Quartz Crystal
Reliability and Application
Copyright, 1998
©
Hong Kong X’tals Limited.
Arthur Lee
Technical Director
1
What is a Quartz Crystal ???
• Production Engineers :
– It is the first component you replace
when you have a failure board !
• Purchasing Officers :
– It is a component that you could
never get a reliable vendor !
• R&D Engineers :
– It is a mysterious little black box !
Hong Kong X’tals Limited
2
What is a Quartz
Crystal ?
A crystal is a Piezo-Electric
Device, i.e. :
Energy is converted between
electrical and mechanical,
millions of times per second.
Hence, its characteristic is very different form
other components, as oppose to :
• Resistor -- Pure Electrical
• Screw -- Pure mechanical
3
Hong Kong X’tals Limited
The Structure of a
Quartz Crystal
Cover
Silver Electrode
on both sides
Quartz Wafer
( Blank )
Silver Epoxy
Sealed with
Clean, Dry,
inert gas
Base
Hong Kong X’tals Limited
Mechanical Energy
Stress and Motion
Electrical Energy
Voltage & Current
4
Equivalent Model of a
Quartz Crystal
L1, C1 :
Mechanical Energy -Stress and Motion
Electrical Energy -Voltage and Current
RS :
Energy Loss
Hong Kong X’tals Limited
Co :
Electrode Capacitance
Rs : (ESR) Equivalent Series Resistance
Co : (Shunt Capacitance) Electrode Capacitance
C1 : (Cm) Motional Capacitance
L1 : (Lm) Motional Inductance
5
Raw Parameters
Rs, Co, C1 and L1 give you all the information
concerning impedance change of a crystal near
by its operating frequency.
Crystal Impedence vs Frequency
J part of
Crystal
Impedence
Frequency
Hong Kong X’tals Limited
6
How to Specify a Quartz Crystal
The first thing to do is to
understand your circuit :
• Application Aspects,
then, your products
Environmental Requirement :
• Reliability Issues.
Hong Kong X’tals Limited
7
Your Application
Copyright, 1998
©
Hong Kong X’tals Limited.
8
Basic Crystal Oscillator Model
Gain=A
Phase=
p
Amplifier
Phase
compensation
network
Crystal
Hong Kong X’tals Limited
3 Main blocks of an
oscillator :
Amplifier, Phase
Compensation
Network,
and Crystal is only
one of the three
parts in the whole
oscillator
feedback loop
Hong Kong X’tals Limited9
Equivalent Model of an Oscillator
Gain=A
Phase=
p
Load
Negative
Capacitance
Resistance
CL
 
Drive Level =
Vx * Ix
Ix
Hong Kong X’tals Limited
Vx
RL
XLe= - XCL
10
Basic Characteristic
of your circuit
The Amplifier and phase compensation
network determines :
• Gain & Phase Shift -> Negative Resistance
• Phase Shift
-> FR or FL ( hence CL )
• Voltage Amplitude -> Drive Level
Hong Kong X’tals Limited
11
How to measure circuit
characteristic ?
Very Difficult !!
The crystal is operating at high impedance and small
capacitance. Hence, inserting any measurement
probes will offset the operating conditions :
• Oscilloscope probes ( 2 pF ~ 20 pF ) : too much
• Vector voltmeter probes ( 50 Ohm ) : too much
Send your PCBA to your vendors for measurement.
Hong Kong X’tals Limited
12
Negative Resistance and
Maximum ESR
Negative Resistance of a oscillator circuit is the maximum
ESR of a crystal at which the circuit will oscillates. However,
this negative resistance may change due to :
•Power supply variations
•Temperature change
•Component variations
•Start up
•The crystal’s ESR may change due to drive level, temperature
So, usually, Max. ESR of crystal =
1/3 ( to 1/10 ) of Negative Resistance
Hong Kong X’tals Limited
13
Drive Level Definition
• By definition, very simple,
Drive Level ( operating ) =
Power dissipated in the crystal =
vector product of voltage Vx and current Ix
• This depends on your circuit, Not the crystal.
• Crystal maker will calibrate / test the crystal at this
drive level
• Specifying Maximum drive level is quite meaningless.
• Current probes will affect phase angle ( CL ).
Send your PCBA to your crystal vendor for measurement
Hong Kong X’tals Limited
14
What is Load Capacitance, CL ?
Load
Negative
Capacitance
Resistance
CL

RL
XLe= - XCL
Hong Kong X’tals Limited
• Load Capacitance describes
the phase shift of your circuit.
• A single crystal can operate at
any phase shift from 0 up to
+90 degree, hence each
crystal can be “Series” and
“Loaded” resonant.
• Crystal makers will calibrate
the crystal’s frequency at that
phase shift.
15
FS, FR, FL and FA
Crystal Impedence vs Frequency
Fa =
Frequency
at which J
part of
impedance
approaches
infinity
Fr ( CL = infinity )
Fs
J part of
Impedence
FL
Frequency
Larger CL
Smaller CL
Not necessary to specify FA
Hong Kong X’tals Limited
16
FS, RS and FR, RR
When at zero phase :
Frequency = FR ( Resonant Frequency )
FS 2.  .
1
( L1. C1 )
Impedance = RR,
No imaginary part
RS
Co
• Below ~60MHz : FS = FR, RR = RS
• Above ~120MHz : FR and RR do not exist, because No
zero phase shift due to impedance of Co 17
Hong Kong X’tals Limited
FL, RL and FP, RP
At Zero phase
(including the external CL)
Frequency = FL also known as FP
RL RS. 1
Co
External CL
( Load Capacitance )
2
CL
Ideal ( no stray )
RL = RP
• Always specify CL with FL
• Due to big measurement errors in RL, DO NOT specify RL
• However, RL is the actual impedance seen by your circuit
Hong Kong X’tals Limited
18
How to specify Frequency
and ESR ?
• Specify the exact Nominal Frequency
that the oscillator is going to operate, e.g.
if you want a 3rd o/t 36MHz, then specify
36MHz, not 12MHz.
• If the circuit is zero phase specify :
– FR, and RR or
• If the circuit is phase lag specify :
– FL, CL and RR
Hong Kong X’tals Limited
19
How to specify Co ?
If your application does not require pulling,
and the frequency is below 100 MHz,
the value of Co is not important.
In such case, putting Co < 7 pF is harmless,
and also quite meaningless.
Hong Kong X’tals Limited
20
Co : in pulling applications
Applications that require to change the oscillator
frequency are pulling applications ( e.g. FM,
VCXO, PLL FSK etc).
• Any ONE of these parameters specifies pulling
uniquely : Ts, FL1-FL2, DF
• These parameters also specify pulling, but
indirectly : C1, L1, Co, Q
Hong Kong X’tals Limited
21
Do NOT Multi-specify
Pulling parameters
Pulling ranges
• Specify one and only one parameter for pulling.
Effective
pulling
range
Co
C1
Ts
• Each crystal has only one pulling characteristic,
more than one definition would only create
confusion or contradiction.
Hong Kong X’tals Limited
22
Do Not Specify Co for pulling
L1, Co, Q are indirect parameters affecting
pulling.
Depending on design, process, and raw material,
crystals with the same Co may exhibit different
pulling characteristics. So, do Not Specify Co
for pulling.
Some crystal makers may use C1 for pulling
control, this is acceptable.
Hong Kong X’tals Limited
23
Co at Higher frequency
At higher frequencies ( about > 100 MHz ), the impedance of
Co becomes very small, so Fr does not exist ( no zero phase
shift ), and the crystal behaves like a capacitor.
Usually an external inductor is required to null the Co, so that
zero phase exist again.
In this case, specify :
Co = Vendor suggested
value +/- 15%

Hong Kong X’tals Limited
1
Co. Lnull
24
Pullability
• If your application is PLL, VCO, FM etc.
must specify either Ts ( ppm/pF ), FL1-FL2,
DF or C1.
1 .
Ts
1
CT
1
C1
CT Co
CT
Consider using Fs in these applications.
• If your application is a simple oscillator,
specify maximum Ts will give you higher
effective Q.
Hong Kong X’tals Limited
CL
25
Temperature Characteristic
AT Cut
• For AT Cut, usually +/- 30 to 50 ppm over -10 to +60
• Always refer to 25 degree C as 0 ppm
50
Frequency Deviation / ppm
30
10
-10
-30
-50
-55
-45
-35
-25
-15
-5
5
15
25
Temperature / Celcius
Hong Kong X’tals Limited
35
45
55
65
75
85
26
Temperature Characteristic
BT Cut
• For BT cut usually over 200 ppm
• Common habit is to refer to 25 degree C as 0 ppm
• Consider seriously to allow +80ppm typical for 25 C
Freq. deviation
( ppm)
90
70
50
30
10
-1
0
-3
0
-5
0
Temperature ( degree C )
0
-100
-200
-300
-400
-500
-600
-700
Hong Kong X’tals Limited
BT Cut Temperature characteristic
27
Overtone or Fundamental ?
Frequency
( AT cut )
Frequency
( BT cut )
ESR
Drop Shock
Performance
Pullability
Circuit
Design
Hong Kong X’tals Limited
Fundamental
1 MHz to 30 MHz
Overtone
18 MHz to over 100 MHz
20 MHz to 40 MHz
Seldom considered
Lower ESR
The higher the frequency,
the worst the drop shock
Can be designed for FM,
PLL, VCO applications
Higher ESR
Better due to much thicker
blanks
Very High Q, hence,
frequency less dependent on
other component variations.
Usually requires filters to
prevent crystal oscillating at
its fundamental frequency
Usually does not require
filter to prevent crystal
oscillating at its overtones
28
Error Budget for Frequency
Tolerances




Factor 1 : The first most important information is :
What is your Total allowable Tolerance from the nominal
frequency ?
Factor 2 : Is the Crystal working in Fs or FL ( what is the CL ? ) ?
This affects the Measurement error.
Factor 3 : Does the circuit require frequency pulling ?
( FM, PLL, VCXO etc. require frequency pulling )
This affect the Ts ( Trimming sensitivity ) of the crystal, hence,
measurement error and frequency change due to components
variations.
Factor 4 : Is there any frequency tuning device ( such as VC or VL )
in the circuit ?
Measurement error and components variations could be
tuned out during production.
Hong Kong X’tals Limited
29
Error Budget ( without VC ) for
Frequency Tolerance
Case A : The circuit does NOT have any tuning device,
such as VC or VL
Note : Measurement error could be large !
Example :
Tolerance at Room Temperature 30 ppm
+
Measurement error (=0.5pFTs of Crystal) 10 ppm
+
Component tolerance (=0.5pFTs of Crystal) 10 ppm
+
Tolerance over Temperature 30 ppm
+
Aging for 1st year
5 ppm
<= Your Total Allowable Tolerance say, 100 ppm 85 ppm
Hong Kong X’tals Limited
30
Error Budget ( with VC ) for
Frequency Tolerance
Case B : If your circuit has some tuning device,
such as VC or VL
Note : If the specification of CL is wrong, you may NOT be
able to tune the frequency to your requirement !
Example :
Tolerance at Room Temperature tuned to 0
+
Measurement error tuned to 0
+
Component tolerance tuned to 0
+
Tolerance over Temperature
30 ppm
+
Aging for 1st year
5 ppm
<= Your Total Allowable Tolerance say, 50 ppm
35 ppm
Hong Kong X’tals Limited
31
Summary :
Basic Crystal Specifications
 The Basic parameters of a Quartz Crystal :
 Nominal Frequency, ESR, CL, Co, L1 and C1
 Pullability
 Room Temperature and over temperature
frequency tolerance
 TC : Over Temperature Characteristic
 AT or BT cut
 Fundamental or Overtones
Hong Kong X’tals Limited
32
Reliability Issues
Copyright, 1998
©
Hong Kong X’tals Limited.
33
Reliability Issues
• Sleeping Crystals
– DLD : Drive Level Dependency
– Starting ESR
• Drop Test
• Spurious modes
• Temperature Cycle
Hong Kong X’tals Limited
34
Sleeping Crystals
• Sleeping crystal is a well known phenomenon in Crystal
Industry :
– The Crystal will NOT start, but after the crystal is started by some
mechanical or electrical excitation, it works fine for some time, then, after
some unknown time, it sleeps again – not functioning.
– The time that it begins to sleep again is unpredictable, could be minutes or
months.
– Not all crystal makers know how to avoid producing sleeping crystals, that’s
why not many crystal makers like to talk about this.
• Root Cause : Improper Cleanliness
• Cure : None, a sleeping crystal will sleep again.
• Test Method : DLD, or high drive
( note high drive is a test, not a cure ).
Hong Kong X’tals Limited
35
DLD or Starting ESR
• Same root cause as Sleeping Crystal, less serious.
• If your circuit has starting problem, you should specify
extra requirement of ESR at a lower drive level.
• This specification depends on your circuit’s drive level, and
starting gain.
• Do NOT over specify, it costs.
7
6
5
4
ESR
3
2
1
0
Hong Kong X’tals Limited
Drive Level
36
At 10nW Rs=37
At 100uW Rs=31, non-ideal process,
depending on applications, could be
good or no good
Crystal DLD
Typical Example
Changes due to
Design limit, not
because of poor
process
Kolinker
KH1200
Test data
Hong Kong X’tals Limited
37
Typical DLD Specification
• Ask your crystal vendor the operating Drive level (DL)
of your circuit, and also whether your circuit has a
good starting gain.
• If your circuit has a good starting gain, specify :
ESR change from 1/3 DL to DL < 30% +2Ohm
• If your circuit does not has a good starting gain,
specify :
ESR change from 1/10 DL to DL <30% +2Ohm
• This specification also safeguard against
Sleeping Crystal
Hong Kong X’tals Limited
38
Drop Test
• Very Important Reliability Specification
• Do Not over specify, it costs !
• Typical Drop shock performance :
Clip Mount :
– Below 12 MHz
– >= 12 MHz
50 cm
35 cm
Spring Mount
– Below 16 MHz
– >= 16 MHz
Hong Kong X’tals Limited
75 cm
50 cm
39
Spurious Modes ( Couple Modes )
• Couple modes always exist !
– Couple modes are caused by energy coupling from the main mode of
vibration ( AT cut thickness shear ) to other unwanted vibration mode.
Just like when you hit a guitar wire, other then the fundamental
frequency, it will always produces other harmonic and inharmonic
frequencies.
• Must specify Minimum ratio of :
ESR(worst unwanted mode) / ESR(operation) > 3
• If the unwanted modes are too active, the oscillator may :
– jump from one frequency to another
– produce impure spectrum that may jam your circuits.
• Do Not over specify, it costs !
Hong Kong X’tals Limited
40
Couple modes at one temperature
Main mode : AT Cut,
Thickness Shear
1.2
1
0.8
Couple Mode is also
known as :
•Spurious mode
•inharmonic
•unwanted mode
0.6
0.4
0.2
0
All other couple modes are NOT AT Cut,
Thickness Shear
Hong Kong X’tals Limited
41
Couple modes at room temperature
Good Example
Main mode : AT
Cut, Thickness
Shear
Kolinker
KH1200
Test data
Hong Kong X’tals Limited
All other couple modes are NOT AT Cut,
Thickness Shear
42
Couple modes over temperature
Main mode
1.2
1
0.8
0.6
0.4
0.2
When Temperature Change :
Frequency changes, ESR changes
All other couple modes change
according to the TC curves of
each of their own modes
0
1.2
1
0.8
0.6
0.4
0.2
0
Hong Kong X’tals Limited
43
The Frequency and ESR of the
Main mode is affected by how the
other couple modes are changing
over temperature.
If a crystal is not designed
properly, the crystal may exhibit
frequency jump or sever ESR
change over temperature.
Hong Kong X’tals Limited
44
Couple modes over temperature
Good Example
Kolinker
KH1200TS
Test data
Hong Kong X’tals Limited
45
Couple modes over temperature
Bad Example
Kolinker
KH1200TS
Test data
Hong Kong X’tals Limited
46
Temperature Cycle
Good Example
Kolinker
KH1200TS
Test data
Hong Kong X’tals Limited
47
Temperature Cycle
Bad Example
Kolinker
KH1200TS
Test data
Hong Kong X’tals Limited
48
Reliability Parameters that Can NOT
be Screened
•
•
•
•
•
Sleeping Crystals :
nearly impossible or too costly
DLD :
too costly
Drop test :
destructive in nature
Spurious modes :
too costly
Temperature Characteristic :
too costly
Reliability performance of commercial crystals can NOT
be 100% screened. Good Production Process and
Design is the only way.
Hong Kong X’tals Limited
49
Summary :
• Do NOT copy specification sheets. Discuss
with your vendor, and send your PCBA to
them for measurement.
• Crystal can NOT be screened, good crystals
must be manufactured with a good process
and control.
Hong Kong X’tals Limited
50
Technical Support from HKC
Send HKC your production failures
Send us your latest PCBA for crystal
specification verification.
E-mail : HKXtals@HongKongCrystal.com
FAX : ( 852 ) 24985908
Copyright, 1998
©
Hong Kong X’tals Limited
51