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Agilent
Balanced Measurement Example:
SAW Filters
Application Note 1373-5
Introduction
SAW filters are commonly used in wireless communi-
cation products because of their very sharp response
characteristics, relatively low insertion loss, and low
cost. In addition, their physical structure is such that
they can be designed with single-ended RF ports, dif-
ferential RF ports, or a combination of the two. This
allows circuit designers to take full advantage of the
benefits of either unbalanced or differential circuit
topologies.
Typically, a SAW filter is most conveniently designed
with relatively high port impedances. This high
impedance works to the benefit of designers of hand-
Port 1 Port 2
held wireless subscriber products who are also con-
cerned about minimizing power consumption.
Port 3 Port 4
The combination of balanced RF ports and non-50
reference impedances tend to make characterizing
SAW filters, and designing them into products, a
challenging undertaking. Agilent's balanced-measure-
ment solutions address this challenge, and make test-
ing and designing with SAW filters as straightforward
as using any 50 single-ended component.
Mixed-mode S-parameters
Single-ended S-parameters do not adequately The 350 data now shows the reflections closer to
describe the performance of balanced devices the center of the Smith chart. The significant capaci-
because the circuit is electrically different depending tive component to the impedance is clearly visible.
on whether it has a single-ended stimulus or a differ-
ential stimulus. Therefore, using conventional single- The transmission parameters now give the user a
ended S-parameters to design differential circuits can better idea of the filter characteristics since much of
lead to misleading and erroneous conclusions. the return loss has been removed.
In addition to multiport single-ended S-parameters, The change in terminating impedance has also affect-
Agilent balanced-measurement systems give the user ed the passband ripple and the rejection characteris-
the ability to examine the mixed-mode S-parameters. tics, as shown in Figure 4.
The mixed-mode S-parameters allow the two modes
of propagation of a balanced device (differential and
common) to be examined independently in both stim-
ulus and response. This is best illustrated by examin-
ing measured data on actual devices. Port 1 Port 2
Single-ended SAW filter S-parameters
Port 3 Port 4
Consider a single-ended SAW filter with port
definitions as shown in Figure 1.
Figure 1. Single-ended SAW filter
One such device, by Sawtek, has port impedances of
700 differential on both balanced ports. The 50
single-ended S-parameters of this device are shown in
Figure 2.
The reflection parameters show that the input imped-
ances on each port are considerably higher than 50,
and slightly capacitive. The transmission parameters
show the bandwidth of the filter and the shape of the
skirts.
The first problem in examining this data is that the
filter, designed for 350 terminations, is measured in
a 50 system. Therefore, the transmission parame-
ters include a considerable amount of mismatch loss
in addition to ohmic loss.
Figure 3 shows data on the same device measured in
a 350 system.
Figure 2. 50 Single-ended S-parameters of a 700 device
2
Mixed-mode SAW filter S-parameters
Since the SAW filter described earlier is driven with
differential signals, the single-ended S-parameters
are misleading. Therefore, we should consider the
mixed-mode S-parameters of this device.
Figure 5 shows the mixed-mode S-parameter data on
the same SAW filter examined in the single-ended
S-parameter example in figure 3. This data has a
350 reference impedance on each terminal.
Therefore, the differential-mode reference impedance
is 700, and the common-mode reference impedance
is 175.
The quadrant in the upper left corner shows the
two-port differential stimulus/differential response
characteristics. The input impedance at the center
frequency of the passband is now in the center of the
Smith chart, and there is no capacitive shift in the
impedance as there is in the single-ended data. The
output impedance shows similar characteristics. The
Figure 3. 350 Single-ended S-parameters of a 700 device differential match is different from the single-ended
matches because the two sides of the balanced pairs
are not isolated from each other.
The transmission characteristics now show the loss
of this device when it is driven differentially. When
inserted into a differential system, the data shows
that the filter will have an insertion loss of about
8.9 dB. The single-ended transmission characteristics,
by comparison, showed a loss of 14.5 dB.
The characteristics of the filter with a common-mode
stimulus and a common-mode response are shown in
the quadrant in the lower right. Both the input and
output ports show a very high reflection (less than