SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
Introduction
For RKE systems “single-chip” receivers and “single-chip” transceivers are used. For
the matching proposals we take the TDA520x series from INFINEON and the
ATA572x series from ATMEL as example.
To improve the selectivity and the image frequency rejection of the system a SAW
front-end filter should be placed between antenna and the receiver. Furthermore, the
problem of receiver blocking by powerful out-of-band interfering signals is strongly
reduced. Narrow-band front-end SAW filters are available for all frequency ranges in
use.
The front-end of RKE receivers
The basic structure of a receiver or of the receive path of a transceiver is realised
with a “superheterodyne approach”, built of a low-noise amplifier (LNA) and a mixer.
Depending on the receiver integrated circuit (Rx IC) the frequency of the local
oscillator LO is defined in such a way that the output signal of the mixer has an
intermediate frequency of some MHz or a few hundred kHz. This intermediate signal
can be handled by the demodulator.
A typical front-end of a RKE system is shown in Figure 1:
Figure 1: Front-end of a RKE receiver path
A SAW filter offers the following benefits: a low insertion loss, a very high selectivity,
blocking of the image frequency and blocking of interfering signals. If the input of a
Rx IC is differential, customized SAW filters can provide the balun function as well.
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SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
Antenna
The typical antenna impedance of a quarter lambda whip antenna is in the range of
50 Ω. The exact input matching of the SAW filter depends on the characteristic
antenna impedance.
A test point, like presented in Figure 2, simplifies the verification of the input and
output matching networks.
Figure 2: Antenna test point
In the following it is supposed that the antenna impedance is 50 Ω.
Matching networks and SAW
Narrow-band front-end SAW filters have input and output impedances different from
50 Ω. Narrow band front-end SAW filters are designed for power matching. Power
matched SAW filters provide a flat pass-band and a low insertion loss.
Power matching
Power matching means that the maximum of the power at the source, e.g. the
antenna, is transferred to the load, e.g. the input port of a SAW filter.
This is the case if the source impedance Zsource = Rsource + jXsource sees its complex
conjugate counterpart Z*=Rsource – jXsource.
1
1
P load = I 2rms⋅R load = ∣I∣2⋅R load =
2

∣V
peak∣
2

1
⋅R load = ⋅
2 ∣Z source Z load∣
2 R
∣V peak∣2
2
2
source R load    X source  X load 
∣V peak∣2
, if R load =R source∪ X load =−X source
max { P load }=
8⋅R load
A general example is given in Figure 3.
Figure 3: Power matching
A matching network transfers the load impedance to Z*.
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SAW CE AE PD
R load
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
Direct and 2 steps matching
The matching between the SAW filter and the Rx IC can be done directly or in two
steps. In two steps means a first matching network transforms the impedance from
one component to 50 Ω and then another matching network realises the
transformation from 50 Ω to the complex conjugate impedance of the following
component, e.g. the internal LNA of the Rx IC (Figure 4).
Direct
2 steps
Figure 4: Matching options, direct and 2 steps matching
Matching
•
direct
•
•
2 steps
Advantages
lower losses in matching
networks
less components in
matching networks
50 Ω environment test
points possible
•
•
•
•
Disadvantage
only overall performance measurable
(e.g. input matching, SAW filter, output
matching, LNA)
higher losses in matching networks
more components in matching
networks
less stable due to a longer
transformation path in the Smith chart
Table 1: Advantages and disadvantages of direct and 2 steps matching
LNA
The internal LNA of the Rx IC is used to amplify the received signal, so that the
signal power at the demodulator offers sufficient strength. However, not only the
signal power is important. A high signal to noise ratio is crucial to avoid errors during
the transmission of the data. That is the reason why the noise behaviour of the LNA
must also be controlled.
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SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
Maximum gain matching vs. optimum noise matching
Every amplifier at a certain operating point has a specific impedance for maximum
gain, which is used for power matching, and a specific impedance for optimum noise,
which is used for noise matching. If these impedances do not coincide, it might
happen that for the configuration with the lowest bit error rate the pass-band of the
SAW filter shows no optimum flatness.
Topology
Figure 5: Circuit topology for maximum gain matching vs. optimum noise matching
In the following the focus of our interest in the circuit topology lies in the interface of
the output matching network of the SAW filter and the LNA of the Rx IC.
The matching of this interface is indicated by the S11 of the input matching network of
the SAW filter. S11 can be measured with a network analyser.
The input of Rx IC has in the pass-band of SAW filter the impedance Z1. So for the
output matching network, the impedance Z1* would be the optimum impedance for
maximum gain matching of the LNA.
However, if the output matching network shows the impedance Z3* to the LNA, the
LNA generates the minimum noise.
The impedance Z2* is the best compromise between maximum gain matching and
minimum noise matching. For this impedance the signal to noise ratio will be the
highest.
One way to quantify the data errors is a sensitivity measurement. Sensitivity
measurements are performed with a data-signal generator. The signal power of the
signal generator, for which the sent and received data are still identical, is plotted
versus the signal frequencies of the signal generator.
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SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
Example of different impedances for maximum gain and optimum noise
Output impedances of the
S11 at input matching
Sensitivity
output matching networks
network
Sensitivity measurements
signal power / dBm
-87.5
Z1*
Z2*
Z3*
-90
-92.5
-95
-97.5
-100
Row 2 Row f12
Row 24
/ Hz
Row 3 Row 13 Row 25
Figure 5a: Circuit behaviour in the case of different impedances for maximum gain and
optimum noise
For Z1* as the output impedance of the output matching network S11 is curled in the
centre of the Smith Chart and the sensitivity level is nearly constant over the whole
pass-band. This situation corresponds to an optimum flat pass-band of the SAW
filter.
If the output matching network offers Z3* to the LNA of the Rx IC, then S11 is no more
located tightly around the centre of the Smith Chart and the pass-band of the SAW
filter will not offer optimum flatness anymore. Consequently sensitivity measurements
will show a curve with a higher ripple. The sensitivity level is affected favourably by a
low noise level and simultaneously it is affected negatively by higher losses in the
matching networks.
Therefore the output impedance of the output matching network Z2* will be the best
compromise of these two effects and best sensitivity measurement results will be
obtained.
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SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
Example of similar impedances for maximum gain and optimum noise
Output impedances of the
S11 at input matching
Sensitivity
output matching networks
network
Sensitivity measurements
signal power / dBm
-87.5
Z1*
Z2*
Z3*
-90
-92.5
-95
-97.5
-100
Row 2 Row f12
/ HzRow 24
Row 3 Row 13 Row 25
Figure 5b: Circuit behaviour in the case of similar impedances for maximum gain and
optimum noise
If the specific impedances for maximum gain and optimum noise are similar, losses
in matching networks and the noise level of the LNA of the Rx IC will be similar too.
As a consequence the sensitivity measurements for all impedances will be alike as
well.
In the case of different impedances, there are two approaches:
- 2 steps matching: The input matching network of the LNA of the Rx IC will
be matched to a 50 Ohm test point, so that the highest signal to noise ratio
are obtained. Then output matching network of the SAW filter is power
matched to the 50 Ohm test point.
- Direct matching: Once the maximum gain impedance is found, the
matching elements have to be changed slightly and measuring the
sensitivity the optimum matching will be found.
In general, the RF power of the network analyser or the signal generator should be
limited to –40dBm so that the LNA works in linear range. If there is an automatic gain
control, it should be adjusted to maximum gain.
In the following we will match to the maximum gain impedance.
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SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
SAW filter matching
There are several matching structures which offer different benefits. As on each PCB
the situation is different and as there are only a limited number of values of the
matching components, some matching structures might suite better than others.
For each design the matching must be optimized.
High and low pass configurations are presented in Figure 6. In Figure 7 we see that
there are notable differences in the wide band view.
High pass configuration
Low pass configuration
0
0
0
0
Figure 6: High pass and low pass configurations
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SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
Figure 7: S21 response for high pass (green) vs. low pass (red) matching configurations
The use of two inductors attenuates low and high frequencies. This configuration has
a band pass characteristic (Figure 8).
band pass configuration
0
0
Figure 8: Band pass configurations
As the impedances of a narrow-band SAW filter and the LNA input of the Rx IC are
rather capacitive than inductive, configurations with two capacitors are not possible.
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SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
The arrangement of two inductors often leads to the lowest insertion loss. Which
configuration provides best stability depends on the tolerances of the components
and on the transformation path in the Smith Chart.
Best results will be obtained if traces are kept short between the SAW filter and
adjacent components, and if inductors with high quality factors are used. For the
specification of the SAW filter the quality factors of the inductors, which are used for
the matching, are usually stated in the data sheet.
The SAW filter provides an additional feature:
Even if the LNA does not include a DC block capacitor, the SAW filter will block the
DC current (see Figure 9). In the static model, EPCOS SAW filters are described by
capacitors, which are connected in serial or in parallel. That is the reason why there
is no direct connection to ground for DC currents.
Figure 9: SAW filters fulfil DC block function
The data sheet includes maximum values of the applicable DC Voltage.
As the SAW filter is an ESD sensitive element, there might be the need of ESD
protection. In the data sheet examples of ESD protection matching networks are
given.
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SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
Optimization of the matching networks
The aim of all power matching approaches is a flat pass-band and a low insertion
attenuation. The S11 in the Smith Chart indicates if the matching is already optimized.
If the pass-band region is not near to the centre of the Smith Chart, the input
matching has to be optimized (Figure 10):
Figure 10: Optimization of the input matching
If the curve in the pass-band is “curled”, than the output matching has to be improved
(Figure 11):
Figure 11: Optimization of the output matching
The green curve, “curled” and centred in the middle of the Smith Chart, shows an
optimized matching.
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SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
Simulation of the matching of EPCOS SAW filters to integrated RKE receivers
Simulations of the matching networks for INFINEON’s TDA 520x series and ATMEL’s
ATA 572x series are presented. On a PCB component values can change slightly
because of the parasitics and the accuracy of the device models in simulations.
For the simulation inductors with quality factors of ~45 for 315MHz, ~50 for 434MHz
and ~60 for 868MHz have been used. The capacitors’ quality factors are higher than
100.
Furthermore micro strip lines (0.5mm common line width, 2mm length) have been
integrated in the simulations for the connection of the components.
SAW Filters
B3741: narrow-band filter, quartz, 315MHz
B3743: narrow-band filter, quartz, 433.92MHz
B3744: narrow-band filter, quartz, 868.3MHz
Please look at the datasheets and at the important notes in the datasheets.
The LNA input impedances are taken from the data sheets of the receivers.
IC ATMEL
315MHz: ATA5723, 26.97 – j158.7 
R//C: 961 // 2p3
433MHz: ATA5724, 19.30 – j113.3 
R//C: 684 // 3p2
868MHz: ATA5728, 14.15 – j73.53 
R//C: 396 // 2p4
IC INFINEON
315MHz: TDA5201,
433MHz: TDA5200,
868MHz: TDA5200,
0.895 / -25.5°  R//C: 859 // 2p3
0.873 / -34.7°  R//C: 672 // 2p3
0.738 / -73.5°  R//C: 216 // 2p6
The shown matching topologies are chosen to present the variety of the matching
networks. It is not guaranteed that the best topology with respect to number of
components, return loss or sensitivity of the components has been taken.
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SAW CE AE PD
LQW18AN43NG00
LQW18AN72NG00
SAW Components
33pF
C0603339F9B200
Matching of EPCOS front-end SAW filters to integrated RKE receivers
LQW18AN15NG00
B3741 – TDA5201
Antenna
TDA5201
B3741
parasitic capacitance
only in simulation
necessary
Smith Chart of S11
2.3pF
15nH
43nH
50 Ohm
33pF
2
0.2pF
72nH
0.2pF
1
LNA input
D=0.5mm
DG=0.5mm
859 Ohm // 2p3F
Return Loss of S11
Transmission narrow band
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SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
B3741 – TDA5201
Transmission medium band
Transmission wide band
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SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
LQW18AN72NG00
LQW18ANR16G00
B3741 – ATA5723 - micro strip line before LNA has a physical length P = 6mm
ATA5723
Antenna
B3741
160nH
P=6mm
LNA input
parasitic capacitance
only in simulation
necessary
Smith Chart of S11
2.3pF
0.2pF
50 Ohm
2
0.2pF
1
72nH
961 Ohm // 2p3F
Return Loss of S11
Transmission narrow band
Page 15
SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
B3741 – ATA5723
Transmission medium band
Transmission wide band
Page 16
SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
B3743 – TDA5200
Antenna
TDA5200
B3743
parasitic capacitance
only in simulation
necessary
Smith Chart of S11
82nH
D=0.5mm
DG=0.5mm
2.3pF
0.68pF
43nH
0.2pF
50 Ohm
2
0.2pF
1
LNA input
672 Ohm // 2p3F
Return Loss of S11
Transmission narrow band
Page 17
SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
B3743 – TDA5200
Transmission medium band
Transmission wide band
Page 18
SAW CE AE PD
LQW18AN24NG00
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
LQW18AN43NG00
B3743 – ATA5724
Antenna
ATA2724
B3743
parasitic capacitance
only in simulation
necessary
Smith Chart of S11
6.8pF
39pF
24nH
50 Ohm
D=0.5mm
DG=0.5mm
3.2pF
2
0.2pF
43nH
0.2pF
1
LNA input
684 Ohm // 3p2F
Return Loss of S11
Transmission narrow band
Page 19
SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
B3743 – ATA5724
Transmission medium band
Transmission wide band
Page 20
SAW CE AE PD
LQW18AN8N2D00
LQW18AN20NG00
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
B3744 – TDA5200
Antenna
TDA5200
1
2
D=0.5mm
DG=0.5mm
parasitic capacitance
only in simulation
necessary
Smith Chart of S11
4.7pF
8.2nH
0.2pF
0.2pF
50 Ohm
2.2pF
20nH
2.6pF
B3744
LNA input
216 Ohm // 2p6F
D=0.5mm
DG=0.5mm
Return Loss of S11
Transmission narrow band
Page 21
SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
B3744 – TDA5200
Transmission medium band
Transmission wide band
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SAW CE AE PD
LQW18AN6N8C00
LQW18AN20NG00
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
B3744 – ATA5728
Antenna
ATA5728
B3744
1
2
D=0.5mm
DG=0.5mm
parasitic capacitance
only in simulation
necessary
Smith Chart of S11
2.4pF
22pF
6.8nH
0.2pF
2.2pF
50 Ohm
0.2pF
20nH
D=0.5mm
DG=0.5mm
LNA input
396 Ohm // 2p4F
Return Loss of S11
Transmission narrow band
Page 23
SAW CE AE PD
SAW Components
Matching of EPCOS front-end SAW filters to integrated RKE receivers
B3744 – ATA5728
Transmission medium band
Transmission wide band
Page 24
SAW CE AE PD
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