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Application Note
SAW-Components
How to choose the optimal SAW filter
App. Note 19
Abstract:
The characteristics of surface acoustic wave (SAW) filters are presented in order to
find a suitable SAW filter for a short range device system.
Version:
Updated:
1.0
January, 2009
Department: SAW WT AE PD
EPCOS AG
SAW Components
D-81617 Munich
P.O. Box 801709
http://www.epcos.com
SAW Components
How to choose the optimal SAW filter
Introduction
The number of radio services has increased much over the last decades. For one
service the radiated signal of another service is an interferer. Therefore frequency
filters are necessary for all reliable radio services. Surface acoustic wave filters are
frequency filters suited from several MHz to 3 GHz, which protect the service from
interferers and ensure that almost all of the wanted signal will be forwarded to the
receiver input or the antenna.
Figure 1: Performance of EPCOS wide band filter (1) and discrete
rd
LC 3 order Chebyshev filter (2)
In figure 1 a comparison of an EPCOS wide band filter and a discrete LC filter are
shown. The LC filter is composed of 3 inductors with quality factors of 55 @915MHz
and 3 capacitors of quality factors of >35@915MHz.
It can be clearly seen that the SAW filter offers a better protection of the service
because of the higher selectivity. Furthermore the signal is more efficiently
transmitted to the receiver input or the antenna due to the lower insertion loss.
In transmitting RF systems SAW filters are used to suppress the radiation of
undesired harmonics.
In receiving paths SAW filters improve the selectivity of the front-end. Image
frequencies are rejected and powerful out-of-band interfering signals are blocked.
In the following text some keywords are italicised. These are key specification
parameters shown in the respective SAW filter datasheets.
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SAW Components
How to choose the optimal SAW filter
Criteria to choose a SAW filter
The application defines the SAW filter characteristics. For each application certain
national or international regulations or end costumer specifications have to be
considered:
• Frequency band
• Electromagnetic interference
• Bandwidth
• Sensitivity
• Maximum ratings, e.g. operable temperature
• Quality standards
Frequency band:
Each application works in one, sometimes in more frequency bands. A SAW filter is
designed for a specific frequency, its center frequency. For example, for GPS
applications the center frequency of the SAW filter would be 1575.42 MHz.
Each country has some frequency bands, in which short range RF systems are
allowed to operate. For standardization reasons the Radiocommunication Sector of
the International Telecommunication Union has defined the industrial, scientific and
medical radio bands (ISM-bands), e.g. 433.05-434.79 MHz for Europe or 902-928
MHz for America. National regulations have to be regarded if there are additional
bands available and what the respective technical requirements are. The most
common institutions are the European Telecommunications Standards Institute
(ETSI) and the Federal Communications Commission (FCC).
Figure 2: Common frequency bands used for short range applications
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SAW Components
How to choose the optimal SAW filter
Electromagnetic interference:
The electromagnetic interference scenario, in which the system operates, will
determine the required level of attenuation outside the pass-band of the SAW filter.
The interferer situation has to be checked both in the used frequency band and in the
close-by frequency bands.
Typical interferer for short range devices are:
Application
Cellular communication
Services
e.g. GSM 850
e.g. WCDMA Band 4
Two way trunk radio
Personal area network
Local area network
Mobile Video Broadcasting
....
e.g. Tetra
e.g. RFID
IEEE 802.12x
DVB-H
Frequency band
824-849MHz (UL)
869-894MHz (DL)
1710-1755MHz (UL)
2110-2155MHz (DL)
...
e.g. 420-430MHz
868-928MHz
2.4-2.4835GHz
e.g. 470-582MHz
Table 1: Examples of some potential interferer services
In most cases one channel is used to transmit the data. This has the advantage that
the bandwidth of the SAW filter can be very narrow and a very high selectivity can be
achieved.
Sometimes more than one channel is used to transmit the data. The advantage is
that although there is an in-band interferer on one channel, the data can still be
correctly transmitted, if the same data is transmitted on a second or third channel.
The disadvantage is that the close-by selectivity is decreased because of the needed
wider bandwidth of a multichannel SAW filter.
Bandwidth
The usable bandwidth, which is correlated with the transmitting bandwidth, can be
calculated by looking at the highest and lowest possible frequency of the transmitting
signal.
The frequency of the transmitting signal varies due to the production tolerances,
temperature shift, aging, variation of the operating voltage and the modulation
spectrum of the transmitter.
The usable bandwidth of the respective filter is specified in the datasheet. The real
filter bandwidth measured @+25°C is larger due to production tolerances and
frequency drift over the temperature range, in which the SAW filter is specified.
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SAW Components
How to choose the optimal SAW filter
Figure 3: Bandwidths of wide-, narrow- and ultra-narrow SAW filters
In general it is possible to offer SAW filters of different bandwidths as different
piezoelectric substrate materials can be used, e.g. quartz or lithium tantalate.
bandwidth
substrate
temperature coefficient
usable bandwidth
impedance
ultra-narrow
quartz
-0,032 ppm/K² (parabolic)
~ 100...120 kHz
~ 400...800
narrow
quartz
-0,032 ppm/K² (parabolic)
~ 300...1100 kHz
~ 250...800
wide
tantalate
~ -35 ppm/K (linear)
~ 600...26000 kHz
50
or customer specific
Table 2: Substrate comparison of quartz and tantalate
Using quartz substrate filters can be designed with a very small usable bandwidth.
Also the temperature shift is smaller, but input and output impedances are higher
than 50 and have a capacitive contribution. Using tantalate as a substrate, 50
input and output impedances are possible, but the usable bandwidth is wider and the
temperature shift of center frequency is higher.
Special attention must be paid to the image frequency. For a superheterodyne Rx
system working at e.g. 315MHz and using 10.7MHz as the intermediate frequency
the image frequency would be 315MHz + 2 * 10.7MHz=336.4MHz, if the system
operate with high side injection. Signals at this frequency will be transferred to the
demodulator and would superimpose to the wanted signal. Therefore the attenuation
at the image frequency is very important.
The far-off selectivity is very good for most SAW filters, but the matching topology,
low, high or band pass, has a major influence at high frequencies.
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SAW Components
How to choose the optimal SAW filter
Sensitivity:
The customer specification requires a certain sensitivity. The sensitivity describes the
ability of the receiver to receive weak signals. Therefore it is important to keep the
losses in the front-end as low as possible. The insertion attenuation of the SAW filter
has a strong impact on the sensitivity.
For worst case calculations the maximum insertion loss of the SAW filter is the
minimum insertion loss plus the ripple in the transmitting bandwidth (narrowband
filter) or the value directly stated in the datasheet as maximum insertion loss for the
corresponding bandwidth (wideband filter).
For each receiver IC the sensitivity is specified by a certain signal input power level
for which the IC is able to decode the signal correctly. As an example a transmitter
radiates a signal with the power of -40dBm. The signal is attenuated in the air by
-25dB and the antenna gain of the receiver unit is -15dB. If the receiver’s sensitivity
level is specified with -85dBm, then there are 5dB insertion loss for the link between
antenna and receiver IC: 5dB = (-40dBm -25dB -15dB)-(-85dBm). If the SAW filter
has a maximum insertion loss in the transmitting bandwidth of -3dB, there would be
2dB for losses in the matching networks.
Maximum Ratings
The maximum ratings concern the operable and storage temperature, the maximum
source power and the DC voltage.
The power handling capability of the SAW filter must be verified. Continuous signals
at the highest operable temperature can be applied over the whole lifetime. With
lower duty cycle over a shorter time period signals of a higher power level can be
applied without direct failure of the SAW filter. These specific values must be
requested separately, if not specified in the datasheet.
In some circuits the SAW filter is also used as a DC Voltage blocker. Typical values
of DC blocking are 0-6V.
Quality standards:
Here we can distinguish automotive qualified and non automotive qualified SAW
filters. In the datasheet it is specified if a SAW filter belongs to an AEC-Q200
qualified family, a qualification standard of the Automotive Electronics Council.
It is also stated in the datasheets that EPCOS SAW filters are compatible with lead
free soldering as specified in J – STD20C, a standard of the JEDEC Solid State
Technology Association, and that EPCOS SAW filters follow the RoHS directive, an
European Union directive on the Restriction of the Use of Certain Hazardous
Substances in Electrical and Electronic Equipment.
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SAW Components
How to choose the optimal SAW filter
Simulation of SAW filter
System calculation with a circuit simulator
For all SAW filters S-parameter files in the touchstone file format, *.snp, are
available.
In a first step ideal matching elements can be taken, but they should be replaced by
models of real matching elements. Transmissions lines should also be taken into
account in a second step of the simulation process.
A simulation taking into account parasitic effects will help to shorten the time in the
laboratory to find the best matching values.
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SAW Components
How to choose the optimal SAW filter
SAW Filter Technology
A SAW (surface acoustic wave) filter’s functionality is based on the piezoelectric
effect. That means, when imposing pressure on a piezoelectric substrate, mechanical
energy is transformed into electrical energy and vice versa. See figure 4 for this
principle.
Figure 4: Explanation of piezoelectric effect
A geometrical structure, like it is shown in figure 5, is deposited on a piezoelectric
substrate. The geometry creates the surface acoustic wave.
Figure 5: Schematic diagram of a SAW component
As the dimension of the structure is inversely proportional to the frequency of the
wave, a wave frequency less than 1 MHz is problematic concerning chip space –
wave frequencies above 3 GHz need finer structures than industry is able to produce.
The transformation of electrical into mechanical energy offers the following
advantages:
• The geometrical structure offers has a very high filter characteristic
• The reproducibility of the structure is very high as well
• The acoustic wavelength is much smaller compared to the electrical
wavelength, which allows very small device dimensions of the piezoelectric
chip.
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SAW Components
How to choose the optimal SAW filter
Annexe – Data Sheets Explained: Narrow-band filter (Quartz substrate)
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SAW WT AE PD
SAW Components
How to choose the optimal SAW filter
Annexe – Data Sheets Explained: Wide-band filter (Tantalate substrate)
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SAW WT AE PD
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