CN-0211 Datasheet, PDF (3/6 Pages) Analog Devices – IF Band-Pass Filter Bank Switching Network for Wireless Infrastructure

Circuit Note
CN-0211
Figure 4 shows test results from the circuit evaluation PCB
when the ADG904R switches are selecting the 856656 SAW
filter. The response compares well to the standalone response,
which is also included in the plot for reference. Due to the loss
of the two ADG904-R switches in series, the insertion loss of
the filter has increased by 1 dB. Ripple in the pass band is of
importance in wireless infrastructure applications. The ripple
across the band is 0.47 dB from the minimum to maximum
peak within the pass band of 135.5 MHz to 144.5 MHz. This
level is well within the data sheet specification for ripple, which
is 0.8 dB max. The high off isolation of the switches is critical in
this application to minimize leakage from one filter channel to
the next. Poor switch off isolation would increase the in-band
insertion loss and ripple of the filter. No evidence of this is
visible in these test results.
Figure 5 and Figure 6 show the 856592 and 856684 SAW filter
channel test results, respectively. The results shown include the
loss from the two ADG904-R switches in series. This loss is
approximately 1.0 dB. Both filter channels show good flat
insertion loss, plus high out-of-band attenuation levels. The
results shown are within the manufacturers data sheet
tolerances and clearly show that the ADG904-R switches are not
introducing any ripple, mismatch, or affecting the flatness of the
filters in any way.
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
105
–7
–8
–9
–10
–11
–12
–13
115 125 135 145 155 165 175
FREQUENCY (MHz)
–14
130 132 134 136 138 140 142 144 146 148 150
FREQUENCY (MHz)
Figure 3. Standalone 140 MHz (Center Frequency), 11.82 MHz (1 dB BW), 856656 SAW Filter Frequency Response, No Switches
0
–7
–10
–20
SAW FILTER + SW
–30
–40
SAW FILTER
–8
–9
–10
SAW FILTER
–50
–11
SAW FILTER + SW
–60
–12
–70
–13
–80
–90
105
115
125
135
145
155
165
175
FREQUENCY (MHz)
–14
130 132 134 136 138 140 142 144 146 148 150
FREQUENCY (MHz)
Figure 4. 140 MHz (Center Frequency), 11.82 MHz (1 dB BW), 856656 SAW Filter Frequency Response Measured on Evaluation Board with and without Switches
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