be reduced vibration sensitivbe accomplished as an “iterative” procedure based upon
ity, along with additional across-the-board improvements
actualdevicefabricationandtesting,
with appropriate in single-sideband phase noise levels.
photomask corrections to “fine tune” the device’s elecIn the following sections of the paper we describe the
trical properties. Obviously the process is aided
by prior current state of the art for high-performance SAW oscilSAW deexperience gainedin the design and testing of devices
with lators.Discussionsofbasicoscillatordesign,
similar characteristics. This point is further illustrated in vice and oscillator packaging, frequency setting, and freFigs. 8 and 9 , which indicate the dependence of substrate quency stability (short- and long-term) are included.
(baresurface)turnovertemperature
upon rotationangle
11. SAW OSCILLATOR
DESIGNAND COMPONENTS
(e) of the cut (121 and the effect of aluminum transducer
metalization thickness and operating frequency upon surThe basic feedback oscillator circuit of Fig. 1 is particface-acoustic-wavevelocity
[ 171, respectively.Many
ularly well suited for two-port SAW devices and is gen-
parasitics. As aresult,one-port SAW oscillatordesigns
[4], [6], [9]-[l l] have been more difficult to implement
thantwo-portdesigns,especiallyforfrequenciesabove
500 MHz. For these reasons the majority of SAW oscillator designs, including our own, have used two-port delay lines or resonators in a basic feedback-loop configuin setration.Thisapproachprovidesconsiderableease
up and also allows for simple diagnosis in the event that
problemsareencountered
in initialoscillatorperformance. This feature of the feedback-loop oscillator architecture will be described in more detail later.
Although SAW oscillators may be designed using any
piezoelectricsubstratematerial,quartzhasreceivedthe
most attention for two primary reasons.
346
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Fig 9 . Dependence o f quartzmetalized surfnce veloc~tyupon lrequency.
a i t h aluminum metal thickness as parameter. Adapted from 1171.
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erallythemostcommonlyusedconfigurationfor
SAW
oscillators. Either delay lines or two-port resonators can
be used. It is also particularly convenient to choose componentsthatwork in a 50-Q environment. By using amplifiers, couplers, and otherelectroniccomponentsthat
have SO-Q input and output impedances, the proper conditions for oscillationcanbequicklyestablishedwitha
vector RF network analyzer. By breaking the oscillator
loop at anyconvenientlocationtheopen-loopgainand
transmissionphase of thecombinedoscillatorcomponentscanbereadily
established.Theconditionsfor oscillation are that there be net small-signal gain through the
loopandthatthetotalphaseshiftaround
the loop bean
integermultipleof 27r. Typicallythesmall-signalopenloopgain is set to 3 & I dB.Theequilibriumoscillator
loop power can be determined by increasing the network
analyzer test signalpowerlevel
until gaincompression
gives 0 dB of open-loop gain. By observing the gain and
347
PARKER A N D MONTRESS.P RECISIONSAWOSCILLATORS
12 GROUPS OF FINGERS
A
10 FINGERS
IN EACH GROUP
phase conditions at frequencies other than the desired oscillation frequency, the potential for spurious oscillations
can also be evaluated.
The simplest SAW oscillator may contain only an amplifier, a SAW device, an output coupler, and a means of
setting loop phaseshift, e.g., a short length of coaxial cable. The saturation of the loop amplifier provides the necessarygaincompression.Morecapableoscillators
may
contain a buffer amplifier, electronic phaseshifter, amplitudelimiter,oranyoftheothercomponentsshown
in
Fig. 1.
The key features of the transducersforaSAWdelay
line designed for oscillator applications are shown in Fig.
10. For goodfrequency stability the delay line shouldhave
as long a delay time as possible. However, the delay line
must also provide a filtering function to limit the amplitude response of the delay line to a relatively narrow frequency range. The long transducer on the left in Fig. I O
serves this purpose and the center-to-center distance between thetransducersdetermines the delaytime 7,q. To
ensure that only one frequency can satisfy the oscillation
conditions at any given time, the combined length of the
two transducers should be no less than approximately 90
percent of the center-to-center distance between the two
transducers. It is generallydesirable to limitthetotal
number of fingers in each transducer to about 120, thus
the long transducer is usually thinned into
a ladder-type
structure [21] as shown in the figure. This gives a device
withonlyabout 5-6 percent of the active acoustic area
covered with aluminum in the transducers. Additional fingers can be used to achieve lower insertion loss, but this
increases the undesirable influence of the metal on turnover temperature, triple transit reflections, and frequency
accuracy.
A number of factors, such as propagation loss, physical
size, and phase errors between groups of fingers contributetolimitingthelengthofaSAWtransducer.While
these upper limits have never been thoroughly explored,
ourexperiencehasshownthatsingle-modedelaylines
800
withcenter-to-centertransducerseparationsupto
120 FINGERS
wavelengths are readily achievable at frequencies below
1 to 1.5 GHz. At 400 MHz an obtainable delay time for
a single-modedelayline is about 2 p . Atypicaldelay
line will have an insertion loss of - 20 dB (in a 50-0 system) if 120fingers are used in eachtransducerandthe
acoustic aperture is approximately 200 wavelengths.
A delay-line oscillator is particularly useful wherea relativelylargetuningrange
is desired. A SAWoscillator
can be tuned over roughly the l-dB bandwidth of the SAW
as indevicewith a simpleelectronicphaseshiftcircuit
dicated in Fig. 1. For asingle-modedelaylinethe
1-dB
~ )is. - 7~
bandwidth is approximately equal to 1 / ( 2 ~ ~This
times larger than the I-dB bandwidth of a resonator with
an equivalent group delay. Thus for deviceswith the same
phase slope (group delay), the delay line has about three
times the tuning range of a resonator [22].
in Fig. I O uses quarter-waveThetransducerdesign
length-widefingers. In productionthislimits the maximum frequency of the delay line to about 1.3 GHz if opticalphotolithography
is used.However,
splitfingers
(one-eighth-wavelength wide) operating at the third harmonic can also be used, and this increases the upper frequency limit to about 1.9 GHz. If the so-called shallowbulk-acoustic-wave (SBAW) mode [23], [24] is used, the
frequency can be further increased to above 3 GHz.
Though delay lines have certain attractive features, the
focal point of our owneffort to develop high-stability lownoise SAW oscillators has become the two-port resonator.
Resonators offer the highest Q (largest group delay) and
thereforethebestfrequencystability.
Fig. 11 showsthe
basic characteristics of the two-port resonator design that
in ourlaboratory.Typically,
hasbeenusedextensively
approximately 50 quarter-wavelength-widefingersare
used in each transducer, andtheacousticaperture
is in
therange of 130-200wavelengths.Thereflectorsare
made up of 1000 etched quarter-wavelength grooves that
deep.Grooved
aretypically 1 percentofawavelength
gratingsgivebetterfrequencystabilitythanmetalgratings since the only metalin the active acoustic area comes
from the transducers. In our resonators the aluminum in
348
IEEE TRANSACTIONSONULTRASONICS.FERROELECTRICS,ANDFREQUENCYCONTROL.VOL.
1000 GROOVES
-
r-
I
-50 FINGERS
Fig. 11. Representativetwo-port
l
l
35. NO. 3. MAY 1988
GROOVES
IOoo
U
-50 FINGERS
SAW resonatordesignshowingtypicaldimensions
wavelengths.
expressed in terms of acoustic
+l 80 -180 Fig. 12. Measuredinsertion
loss andphasecharacteristics
for 425-MHz two-port SAW resonatordesignas
Fig 11.
the transducers is usually about 1 percent of a wavelength
thick and is recessed into etched groovesof the same depth
as the gratings.
A unique feature of this design is the large space between the transducers ( 200X). This gives an equivalent
cavitylengthofabout 350 wavelengths. A consequence
of this large cavity lengthis that the resonator will support
threecavitymodes. A typicalplot of insertion loss and
transmissionphase is shown in Fig. 12 for a425-MHz
two-port SAW resonator. The threeresonances are clearly
evident, but notethatthephase
of the twoouterresonances is shifted by 180" fromthephaseofthecenter
peak. For mostfilterapplicationsthethreepeakswould
be unacceptable, but for oscillators the phase reversal totally eliminates the outer peaks from consideration if the
oscillator loop phase is adjusted for the center peak. The
large cavity length reduces the fraction of the cavity covered by metal to about 7 percent, and in most cases eliminates the need to apodize the transducers [ 2 5 ] ,even for
the relatively wideapertures used in our designs. Some
evidenceoftransversemodescanbeseenonthehighfrequency side of the resonance peaks in Fig. 12, but they
have little or no effect onoscillatorfrequencystability
-
illustrated i n
since they are well outside the I-dB bandwidth. Another
advantage of a large cavity size (both width and length)
of the
is that it increasesthepower-handlingcapability
resonator [26].
Devicesbasedon this design havetypicallyexhibited
4-10 dB of insertion loss, dependingon frequencyand
package type. The unloaded Q, Ql, is usually 80-95 percent of the material limited value and the loaded Q, Q, is
about one-third to three-quarters of Q,. For ST-cut quartz
Q, is inverselyproportional to the
the materiallimitto
F , (in MHz)and is given by the
resonantfrequency
expression
Q,,, = (1.05
X
107)/F,).
(1)
Theequivalentcircuit of atwo-portSAWresonator
is
shown in Fig. 13, and the typical range of component values is shown in Table I1 for devices in the frequency range
300-1000 MHz. C,, representsthestaticcapacitance
of
eachtransducer.Theone-portresonatorshown
in Fig.
2(c) is not as convenient to use as the two-port resonator
in most applications, but it does have the advantage that
lower values of R , can be obtained for an equivalent number of fingers [ 2 5 ] .
PARKERAKDMOKTRESS:PRECISION
349
SAW OSCILLATORS
+15VDC
1
I
_L _L
2 Fig. 13. Equivalentcircuit for two-port S A W resonator.
TABLE I1
TYPICAL
R A N G EOF VALUES FOR TWO-PORT
SAW
RESONATOR
E QUIVALENT-CIRCUIT
PARAMETERS
50 Q
0.5 mH
0.05 f F
1.0 pF
<
<
<
<
R, < 350 fl
L , < 1.5 mH
C, < 0.2 f F
C,,< 3.0 pF
The photolithographic requirements are essentially
the
sameforresonators as for delaylines,butthereareno
effective resonator designs that work at harmonics of the
grating periodicity, i.e., there is no groove equivalent to
asplitfinger.Alsoshallow
bulk acoustic wavesdo not
work well in resonators (it is difficult to design efficient
reflectors), so there are no tricks to obtaining higher frequencies. Therefore, in production, SAWresonatorsare
limited to a maximum frequency of about 1.3 GHz with
optical photolithography. Of course, this upper limit may
be increased through the use of E-beam lithography, although this is not yet a fully developed production techniquefor SAWdevices.The low endofthefrequency
range is set at about 100 MHz by the large physical size
of thedeviceandthe
ready availability of high-quality
BAW resonators.
ThoughtheSAWdevicehas
by far the largest delay
time or phase slope of all the oscillator components, the
other components still play an important role in the frequencystabilityofaSAWoscillator.Incomparisonto
low-frequency BAW resonators
the
higher
frequency
SAW devices have a one to two order-of-magnitude lower
Q, and therefore the other oscillator components have a
proportionately greater influence on the frequency stability of the SAW oscillator. To minimize the influence of
theloopamplifieronfrequencystability,theamplifier
shouldhavea very large bandwidth (low Q) andshould
use negative feedback for greater gain and phase stability.
It is convenient to work in a 5 0 4 environment, and there
are a number of commercially available TO-S packaged
amplifiers that have demonstrated excellent performance
in SAW oscillators. Fig. 14 shows a circuit diagram of a
typicalhigh-performancehybrid MIC amplifierthathas
active current biasing of the RF transistor. The best performance is obtained if bipolar silicon transistors are used,
FETs.
since they give lower flicker noise levels than GaAs
(See Section V for a further discussion of noise levels in
amplifiers.) A somewhat simpler amplifier circuit using a
Darlington pair is shown in Fig. 15, and it has also demonstratedgoodperformance
in SAWoscillators.These
circuits can have bandwidths greater than 1 GHz, are very
stable over timeandtemperature,andare
well behaved
3
Fig. 14. Schematic diagram of broadband hybrid circuit amplifier suitable
for high-performance SAW oscillator applications.
P
TI
CB
Rf
.
OUT
Cac
Fig. 15. Schematic diagram of br0adbar.d hybrid circuit amplifier suitable
for moderate performance SAW oscillator applications.
when driven into gaincompression.Alsotheirperformance is not particularly sensitive to a source or load that
is not exactly 50 Q.In most cases the SAW devices cannot
be designed to have input and output impedances of 50 Q ,
even though they are intended to work in a 50-Q environment.
Commercial amplifiers such as the Avantek UTO-502
and Watkins-Johnson A5 perform very well in SAW oscillator applications and have been employed where size
requirements were such that printed circuit board mounted
components could be used. Generally, the other necessary
componentssuch as high-quality 50-Q couplers,phaseshifters, and buffer amplifiers are also readily available.
SAWoscillatorsbuiltentirely
with commercially availableamplifiersandpowersplittershavedemonstrated
state-of-the-art performance when high-quality SAW devicesareused.
However,performancerequirementsor
size constraints can sometimes not be met with commercial components and custom-designed circuits may be required. The entire SAW oscillator can then be assembled
on a single hybrid circuit, and this has been a major direction of our own effort recently. The novel “all-quartz
350
IEEE TRANSACTIONS ON ULTRASONICS.
Fig. 16. Photograph
FERROELECTRICS. ANDFREQUENCY
CONTROL. VOL 35. NO 3. MAY 1988
of two-port SAW r e s o n a t o r m o u n t e d o n TO-8 cold-weld package header prior to s e a l i n g
package” (AQP) for the SAW device (see Section 111) is
particularly appropriate for hybrid circuits.
The TO-8 type enclosure
is most appropriate for printed
circuit board mounting applications where discrete packagedcomponentsaregoingtobeusedtoconfigurethe
P ACKAGING
111. SAW DEVICE
overall oscillator. If small oscillator size is a design obThe long-term frequency stability (aging) of a SAW os- jective, then an alternative packaging technique is essencillator is intimately related to the SAW packaging techtial. The incorporation of an unsealed SAW device into a
nology used. Earlyresultsconcerningthelong-termstaresistance weld-sealed hermetic package, along with elecby organic troniccomponents,does
bility of SAW oscillatorsweredegraded
not lenditselftogoodlongmaterialsthatwereusedfor
SAW substratemounting
3-15
term frequency
stability
applications.
Typically
within the packaging enclosure [27]. After it became ap- PPM/year aging ratesareroutinelyachievedusingthis
parent that an ultra-clean, high-vacuum-sealing technique technique, an unacceptably high level of aging for many
was requisite toobtainsatisfactoryagingresults,steady
applications. An alternative packaging technique for SAW
improvementsweremade as reported in a successionof
devices has been reported in the literature [37], [46], [50][53]. Theseearlyattempts toencapsulate SAW devices
papers (261,[28]-(471. Excellentperformancehasbeen
obtainedfordevicesthathave
been cold-weldvacuum
using an all-quartz package concept met with somewhat
16 limited success. To the best of our knowledge, either the
sealed in aTO-8orequivalenttypeenclosure.Fig.
shows a typical TO-8 type cold-weld package (HC-37U) aging performance achieved did not compare at all favorcontainingatwo-port
SAW resonatordevice.Afterthe
ably with that obtainable using the aforementioned coldappropriate
cleaning
procedures,
including
UV/ozone
weld package-sealing technique, or else they were very
[48], the header and cover are placed in a special set of difficult to fabricate.
sealing
dies
within
cryogenically
a
pumped
vacuum
More recently our own efforts to develop a two-piece,
chamber. An extended bakeout period (6-24 h) at an el- all-quartz package (AQP) sealing technique which would
evated temperature ( > 360°C) prior to package sealing is be compatible with high vacuum processing has led to the
essential if good device aging is to be achieved [49]. (See approach shown schematically in Fig. 17 (471. The SAW
Section V . ) The dies arethen brought together under pres- substrate is typically 0.6 in long, 0.5 in wide, and 0.035
is
sure and special knife edges deform the lips of the header in thick.The identicallyorientedquartzcoverplate
as the SAW
and cover to accomplish an essentially intermetallic bond generallyofthesamelengthandthickness
that is hermetic.
substrate, but somewhat narrower to allow electrical con-
PARKERAND
35 I
MONTRESS PRECISIONSAWOSCILLATORS
-T-
SUBSTRAI'E
COVER
I
- .
EXPOSED
BUSBARS
GLASS FRIT
(h) Fig. 17. Schematicdlagrdm
o f all-quam package tor SAW &\ice\. a) Cross-sectional view. ( b ) Top view. Fig. 1R. Photograph olsealed all-quartz packaged 401-MH7S A W revjnator
nections to theexposedbusbarsconnectedtothe
SAW
devicetransducers. A glassfrit is thenusedtosealthe
two quartzplatestogether
in a vacuum chamber. Both
vitrifying and devitrifying frit types have been used. Fig.
18 contains a photograph of an all-quartz packaged 400MHz SAW resonator device. The technique has beenused
successfully to package both SAW resonatorsanddelay
lines. A number of sealed all-quartz packages have been
subjected to both coarse-leaktesting(bubbletester)
and
fine-leaktesting (heliumbombing) with anultimate instrument detection limit of - 6 X I O - ' ' atm-cc/s with no
measurable leaks observed. The transparency of the package cover assists in the obscrvation of coarse leaks due to
eitherinadequatefritcoverageorbubblesdeveloping
within the frir during thc sealing process.
Several all-quartz packaged devices have been repeatedly subjected to temperature cycling over the -40°C to
+8O"C rangewithoutanyleaksdeveloping
as aconse-
352
1
-40 -33 - 2 0
I
1
I
I
I
I
I
I
1
-10
0
10
20
30
40
50
60
70
TEMPERATURE ("Cl
Fig. 19. Typical fractionalfrequency change versus temperature for allquartz packaged SAW resonator device. Heating and cooling rates were
approximately 1"C/min.
TABLE 111
SEALING-INDUCEDRESONANTFREQUENCY
SHIFTS FOR ALL-QUARTZ
PACKAGED SAW RESONATORS
187MHz
2 Devices
Resonant
Frequency
Shift (kHz)
350 MHz
40 Devices
Ave.
Range
Ave.
+25
+20 to +30
+l4
Range
+ 5 to + 2+0l 2
401 MHz
3 Devices
353-397 MHz
20 Devices
Ave.
Range
Ave.
0 to +25
+1l
425 MHz
5 Devices
Range
+ 7 to
+ 18
Ave.
Range
+ 15
-10 to +30
sensitivity while maintaining excellent long-term oscillaquence of these temperature excursions. Fig. 19 shows a
tor stability.
typical resonant frequency versus temperature characterThe aging, phase noise, and vibration sensitivity charisticfora310-MHz
AQPSAWresonatordevice.The
maximum hysteresis between heating and cooling runs is acteristics for both cold-weld and AQP packaged devices
only 1 PPM, typical for the 1"C/min heating and cool- are discussed in Section V.
ing rates used for the measurement. Similar results have
IV. FREQUENCY
ACCURACY
been observed for devices packaged in cold-weld-sealed
Most, if not all,oscillator specificationsplacelimits
to thesametemperature
TO-8enclosuresandsubjected
cycling.
upon the acceptable frequency tolerance due
to all causes.
Typicallymorethan +250 PPM is considered fairly reA typical frequency shift of
+35 PPM (e.g., approximately 14 kHz for a 400-MHz SAW device) has been laxed, +25 to +250 PPM is considered moderately tight,
observed to result from the all-quartz package sealing pro- whilelessthan +25 PPM is considered quite stringent.
111 sum- A major factor in achieving good overall frequency tolcess when applied to aSAWresonator.Table
marizes our measured package-sealing induced-frequency erance is an accurate initial set-on of the oscillator's frequency. Generally, two issues are involved in the proper
shifts for a number of SAW resonators designed for opset-up of an oscillator to a particular frequency: 1) fabrieration in the 187-425 MHzfrequencyrange.Thisfrecating the SAW device (resonator or delay line) with sufquencyshift is somewhat lessthanthattypicallyfound
ficient accuracy to guarantee that the oscillator's desired
for devices packaged in cold-weld-sealed TO-8 type enoperatingfrequencyfallswithinthel-dBinsertionloss
closures, and eliminates completely those frequency shifts
associated with device mounting and wire bonding tech- bandwidth at the intended operating temperature, and 2)
that provisions are made to adjust the net loop transmisniques in the cold-weld-sealed TO-8 typepackage as well.
Another benefit of the all-quartz package is the consid- sion phase in order to establish oscillator operation at the
erable latitude it affords in the choice of SAW substrate appropriate initial set-on frequency. The second issue inis dependent upon how much
mounting materials and techniques sinceit provides a her- volvescircuitdesignand
effort (and money) one is willing to put into the task of
metic enclosure for the SAW device. This holds out the
accuratelyadjustingtheoscillator'sinitialset-onfrepossibility for further reducing SAW oscillator vibration
+
+
PARKER ANDMONTRESS:PRECISIONSAWOSCILLATORS
353
Before Etching
quency. For example, a 400-MHz SAW resonator oscilAlummum Electrodes
lator’s specification might require f 1 PPM set-on accuracy. At thisfrequencyatypicalresonatorgroupdelay
(78)might be about 9 ,us, thusrequiringanincremental
phaseadjustmentcapabilityofapproximately
0.5” to
achieve this level of frequency set-on accuracy. This can
usually be accomplished using either tapped meander line
or lumped-element phaseshifter circuit design techniques.
The problem of accurate device fabrication presents an
entirely different situation. Typically, only f200 to i-500
After Etching PPM control of a SAW resonator’s frequency is possible
Aluminum Electrodes after initialfabrication.Thewideresonantfrequency
range is due to substrate-to-substrate variation as well as
a lack ofperfect control over processing variables such as
IDT
groove depth, metalization thickness, accurate alignment
Grooved Reflector
Quartz Substrate
Grooved
of the devicewith the substrate, etc. Also, depending upon
(b)
thepackagingtechniqueused,mountingstressesand
Fig. 20. Cross-sectional views illustrating reactive ion-etching process for
package sealing can lead to additional uncontrolled shifts
a SAW resonator. (a) Recessed electrodes before etching. (b) Recessed
in a device’s resonant frequency. Much of the foregoing
electrodes after etching.
discussion also applies to SAWdelay line devices as well;
however,theirinherentlywiderbandwidthusuallyalleprocedureinorderto
accuratelyset theresonantfreviates most fabricational tolerance effects.
Reactive ion etching (RIE) with CF, is commonly used quency of sealed devices.
As a direct result of our exploitation of the RIE trimtoaccuratelytrimaSAWresonator’scenterfrequency
afterinitialfabrication[54]-[56].
Fig. 20 illustrates the ming technique, we are now able to accurately fabricate
techniquefor arecessedtransducerconfiguration.
The AQP devices to & 10 to +50 PPM accuracy in the 200trimming process depends upon the fact that quartz etches 800 MHz frequency range. We are currently working on
post-seal trimming techniques to improve this accuracy to
approximatelyseventimesfasterthanaluminum.The
technique is unidirectional, that is, it may only be used to i- l to +_ 10 PPM, since for voltage-controlled oscillator
trimthedevicedown
in frequency. An analysis carried
applicationsanevenmorestringentfrequencytolerance
is necessary to maintain proper oscillator operation
over
out on a 465-MHz SAW resonator design indicates that
the sensitivity is approximately - 3 to -6 PPM/A. Thus the full tuning range.
very little material is actually removed from the substrate
V . FREQUENCYSTABILITY
surface. We have applied the process to a large number
of devices in the 300-1000 MHz frequencyrange with
The frequency stability of a precision SAW oscillator
excellent results. Generally, we try to limit the maximum is one of its most important performance parameters. After
trimming required to 100-200 PPM. If more trimming is the initial frequency of an oscillator is accurately set, it
necessary a corrected photomask is first procured in order mustthenstaywithintheallottedfrequencyrangeand
to reduce the absolute amount of frequency trimming re- also exhibit specified spectral characteristics. In this disquired. Recently more than 50 350-MHz AQP resonators cussionoffrequencystability,theobservedfrequency
wereallfabricatedandsuccessfullytrimmedusingthis
changes of an oscillatorwill be classified as either systemtechnique with no observed degradation in performance, atic or random effects. Systematic effects are such that a
e.g., phasenoise,aging,etc.Thesedevices
wereall
given set of conditions will have a predictable effect on
trimmedusingthefixtureshown
in Fig. 21. The fixture oscillator frequency. For example, the frequency-versushas provisions for in situ monitoring of the resonant fretemperaturecharacteristicofanoscillatorfallsintothis
quency during the etching process and also has a built-in
category. Certain aspects of long-term stability (or aging)
dc proportionalcontrolheaterthatpermitstrimming
also fall into this category in the sense that observation of
an oscillator’s frequency for a period of time
may allow
whilethedevice is maintained atitsspecifiedoperating
ob- one to accurately predict the magnitude and direction of
or turnover temperature. Using this fixture we have
tained excellent results for sealed AQP resonators where
future frequency shifts by extrapolating from past performore than 60-70 percent of the sealed devices possess an
mance. Random effects, on the other hand, are different
insertionlossresponse in which the 1-dB bandwidth (at in that the prediction of the magnitude and direction of a
operating or turnover temperature) includes the intended
frequency change at a specified time is not possible. Here
fixed oscillator frequency. This is equivalent to _+ 15 PPM onlystatisticalparameterssuch
as spectraldensitiesor
controlovertheresonantfrequency
for thesealeddevariancescanbeused.Systematiceffects
will bedisvices. Typically a sealing-induced upward frequency shift
cussed first.
of + 10 kHz (i-4 kHz) was observed for these devices.
The parameter that has one of the largest effects on a
This sealing-induced shift can be anticipated in the trim SAW oscillator’s frequency is temperature. This was dis-
-7-
354
IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS. A N D FREQUENCY CONTROL, VOL. 35. NO. 3 , MAY 1988
Fig. 21. Photograph of fixture used for reactive ion etching of SAW devices. Fixture permits in siru monitoring of frequency
duringtrimmingprocess.
DC proportionaltemperaturecontrollersallowtrimming
to be carriedout at device'sturnover
temperature.
spect to timeis large enough, asecond type of temperature
effect knownasdynamictemperaturedependencebecomes important [6 l ] . Here a fractional frequency change
of temperature is
proportionaltothetimederivative
caused by stressesproduced in thequartzsubstrate
by
time-dependent temperature gradients. The magnitude of
the dynamic temperaturecoefficient is strongly dependent
on the details of the SAW substrate mounting and packaging,
but for the ST-cut family the dynamic coefficient
A F / F = A ( T - T,))'
(21
usuallyfalls in therangeof 1 to I O X IOPh/"C/s [62].
where A is the quadratic temperature coefficient. The coef- The sign of this coefficient can be either positive or negficient A is not a strong function of cut angle and has the ative.
value
-35 X
'C)*. However,
as
approximate
of
After the static temperature dependence, the next largmentioned earlier,otherparameters,suchastransducer
est contributor to frequency error is long-term stability or
metalization and the sensitivity of various circuit paramaging. Frequency shifts that occur over a long period
of
eters to temperature, canalso influencethetemperature
time(weeks to years)areusuallycaused
by relaxation
dependence of the oscillator's frequency. There are other
phenomena in or around the SAW device. Stress relaxacuts of quartz that give somewhat better temperature station in mounting structures or transducer metalization are
bility [ 1.51, [57]-[60]but their use has not become widecommoncauses of long-term frequencydrift,asareadspread because the improvement is only on the order of a
sorptionanddesorptionofforeignmaterialsonthesurfactor of 2 or 3. Furthermore, the cut-angle tolerances are face of the SAW substrate. These processes usually prosubstantially tighter, thusmakingthesubstratesconsidfractional
duce
a frequency
shift
that
increases
erably more expensive to fabricate.
logarithmically with time.The high frequency(smaller
The frequency-versus-temperature characteristic shown wavelength) of SAW devices as compared to BAW resoi n Fig. 6 is referredto asthestatictemperaturedepennatorsmakessurfacephenomenasignificantlymore
imdence and applies only if the time rate of change of tem- portantandwillperhapslimit
the long-termfrequency
perature is slow. If the derivative of temperature with re- drift of SAW oscillators to levels that are one to two or-
cussedearlier, andthebasic
frequency-versus-temperatureCharacteristic ofST-X quartz was shown in Fig. 6.
Thischaracteristic is largelydetermined by thecut of
quartz. The turnover temperature T,, can be varied over a
range of temperatures by choosing the proper cut angles.
The fractional change in frequency for temperature variations about To is given to a good approximation by
PARKER A N D MONTKESS:P RECISION
355
SAW OSCILLATORS
I
l
I
-05
0
l
HIGH TEMPERATURE BAKE
0.5 20
60
40
80
100
l
TIME (WEEKS)
Fig. 2 2 . Long-term fractional frcquenc) stability of five TO-8 packaged 4 2 5 - M H z S.4W resonator oscillators. M a x i m u m bake
temperature is shown for each SAW resonator.
oscillators
ders of magnitude larger than that of precision 5 - or 10MHz BAW oscillators.
A high-temperature bake priorto the sealing of the SAW
device is veryimportant for goodlong-termfrequency
stability (491. and this is illustrated in Fig. 22. This figure
shows the time-dependent fractional frequency change in
partspermillion (PPM) for five cold-weld TO-S sealed
425-MHz SAW resonators operated in oscillator circuits.
The maximum bake temperature prior to sealingis shown
in the figure for each SAW device. For temperatures above
360°C there is a substantialimprovement in. long-term
frequency stability. (Note the order-of-magnitude change
in scalebetweentheupperandlowercurves.)Amaximum frequency change on the order of a few tenths of a
PPM over a period of two to three years of operation at a
constant temperature of 60°C is typical for both TO-8
packagedand AQPresonators in the150-MHz to 500MHz range. As will be discussed in more detail later, there
are also random frequency fluctuations that contribute to
long-termfrequencystability 1451. and thesecannotalways be ignored when the systematic drift is reduced to a
-
low level. The random frequencyfluctuationsevident in
the lower two curves of Fig.22 are not significantly larger
than the systematic drift, but for delay-line oscillators in
the same frequency range this is not true 1451. For delayline oscillators i n the same frequency range this is not true
[45]. For delaylines it is notusual toseerandomfrequency fluctuations on the order of I PPM over a period
of two years, even though the systematic drift may only
be on the order of a few tenths of a PPM/year.
For SAW resonators in the 700-1000 MHz range we
haveobservedthatthelong-termfrequencydrift
is significantly worse than that measured for comparably packaged400-MHzresonators.Fig.23showstypicalaging
curvesfor five TO-S packaged(high-temperaturebake)
SAWresonators in thisfrequencyrange.Similarresults
have also been obtained for high-frequency AQP resonators. At this time it is not clear why this strong degradation in long-term stability occurs with the increase in frequency, but it should be noted that the phase slope (group
delay) decreases by a factor of 4 for every doubling the
of
resonator's frequency. This significantly increases the im-
356
IEEE TRANSACTIONS ON ULTRASONICS. FERROELECTRICS, A N D FREQUENCY CONTROL. VOL 35. NO. 3 , MAY 1988
portanceofthephasestabilityoftheoscillator's
nonacoustic components.
Another very important systematic effect is acceleration
or vibration sensitivity. Like dynamic temperature dependence, vibration sensitivity is strongly dependent on the
details of how the SAW device is mounted and packaged.
Acommonlyobservedlevel
of vibrationsensitivity for
ST-cut SAW devices is A F / F = 1 x 10-9/g [63]. Levelsas low as 1 X 1O-Io/g havebeen observed, but the
the
levelcanalsobe
as largeas 1 X lO-'/g.Though
magnitudeofvibrationsensitivity
is smallcomparedto
temperatureeffectsandlong-termdrift,itsimportance
arisesfromitsinfluenceonspectral
purity in a high-vibrationenvironment.Asinusoidalvibration
will create
discrete FM sidebands on an oscillator signal, and random
vibration will cause random sidebands. This latter effect
will be discussed further whenthetopicofrandom-frequency fluctuations is considered.
There are also other parameters that can influence oscillator frequency. For example, the typical AQP device
is sensitivetoatmosphericpressureatalevel
of 1-3
PPM/atmosphere.TO-8 packageddevicesaresubstantiallylesssensitive. The frequency ofa SAW oscillator
will also vary with dc power supply voltageby something
on the order of 0.1-10 PPM/V depending on the type of
SAW device used and the voltagesensitivity of the phaseshift through the oscillator circuitry. Load impedance can
alsoinfluencetheoscillator'sfrequency,
but this effect
can be reduced to under a few tenths of a PPM for a 50percent change in load resistance if a high Q resonator is
used along with a good output buffering circuit.
Random-frequencyfluctuations
are more difficult to
quantify than systematic effects since one cannot simply
state that the frequency will change a certain amount in a
given situation or in a specified time. Even the standard
deviation, a common statistical parameter, isnot typically
applied to random-frequency fluctuations since, for some
common noise processes, its value will depend on either
the number of samples or the total length of the measurement time. The mostcommonparameters
now used to
characterize
random-frequency
fluctuations
are
either
phase or frequency spectral densities and the two-sample
[a].
Thespectral densityoffre(orAllan)variance
quency fluctuations SAF(f ) is simply the magnitude of
the mean squarefrequencyfluctuation
in a l-Hz bandwidth at the noise or carrier offset frequencyf. SAF(f ) is
given in unitsof Hz2perl-Hzbandwidth.Thepower
spectral density of phase fluctuations S , ( f ) is the mean
squarephasefluctuation in a l-Hz bandwidth andisrelated to S A F (f ) by
S J f ) = S*F(f)/f2.
(3)
S , ( f ) is given in units of radians' per l-Hz bandwidth.
1 X
asmallangleapproxIf S,( f ) is lessthan
imation can be made and S, ( f ) may be interpreted as the
double-sideband
noise-to-carrier
ratio.
The
quantity
6:( f ) is used to denote the single-sideband noise-to-car-
-
rierratio in dBc/Hz [65], and is expressed in terms of
S m ( f ) by
6:(f)
= 10 log ( % A f ) / 2 ) .
(4)
Strictly speaking, 6: ( f ) should only be used if its value
is less than
-20 dBc/Hz, but it is commonly used even
for much highervalues. S, ( f ) and SAF(f ) are, of
course, not limited to smallvalues,sinceit
is onlythe
interpretation of phase (or frequency) fluctuations as FM
sidebands that is limited to small values.
Another parameter used to quantify random-frequency
fluctuations is the Allan variance 0;( T ) [66]. This parameter is the average value
of one half the square of the fractional change in frequencybetweentwoadjacentfrein atimeinterval
r.
quencymeasurements,eachmade
Actually the parameter most often specified for an oscillator is the square root of the Allan variance,U,,( T ) , which
is conceptually easy to understand as 0.7 times the average fractional frequency change for the time interval T.
In most free-running (non-phase-locked) SAW oscillators the spectral densities have a characteristic power law
dependence on the noise (or offset) frequency f. This dependence is illustrated in Fig. 24 for S4( f ) and SAF(f ) .
Normallythe
(whitefrequency)and
f-' (flicker
phase) sections of S, ( f ) are not present simultaneously,
and in most SAWoscillatorsthe f - I section is absent.
For each characteristic slope in Fig. 24 one can calculate
values of u v ( r ) from either S , ( f ) or S A F (f ) [64]. Fig.
25 shows thatc r y ( r ) has a powerlaw dependence on r that
is analogous to-that of the spectral density dependence on
f. Thepowerlawdependenceon
f (or r ) is generic in
nature,andforindividualoscillatorstheexponents
may
not beexactlyintegers(orhalf-integers)
but may vary
above or below the indicated values.
The sources of phase or frequencynoise in SAW oscillators(acousticoscillators in general)aregenerally not
well understood. In some cases the noise source
maybe
external to the oscillator, such as frequency noise caused
by randomtemperaturefluctuationsorvibration.However, in many cases the noise is spontaneous in the oscillator and is generally thermal in origin. One of the better
understoodsources of noise is thermalvoltagefluctuations, or Johnsonnoise.
To helpunderstand how frequency fluctuations arise it is instructive to use the model
of Leeson [g]. Fig. 26 shows a typical SAW oscillator in
both the open- and closed-loop configurations.
If an RF
signal at the intended oscillator frequency and powerlevel
is passed through the oscillator components in the openloop configuration, phase fluctuations will be imparted to
the RF signal.The thermallygeneratedvoltagefluctuations due to Johnson noise will be added to the RF signal
and will cause both amplitudeandphasefluctuations.
However, one of the oscillator components will be operating in a gain compression mode, and this tends to suppress the AM noise. S, ( f ), or the double-sideband noiseto-carrier ratio, for Johnson noise is given to a good approximation by
S, ( f ) = 2GFKT/P,,
(5)
-
f-'
357
PARKERANDMONTRESS:PRECISIONSAWOSCILLATORS
+-+--L
Open-loop
A+
+- 26 FKT
PO
l
1
I -
I
I
I
IFlicker I White
I Phose I Phase
I
l
l
1
1
I
Random !Flicker IWhite
Wdk Freq I Freq.
Freq.
I
l
f
1
I
-
f
Fig. 24. Characteristic power law dependence of simple oscillator spectral
densities.
t
2GFKl
Po
f
Fig. 26. Spectral density of phase fluctuations for SAW oscillator i n both
open- and closed-loop configurations.
white
Phase
I
I
I
I
I
I
27r. The oscillator frequency then tracks the phase variations and the white phase noise becomes white frequency
noise. This imparts a 1 / f ' slope to the oscillator's phase
noise as shown in Fig. 26 (see ( 3 ) ) . For resonators,f, can
also be expressed as
z
I
I
Fllcker I white I flicker I Random
I Phase 1 Freq. I Freq. I Walk Freq.
'
Fig. 2 5 . Characteristicpowerlawdependence
of simpleoscillatorfractional frequency stabilities.
where G is the compressed power gain of the loop amplifier, F is the noisefactor of theamplifier, K is Boltzmann's constant, Tis the temperature in "K, and P, is the
carrierpowerlevel (in watts) at the output oftheloop
amplifier. For 6:( f ) (in units of dBc/Hz) this becomes
C ( f ) = -174
+ G + F - P,
(6)
where G and F are expressed in dB and P , is in dBm. For
offset or noisefrequenciesfarfromtheoscillator
frequency this noise level is the same for both the open- and
closed-loopconditions.SinceJohnsonnoise
is independent of noise frequency, this results in a flat spectrum for
S,( f ), as shown in Fig. 26, and is referred to as white
phase noise.
As the noise frequency gets closer to the oscillator frequency, a significant difference in phase noise levels appears for the open- and closed-loop cases. At a noise frequency f,equal to 1 /( 27r7,), where 713is the group delay
or phase slope of the acoustic device, the phase fluctuations caused by the Johnson noise begin to influence the
oscillatorfrequencythroughtherequirementthatthe
Dhaseshift around the IOOD must be an inteeer multiDle of
f,= Fo/(2Q~).
(7)
which is equal to one half the 3-dB bandwidth of the resonator.
Since the open- and closed-loop noise levels are significantly different close to the carrier, it is important to distinguishbetweenthe two. Therefore from this point on
S,( f ) willbe used todenoteclosed-loop(oscillator)
phase noise while S;( f ) will be used to denote open-loop
phase noise. Thus for Johnson noise
S$( f
1 = 2GFKT/P, (8)
and
or
%(f)
=
[ 2 G ~ K T / P , I I [ ( F ~ ~ / ( 2 Q L+f )1)1' .
(10)
To reduce oscillator phase noise caused by thermal noise,
the noise factor and required gain of the amplifier should
be minimized and P,, should be maximized. Close to the
carrier, 7R (or Q,, ) alsobecomes a factor.However, it
should be noted that increasing 7,. or 0 , (for a given O , , )
358
IEEE
TRANSACTIONS
ON LLTRASONICS.
FERROELECTRICS.
AND
FREQUENCY
CONTROL,
will increasetheinsertion loss of theSAWdeviceand
more amplifier gain will be required. For resonators, the
optimum noise level in the white frequency range is obtained if the insertion loss is approximately 6 dB 1291.
For noise frequencies even closerto the carrier, another
type of noise process sets
in. This process usually has a
1 lfdependence in S,$( f ) and is commonly called flicker
or 1 lfnoise. This process is fundamentally different from
Johnson noise in that it is usually not a noise voltage but
a direct modulation of the phaseshift through one
of the
oscillator components. If the transition to 1 /f noise, fcl,
in Fig. 26 occurs at a frequency less than or equal to fT,
there will be no region in S,( f ) that has a 1 lfdependence. If fa is greater than f,. there will be a 1 /f region
but no 1 /f2region. For most SAW oscillators, f, <
and as shown in Fig. 26 the 1 /f dependence in S$ (f)
becomes a 1 / f 3 dependence in S, ( f ) . Note that for these
conditions a 1 /f dependence in Si( f ) results in a 1 /f
dependence in S A F (f ) since the oscillator frequency will
track the open-loop phase variations.
While little is known about the causes of l/f noise in
SAW oscillators, we do know that it is usually the SAW
of flicker noise. The
device that is the dominant source
open-loopconfiguration in Fig. 26 is particularly useful
for determining the source of 1 /f noise since the oscillator components can be measured individually. Typically
a good-quality silicon bipolar transistor amplifierwill have
3 dB of gain compression)of 6‘( f
a flicker noise level (at
= 1 Hz) = - 135 dBc/Hz. For lessgaincompression
the amplifier flicker noise levelwill usually be even lower.
SAW resonators or delay lines, however, are typically in
the - 130 to - 110 dBc/Hz range atf = 1 Hz [67]-[71].
Since 1 lfnoise is generally a modulation process, and not
caused by an additive voltage noise, increasing the loop
power level does not result in a reduction of noise level
and usually has little effect at all [67], [68]. The 1 lfphase
noise in SAW resonators appears to be caused by fluctuations in the resonant frequency, and it has been observed
thatchanging QL by impedancematchingalso has little
effect on theoscillator’sphasenoiselevel(721.Only
if
theamplifier (orsomeothernonacousticcomponent)
is
the dominant noise source will an increase in QL result in
reduced oscillator flicker noise.
The source, or sources, of 1 /f noise in SAW devices
have not yet been identified, but it has been clearly established that fabrication variables do influence the I lfnoise
level 1691, [70]. Generally there is considerable scatter in
noiselevelsamongotherwise“identical”devices,
so it
is diHicult to make accurate predictions of 1 lfnoise levels.However, sufficientnumbers of deviceshave been
measured to provide some general guidelines for estimating approximatenoiselevels.Thefactthattheaverage
valueof C ’ ( f = 1 Hz) is roughlyconstant ( - 125
dBc/Hz) for SAW resonators over the frequency
range
of 100 MHzto 1000 MHzresults in anapproximate F:
dependencefor S, ( f ) [72],[73].Frommeasurements
made on a large number of devices, an empirical relation
in
has beenderivedforestimatingflickernoiselevels
fT,
-
VOL. 35. NO. 3 , MAY 198X
SAW resonator oscillators
S,( f ) = [2
F:] /f’
X
(11)
where F,, is in Hz. Actual noise levels may be as much as
a factor of two or three less than the value calculated from
(1 1) or even several orders of magnitude larger, but most of our resonators fall within a factorof 3 of this estimate. Equation (1 1) hasbeenfoundtoapplyequallywellfor BAW resonators [72], [74], [75]. For most SAW resonator oscillators. wheref,, IfT, (10)
and ( 1 l ) can be combined to give an expression for the
to a
overallphasenoisespectrumwhichisvaliddown
noise frequency of about 0.1 Hz
SJf) =
+
(&/f3)
*
[2GFWP,I
[ ( F 0 / ( 2 Q ~ f ) ) *+ 11
(12)
where a = 2 X IO-”‘. Fig. 27 shows 6:( f ) for a 500MHz SAW resonator oscillator over the noise frequency
range of 1 Hz to 10 MHz. As is common in many SAW
oscillators,there is little or noregion of 1 / f * dependence, and the noise spectrum is comprised primarily of
flicker noise ( 1 / f 3 ) and the white phase noise floor. For
thisoscillator,G = 15.8 (12 dB), F = 4 (6 dB), P,, =
0.1 W ( +20 dBm), F, = 500 MHz, QL = 6000, and (Y
= 8 X
(experimentally
determined).
The spectral density of flicker frequency noise can be
converted to uy(7 ) [64] by using the relation
U\.(.)
= [ 2 ln ( 2 )
7
k,/F,]
1/2
(13)
where k , = S , ( f ) f ’ . Combining (1 1) and ( 1 3) gives an
estimate of cy(7 ) for SAW resonator oscillators
u,(7)
=5
X
lo-’” F ,
(14)
where again F,, is in Hz. Note that uy( 7 ) is independent
of measurement time 7 for flicker frequency noise. This
is commonly referred to as the “flicker floor.”
An expression similar to ( 1 1) has also been empirically
derived for delay line oscillators [76] and is given by
S,(f) = 2
X
10-13/(7:f3)
(15)
where 71:is in seconds. As with resonators, this is only an
estimate of flicker noise level and there will be substantial
variation from device to device. Note that the flicker
noise
in delay lines does not depend on device frequency, but
only on the delay time 7R. The 1 / T ; dependence in (15)
has not been firmly established and may not be valid over
a wide range of 7 ? [71], but for values between 0.5 and 2
ps it does give reasonable results for the observed flicker
noise level in delay-line oscillators.
I n a high-vibration environment. which is often present
in airborne and missile applications, the close-to-carrier
phase noise maybe dominated by the effect of vibration
onoscillatorfrequency[63],[77].
If the vibration is sinusoidal, the induced single-sideband noise-to-carrier ratio, in dBc, will be
Pssb/P,,
=
10 log [irF,1R)/(2h,)123
(16)
PARKERAND
359
MOKTRESS: PRECISION SAWOSCILLATORS
-20 I
I
1
1
10
1o2
-200
l
Id
:c?
f
l?
1 o6
(Hz)
Fig. 27. Single-sideband phase noise-to-carrier ratio for 500-MHz S A W oscillator.
where y is the vibration sensitivity of the SAW oscillator,
g is the peak acceleration in g ’ s (units of acceleration equal to the Earth’s gravitational acceleration). For a random vibration
h, is the vibration frequency, and
where G is the spectraldensityoftheappliedvibration
level in g2/Hz at frequency f . The vibration sensitivity y
is in fact a vector and must be defined for three mutually
orthogonal axes [78]. As mentioned earlier, for SAW devices y is generally in the range of IO-]* to 10-9/g for
each axis.
For frequencies very close to the carrier( f < lop2Hz)
it has been observed that SAW oscillators tend to exhibit
a l /f4 dependence in S , ( f ) which corresponds to a random walk in frequency ( S A F (f ) cc 1 / f 2 ) . Fig. 28 shows
typicallevels of random-walknoisefordelay-lineand
resonator oscillators near 400 MHz [45]. The presence of
this random-walk noise causes random frequency fluctuations to appear in aging data (two-year measurement pef l PPMfordelaylinesand
riod)atlevelsaslargeas
about fO.05 PPMforresonators.Random-walkfrequency fluctuations have also been observed in BAW oscillators [79] and were traced to temperature fluctuations.
However, the randomwalk noise level in SAW oscillators
is in most cases toolarge to be caused by temperature
fluctuations. It has not yet been determined whether other
environmentalfactorsmaybecausingthesefrequency
fluctuations or if the noise originates in the SAW device
as it does with flicker noise.
One final point worth noting is the effect of frequency
multiplication on oscillator noise. Regardless of the source
N increases
of thenoise,multiplication
by afactorof
S , ( f ) by N’; or for d:( f ) the effect of multiplication is
to add 20 log ( N ). Since the white phase noise floor of
SAW oscillators is comparable to thatofquartz-crystal
BAW oscillators, the process of multiplying a
BAW oscillator up to SAW frequencies will cause the multiplied
noise floor to be substantially higher than a SAW oscil-
IO
-
-
8 -
-
400 MHz SAW DEVICES
0
Delay
Llne
A
Delay
Line
B
6 -
-
4 -
@ 2 U
ma
-
0
so-2
-
-
-
-4 -
-6
-
-8
-
Fig. 28. Spectraldensity of frequencyfluctuations for several 400-MHz
SAW oscillators.
lator’s noise floor. However, in the flicker frequency region of the noise spectrum the opposite is true. Here the
result of using a factor of N higher oscillator frequency is
toraisethephasenoiselevelof
N4 (see ( 1 1)) whereas
multiplication by N will increase the phase noise level by
only N 2 . For vibration-induced phase noise the increase
360
IEEE TRANSACTIONS ON ULTRASONICS,
FERROELECTRICS,
AND
FREQUENCY
CONTROL,
in noiseisthesamewhethertheoscillatorfrequency
is
increased or frequency multiplication is used (see (17)).
The only relevant parameters here are the respective vibration sensitivities for the two oscillators.
VOL. 35, NO. 3. MAY 1988
cases the frequency window for all causes over ten years
may be as tight as & 10 PPM. Obviously a requirement at
this level can only be met if the oscillator is ovenized or
temperature compensated [80] and aging rates well under
k 1 PPM/year canbeconsistentlyachieved.FurtherVI.A PPLICATIONS
more, the oscillator's frequency must be accurately set in
TheSAWoscillatorapplicationsthathavebeenadorder not to use up a significant fraction of the frequency
dressed over the years by the authors have primarily been window with set-on errors. In the authors' experience the
in high-performancesystems.Theseapplicationshave
importance to system performance of reduced oscillator
generally fallen into two broad categories; namely, air de- noiselevelshasbeensufficientincentivetosustainthe
or positionlocation
fenseradarsystemsandnavigation
effort to improvethelong-termstabilityandfrequencysystems.Theperformancerequirements
placed onthe
setting technology of SAW oscillators. This
effort has paid
SAW oscillators canbesubstantiallydifferentforthese
off,and SAWoscillatortechnology is now available to
twocategories.Modemradarsystemsrequire
verylow
meet these requirements.
phase noise levels for noise frequencies greater than apIn ground-based radar systems the vibration levels are
navigation relatively low, and space and weight requirements are not
proximately 1 kHz.Forcommunicationand
systems the most stringent specifications on phase noise
particularly tight. Here TO-8 packaged SAW devices and
performance are for noise frequencies less than approxiprintedcircuitboardelectronicshavebeen
acceptable.
mately l kHz. In both cases the requirements for medium- However, in airborne applications the vibration levels are
andlong-termfrequencystabilitycanbequitedemandquite high and space and weight limitations are tight. This
ing. We will first address the use of SAW oscillators
in is an area of current interest. In a missile environment the
radar systems.
vibration level may be as high as0.2 g2/Hz in the 50-Hz
40-50 dB degraIn a modem air defense radar the Doppler shift of the to 3-kHz range, and this will lead to a
dation in phase noise over this frequency range for a highreturn signal is used to distinguishmovingtargetsfrom
quality SAW resonator oscillator with a vibration sensistationarybackground.However,
in thepresenceofa
strong stationary return signal it is very important that the tivityof 2 X lOW9/g. An importantarea ofcurrent retransmitted signal (and hence the return signal) have very searchisthereductionofvibrationsensitivityinSAW
oscillators.Inadditiontovibrationsensitivity,sizeand
low noise sidebands so that a weak but frequency-shifted
target return can be detected. Typically this requirement
weight are also important considerations, and here theuse
manifestsitself
in phasenoisespecificationsforboth
ofhybridcircuittechnologyis
ofconsiderableinterest.
transmitter and receiver oscillators that are very stringent The all-quartz package is particularly attractive for use in
for noise frequencies greater than about 1 kHz. The main hybrid circuit SAW oscillators since it offers state-of-theadvantage of SAW oscillators for this type of application art long-term frequency stability in a small volume and in
a package configuration convenient for hybrid circuits. It
is that quartz-crystalstabilitycanbeachievedathigher
is also compatible withavarietyofmountingmaterials
frequencies,andthereforelessfrequencymultiplication
is needed to achieve final transmitted frequencies at L- or and techniques that might be used for reducing vibration
X-band. A lower multiplication factor means a lower noise sensitivity. Fig. 30 shows a 400-MHzhybrid circuit SAW
[8 l ] .
floor. Fig. 29 shows a typical phase noise requirement for oscillator using an all-quartz package SAW resonator
In navigation or communication system applications the
an X-band radar and the predicted performance of a 500MHzSAWoscillatorfollowed by a times 20 multiplier phasenoiserequirementsfornoisefrequenciesgreater
with theoretical performance. The measured phase noise
than approximately l kHz are not usually very tight, but
of the 500-MHz oscillator shown in Fig. 27 was used as it is for frequencies less than about 1 kHz where the stathebasisforcalculatingthemultipliedSAWoscillator
bilityrequirementsarestringent.Ingeneral,
multiplied
noise in Fig. 29. If a 10-MHz BAW oscillator were used low-frequency BAW oscillators give the best noise
peras the master frequency source instead of the SAW oscil- formance for these applications since lower close-to-carlator, an additional multiplication factor of 50 would be rier noise levels are obtained by multiplying a high-qualrequired,andthiswouldresult
in afurthernoise
floor ity low-frequency BAW oscillator than by using a higherdegradation of 34 dB.
frequency SAW or BAW oscillator with less multiplicaIn some cases, low-noise SAW oscillators are replacing tion. However, some position location applications have
lumped element L-C oscillators. In these instances the re- been addressedwheresizeand/orpowerconsumption
quirementsformedium-andlong-termstabilityare
not constraints have precluded the use of long multipliedfilter
very stringent. In one application the frequency window
chains. In one case a major requirement was that a very
smallfixed-frequencyoscillator
in the 350-475 MHz
was large enough that neither frequency trimming of the
SAW devices nor temperature control was needed for derangehaveafrequencystability
of 3 X lo-'' formeasurement times ranging from l to 100 S . The first step in
vicesfabricatedon ST-cut quartz. However, more often
than not oscillator requirements are derived with the perachieving this is,of course, good temperature control,but
even when this is accomplished the oscillator must have
formanceof BAW oscillators in mind and themediumor betterthan 3
aninherentfrequencystabilityequalto
andlong-termstabilityspecificationsaretight.Inthese
PARKERANDMONTRESS:P RECISIONSAWOSCILLATORS
36 1
Typical X-Band Exciter Requirement
-20
I
2
a
%
-40-
-
60-
-
-a0
k
- 1 00 -
-120 -140 -
-
-160
180
-200
500 MHz
Hlgh Power SAW
Referencedto
X-Band
I
l
10
lo'
13
lo"
f
Id
1I
1 GE
(Hz)
Fig. 29. Single-sideband phase noise-to-carrier ratio for typical X-band radar exclter requirement and predicted perforn1anc.e o i
500-MHz S A W oscillator after multiplication by > ; N .
Fig. 30. Photograph of hybrid circuit 400-MHz SAW resonator oscillator.
partsin 10". This is not at alldifficultforamultiplied
low-frequency BAW oscillator, but this performance level
canalso be met by a high-quality SAW resonatoroscillator.Fig. 31 showsthemeasuredfractionalfrequency
stability uv( 7 )of a 425-MHz resonator oscillator for meaI O ms to over 1 X 10' S .
surement-timesrangingfrom
This oscillator comfortably meets the required frequency
stability.
-
A second position location application required a 400
15 min of
MHz oscillator with a frequency stability over
1 X
and
maximum
deviation
of
f 2 X lop9 about
the average slope over 15 min [81]. Again a major concern is temperaturecontrol,butoncethis
is achieveda
SAW resonatoroscillatorcancomfortably
meettherequirement.Fig. 32showsthemeasuredfractionalfrequency change of a 425-MHz SAW oscillator for a period
362 IEEE TRANSACTIONS ON ULTRASONICS,F ERROELECTRICS,A NDFREQUENCYCONTROL,
VOL. 35, NO. 3, MAY 1988
h
P
v
10‘0 FLICKER FREQUENCY RANDOMWALK
FREQUENCY
10”
l
I
1 102
I
103
101
I
I
I
I
102
I
104
10
T (seconds)
Fig. 31. Fractional frequency stability of 425-MHz SAW resonator oscillator.
1.S
LL
oa
1.o
-1
%
P
c
2
Q
Q
05
l
-2
0.0
-3
4.5
0
I
I
I
I
2Ooo
4000
6000
8WO
10000
TIME (seconds)
Fig. 32. Observed temperature and fractional frequency change with time for 425-MHz SAW resonator oscillator.
of 1 X lo4 S (2.8 h). With a temperature stability of
+l
X 10-30C, the SAW oscillator frequency was stable to
about 1 part in lo9 for the entire measurement period.
VII.CONCLUSION
At thistimeSAWoscillatorshaveestablishedthemselvesasviableVHF,
UHF, andlowmicrowavefrequency sources. They offer medium- and long-term frequencystabilitiesapproachingthoseusuallyassociated
with low-frequency quartz-crystal BAW oscillators, while
at the same time offering substantial improvements when
compared to multiplied BAW oscillators over certain portions of the phase noise spectrum. Furthermore, two-port
SAW devices are very convenient and easy to use in feedback-loop oscillator circuits. The proper oscillation conditions for this type of oscillator are readily established
and problemscanberapidlydiagnosed.Inmanycases
alternative technologies are pushed to their limits in complexity and performance in order to meetlownoiserequirements. The much simpler SAW oscillator can many
timescomfortablymeetthenoiserequirements
and will
It is
ultimately lead to a more reliable frequency source.
theauthors’beliefthatSAWoscillators
will eventually
become the standard frequency source for many applications requiring stable oscillators in the VHF, UHF, and
low microwave frequency ranges.
ACKNOWLEDGMENT
The authors are grateful for the valuable contributions
of the following individuals to the research and development reported herein: Mr. M. Bennett, Dr. J . Callerame,
Ms. J. Columbus, Dr. J. Greer,Mr. P. Harkins,Mr. J .
Lang, Mr. M. Loboda, and Mr. E. Sabatino. The authors
are also indebted to Regina Guerin for her patience
in preparing this manuscript.
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pp. 105-120. May 1971.
I . H. Shodf. D. Halford, and A . S. Risley. “Frequency stability spec(tication and measurement. High frequency and microwave signals,”
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Ian.1 973.
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J . L. Henderson, “Annealing behavior and phase noise performance of SAW resonator\,” in Proc. IEEE Ultrason. Symp.. 1985,vol. 1. pp. 247-252. W J . Tanskl. “The inHuencc o f a chrome film bonding layer on SAW resonator pcrtormance,” i n Proc. IEEE Ulrruson. S y m p . , 1985, vol. I . pp. 253-257. R . L . Jungcrnran. K . L. Baer,andR. C. Bray,“Delaydependence
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[BO] M . F . Lewis,“TemperaturecompensationtechniquesforSAWdevices,” in Proc. IEEE Ultrusorz. Symp., 1979, pp. 612-622.
[Sl] G . K . Montress. T. E. Parker, and J . Callerame. “A miniature hybrid
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Thomas E . Parker (”79-SM’86)was
born in
Natrona Heights. PA, o n September 17, 194.5. He
receivedtheB.S.degree
i n physicsfromAllegheny College in 1967. He receivedthe M . S degree i n 1969and the Ph.D. degree i n 1973, both
i n physics, from Purdue University. His doctoral
thesiswasa Brillouin scattering study of acoustoelectric domains in GaAs.
In August 1973 he joined the stafl of the RaytheonResearchDlvision.working
with the GcneralizedFiltersandMicrowaveAcoustics(now
Stable Sources) Group. lnltially his work was mainly related to the
dcvelopment of improvedtemperaturestablesurface
wave materials. Hewas
responsible for the development of the fused silica-lithium tantalate structure, which not only has higher piezoelectric coupling, but also has only
one-tenth the temperaturesensitivity ofST-cut quartz.More recently he
has been responsible for the sur~ace-wave-controlled oscillator programat
the Research Division.His primary interest has been frequency stability.
with emphasis on l l f n o i s e , temperature stability, and aging.
Dr. Parker is a member of the IEEE, Sigma Pi Sigma, and Sigma X I .
He has served on the Technical Program Committees for
both the UltrasonicsSymposiumandtheFrequencyControlSymposium.Hcwas
FInanceChairmanforthe1980UltrasonicsSymposiumand
is thecurrent
Finance Chairman for the Frequency Control Symposium.
Gary K . Montress (S’66-M’76-SM’87) was born
in East Orange,NJ, o n April 10, 1947. He re-
ceived the B.S.E.E., M.S. E . E . , Electrical Engineer, and Ph.D. degrees from theMassachusetts
Institute of Technology, Cambridge, MA, in
1969.1971.1971,and 1976. respectively.
From1969to
1972,while at MIT. he was a
Teaching Assistant i n thc ElectricalEngineering
Department.where he taught courses on s o l d
state electronics and circuit design and also pursuedresearch i n the area of p-n junct~onbreahdown phenomena. From 1972 to 1975 he was a n Instructor i n the Electrical
EngineeringDepartment,teachingandsupervisingcourses
in solid-state
physicsandmicroelectronics.From 1975 to 1976. while a Research Assistant in theResearchLaboratoryforElectronics
at MIT. he completed
his Ph.D. thesis research and dissertation in the area o f wlid-state microwavedevices.From1976to
1984 he wasamemberoftheprofcssional
staff at the United Technologies Research Center, East Hartford. CT, where
he was involved in research anddevelopmentactivitlesrelatedtosolidstateelectronics,SAWfrequencycontrol.and
signal processingcomponents, and GaAs material and device technologies for SAW and electronic
device applications. Since October 1984 he has been a membcr of the professional staff at the Raytheon Research Division. He i \ currently engaged
in researchanddevelopment
activities relatedto stableVHF,UHF.and
microwave frequency sources, including both SAW and dielectric resonator based oscillators andsynthesizers.Hisresearch
interejth also include
thedevelopmentof
low noise hybridand !vlMIC circuitry using \ilicon
bipolar transistors, for applicationto extremely low noise frequency sources
operating in the 100-MHz to 20-GHz frequency range.
Dr. Montress is a member of Eta Kappa N u , Sigma X I . andTau Beta
Pi. His IEEE activities include currently serving as an oHicer o t the Boston
Chapter of UFFCS and as a member of the Technlcal Program Commlttcc
for the annual Ultrasonics Symposium.