Vol.1 October 2004
8 A Comparison of a PLL-based Transmitter Design and a SAW-resonator-based Design
Atmel Automotive Compilation 9
SECURITY
ALARM
CAR ACCESS
The so-called SRDs (Short Range Devices) feature
A COMPARISON OF A PLL-BASED
TRANSMITTER DESIGN AND
A SAW-RESONATOR-BASED DESIGN
PERFORMANCE VERSUS COST
• Short transmitting range up to 100 m
• Low data rates (1 kBd to 10 kBd)
• Low current consumption (< 10 mA)
• Mostly battery driven (transmitters), e.g. 3-V cells
• Suitable for hand-held operation (transmitters),
Johannes Baur
small application size, dedicated antennas
ABSTRACT
The technological progress and cost reduction of integrated RF circuits allow the full integration of
frequency synthesis circuits such as PLLs even for the use in RF remote control applications.
NATIONAL REGULATIONS
Well-accepted and still technically competing are cost-reduced SAW-resonator-based applications.
The national approval organizations (ETSI - European
Telecommunications Standards Institute and FCC –
INTRODUCTION
Federal Communications Commission, US)
take care
Due to growing density of RF radiators, product com-
Buying a television set without a
approach this cost-sensitive market.
The widely accepted RKE (Remote
about regulations dedicated to all RF applications. The
pliance and concept suitability to those standards
remote control? Unbelievable, these
First RF transmitters for remote con-
Keyless Entry) applications based on
availability of channels, prescribed radiation characteris-
became more important – the spectral performance
applications are 100% equipped
trols covering the nowadays well-
IR systems in the automotive market
tics and licensing is given by EN 300 220 (EU) and FCC
dominates over system cost.
with InfraRed (IR) remote controls
accepted
Scientific
paved the way for developments
part 15 (US).
– and it’s convenient. In many appli-
Medical (ISM) bands used discrete
using integrated RF circuits with the
cations, the use of an optical link is
circuits. The core of an RF transmit-
advantage of applying PLL-based
Available ISM bands around 315 MHz, 433.92 MHz (see
sufficient. But what about garage
ter design is the frequency synthesis,
frequency
systems.
Figure 1) and 868.3 MHz were assigned for the operation
door openers, or weather stations?
those discrete solutions used a
Currently, brand-new cars do have
of SRD applications. They are restricted to +10 dBm
These applications are supposed to
ceramic or a SAW resonator. The
RF remote controls, most of them
equivalent radiated power (ERP) and a bandwidth of
bring comfort to the user, and this is
cost of the bill of material was
use PLL-based systems. Atmel has
(870 kHz, where the usage is not application specific (all
only possible in case of a successful
acceptable for these convenience
been one of the very first manufac-
valid for 433-MHz band) and license-free. Spurious emis-
data link through the surrounding,
applications; however, the RF per-
turers supplying fully integrated RF
sions are restricted as well, thus, adjacent channel users
such as garage doors, walls and
formance was just at a low technical
transmitters and receivers.
are protected up to a certain power level.
even furniture. RF technology was
level.
for a long time too expensive to
www.atmel.com
Industrial
synthesis
Figure 1.
Extract of
EN 300 220
Concerning
Power,
Bandwidth and
Antenna
Atmel Automotive Compilation 11
Vol.1 October 2004
10 Example of a PLL-based Transmitter Solution
Figure 2. ASK PLL
Transmitter Solution
T5754 and Peripherals
SECURITY
Due to the wide operating range of
Note:
some VCO designs, a robust power
Generated
supply rejection is required to guaran-
spurious
tee a spectral clean ASK switching.
emissions
ALARM
CAR ACCESS
at ±n xfclk
Regarding the integer division VCO
have to meet
down to the PLL or a possible divi-
the regulations.
sion between XTO and PLL, a microcontroller-controlled
EXAMPLE
OF A
transmitter (Tx) is possible.
PLL-BASED TRANSMITTER SOLUTION
A spin-off of an integrated PLL-Tx is a
Major blocks of a transmitter and the functional demands:
• Frequency synthesis
quartz crystal accurate clock which
• Power amplification
- Accuracy
- High efficiency
- Stability
- Less VCC and temperature dependency
- VCC and temperature dependency
- High isolation
- Phase noise and spurious generation
multi-channel
clocks a microcontroller.
Figure 2
proves that this reduces the application cost and the overall tolerance of
the RF telegram timing.
• Modulation capability of the RF carrier
- ASK: fast on-off keying without cross-talk
- FSK: flexible pulling of frequency reference
Actual design uses an RC VCO,
ity, it is sufficient to integrate just an
alternatively an LC structure could be
RC VCO. A screen shot is shown in
The design of integrated amplification
used. The difference is the amount of
Figure 3. The T5754 datasheet spec-
stages is less VCC- and temperature-
The frequency reference is a quartz
and AT cut-cubical dependency. The
A quality factor is the suitability for
generated phase noise besides the
ifies the in-loop phase noise PLL at
dependent and uses highly efficient
crystal driven by an oscillator circuit
design is centered to 25°C with the
low drive levels (100-200 nW).
carrier, which is important since the
20 kHz, i.e., the amount of noise
operation modes. This might result in
(XTO). To achieve fast settling-times
consequence that a cubical cut
Contamination during fabrication
signal is likely to be received by adja-
generated by the VCO, which is
an increase of the harmonics and
(less current consumption and short
results in less overall dependency.
results in DLD (drive level dependen-
cent channels. This is subject of the
reduced by the function of the PLL
thus requires a low-pass characteris-
cy) and may cause an increase of RS
ETSI/FCC regulations, too. The
and loop filtering.
tic of the antenna matching circuit.
response time), a phase comparator
frequency at 13.56 MHz was chosen.
Ordering parameters:
– the start-up process fails.
allowed bandwidth according to
Settling time is ts ~ 1/fXTAL2.
• Frequency tolerance:
The next part of the frequency syn-
ETSI is ±870 kHz,
centered at
Since the form factors of hand-held
initial/temperature/aging
thesis is the fully integrated PLL,
433.92 MHz, and ±0.25% of the car-
devices is crucial, a further advan-
(e.g. ±30, ±30, ±10)
charge pump, loop filter, VCO and
rier
integer divider for an accurate and
(±787 kHz) according to the FCC
stable generation of the RF frequen-
regulations.
tage is the possibility to design a
crystal blank in a smaller and lighter
packages. Applied quartz crystals
• Maximum series resistance RS
(< 110 Ω)
• Load capacitance Cl (e.g. 12 pF)
frequency
at
315
MHz
Figure 3. Radiated
cy, see Figure 2.
are available in different blank cuts,
Regarding the demands of SRD
Carrier: In-loop Phase
this determines the drift vs. tempera-
applications in terms of radiated out-
Noise PLL at
ture: BT cut-quadrature dependency
put power and product cost sensitiv-
+20 kHz : -85 dBc/Hz
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Atmel Automotive Compilation 13
Vol.1 October 2004
12 Example of a PLL-based Transmitter Solution
The function of SAW devices is based
on the piezo-electrical effect. Figure 5
shows a schematical view: The two
interweaved comb structures of IN
and OUT in a distance of the wavelength λ result in a Surface Acoustic
Wave (SAW). Due to the very high
SECURITY
permittivity of the substrate
ALARM
CAR ACCESS
(εr ~ 10.000), a small component size
is possible.
The chosen substrate determines the
temperature dependency and opera-
EXAMPLE
OF A
PLL-BASED TRANSMITTER SOLUTION
(cont´d)
tion bandwidth. The deviation df/f0
ASK modulation (1 kHz to 32 kHz) is
additional serial or parallel cap, using
Table 1 provides a representative
depends on -1/K2 and is typically
done through a TTL input signal,
a present microcontroller port as
overview about the cost of such a
centered at 25°C. Bear in mind that
FSK modulation is achieved by
switch. Typically, a pulling of up to
design. Only the RF section is con-
automotive demands regarding the
pulling the quartz crystal with an
±150 ppm is possible (see Table 3).
sidered.
operating temperature ranges of
transmitters (-40°C to +85°C) and
Table 1. BOM
receivers (-40°C to +105°C) are really
PLL Transmitter
tough to meet. Using LiTaO3 or
LiNbO3 substrates, it is not possible
Figure 4. Discrete ASK SAW Transmitter Solution
to design a state-of-the-art SRD
system.
Ordering parameters of a SAW
resonator:
• Frequency tolerance: initial/aging
(e.g. ±75 kHz, -10/+ 50 ppm)
For further application and product details, see Table 3.
EXAMPLE
OF A
The frequency reference is a SAW
General assumption:
Compared to a PLL design, the nec-
resonator as the core of either a
• ASK operation using a 1-port
essary building blocks are the same,
Colpitts oscillator or a Pierce oscillator circuit. Oscillation stability, VCC
and temperature dependency as well
resonator in a Colpitts oscillator
• FSK operation using a 2-port
resonator in a Pierce oscillator
ture coefficient
(e.g. -0.032 ppm/K2 )
• Motional resistance RS
but all blocks are included in a single
Figure 5:
transistor stage. This is possible
Structure of a
since the SAW resonator already
as the modulation capability depend
oscillates at the actual RF frequency.
on the chosen basis circuit topology.
Nevertheless, for a competitive output power and a stable application, a
second stage acting as amplifier is
necessary, see Figure 4.
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• Frequency tolerance: tempera-
SAW-BASED TRANSMITTER SOLUTION
• Piezo-electrical substrate
(quartz crystal, LiTaO3, LiNbO3)
1-port SAW
The maximum data rates of ASK
resonator
systems are limited to roughly
4 kBd due to settling times of
15 to 20 µs.
Atmel Automotive Compilation 15
Vol.1 October 2004
14 Example of a PLL-based Transmitter Solution
EXAMPLE
SECURITY
OF A
SAW-BASED TRANSMITTER SOLUTION
(cont´d)
Designing such a discrete applica-
modifications, e.g., of the harmonic
a Pierce oscillator circuit, is needed.
tion is difficult since it requires expe-
level generation.
Pulling the frequency reference is
rienced oscillator and analog design
When
skills. Nevertheless, this solution is
designers must spend some effort in
diode.
more flexible than a fully integrated
achieving a sufficient frequency devi-
Table 2 provides a representative
one, and it enables quick and easy
ation. A 2-port resonator, designed in
overview on the cost of such a design.
using
FSK
modulation,
done by a serial varactor or a PIN
ALARM
CAR ACCESS
Table 2.
BOM - SAW
EXAMPLE
OF A
SAW-BASED TRANSMITTER SOLUTION
Transmitter
(cont´d)
One major advantage of SAW resonator designs is the reduced phase noise. The comparison between Figure 3 and
Figure 6 shows the narrow-band spectral view (measurement ResBW = 1 kHz). Note the noise floor of the spectrum
analyzer besides the carrier.
Figure 6.
Radiated Carrier
of SAW Tx:
Reduced Phase
Noise
For further application and product details, see Table 3
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Atmel Automotive Compilation 17
Vol.1 October 2004
16 Comparison of Both Solutions - Conclusion
Table 3.
Comparison
PLL-based
Tx vs. SAWbased Tx
SECURITY
ALARM
CAR ACCESS
COMPARISON
OF
BOTH SOLUTIONS - CONCLUSION
The advantages of the PLL-based
High
temperature
applications
design are related to performance:
(even up to +125°C) and modulation
designs is the lower total component
the quality in terms of frequency
rates up to 32 kHz, an important
cost. SAW applications are about
synthesis. Current and future appli-
current-saving feature, can be achieved.
13%
cations move to narrow-band RF
The
advantage
cheaper
of
than
SAW-based
PLL-based
designs. (see Table 1, Table 2). When
systems, the RF bands are more and
It is obvious that applications with
considering the power consumption
more overcrowded. Therefore, excel-
an integrated PLL-Tx with a mini-
efficiency, the SAW TX side pro-
lent selectivity and sensitivity on the
mum amount of external compo-
vides better results (33% vs. 22%)
receiver side is necessary, which is
nents and reduced need for analog
thanks to the missing current con-
only possible with narrow-band PLL
design skills such as theory of oscil-
sumption of the PLL circuit.
solutions.
lators and PLL are much more easy
than SAW-based designs.
Furthermore, a flexible channel
OUTLOOK
It seems that even some automotive
Current automotive developments are based on bi-
most recent extension of the PLL-based synthesizer is
suppliers have switched to discrete
directional RF links to improve the feature set and con-
the fractional-N device. These new devices allow the
selection can only be achieved with
Finally, the reliability regarding mass
solutions since cost considerations
venience. Since the amount of RF users has been
generation of pure high-frequency sources with flexible
quartz-crystal-based
production,
is
have currently a priority over per-
steadily increasing, and the usage of the available
channel spacing from a few hundred Hz to several hun-
These devices are more likely to get
increased, which reduces time-to-
formance issues, which will certainly
bands needs to be more efficient, channel-spacing and
dreds of kHz.
ordered in a customized way than
market.
not be the trend in long-term.
multi-channel devices are the next evolution step. The
solutions.
EMI
and
RFI
SAW devices. Some bands are not
even available to operate with SAW
apps.
CONTACT
Atmel Germany GmbH, Theresienstr. 2, 74072 Heilbronn, Tel. 07131 67-2081, Fax: 07131 67-2423
www.atmel.com