Circuit Note
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit
www.analog.com/CN0211.
Devices Connected/Referenced
ADG904-R
Wideband 2.5 GHz, 37 dB Isolation at 1 GHz,
CMOS, 1.65 V to 2.75 V, 4:1 Mux/SP4T
CN-0211
IF Band-Pass Filter Bank Switching Network for Wireless Infrastructure
EVALUATION AND DESIGN SUPPORT
Circuit Evaluation Boards
CN-0211 Circuit Evaluation Board (EVAL-CN0211-EB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
CIRCUIT FUNCTION AND BENEFITS
The function of the circuit, shown in Figure 1, is to route an RF
signal through an IF band-pass filter bank. The circuit uses
three 140 MHz IF SAW filters with different bandwidths and
two
ADG904-R
SP4T CMOS RF switches. The
ADG904-R
switches control which band-pass filter the RF signal will pass
through. Only one filter is selectable at a time. The typical name
given to this type of switching is “filter bank switching.”
This application is very common in wireless infrastructure
applications, such as cell phone base stations and point-to-
point radio RF front ends. Selecting IF filters with different
bandwidths results in more flexibility in controlling the various
data rates in these systems.
Insertion loss versus frequency in the IF band must be flat to
achieve optimum system performance. The
ADG904-R
switch
is ideally suited for this circuit because it has very flat insertion
loss versus frequency. The insertion loss also remains flat over
the supply voltage and temperature range. Switch off isolation
is another very important parameter to maximize the
performance of this circuit. The
ADG904-R
has excellent
off isolation of greater than −50 dB up to 200 MHz.
Triquint 856592 SAW filter. This filter is a single ended filter
with a characteristic impedance of 50 Ω. It has a center
frequency of 140 MHz and a typical 1 dB bandwidth of
20.7 MHz. Note that lumped element matching is required to
achieve the optimum 50 Ω termination impedance. The output
of the filter is connected to pin 17 (RF2) of the
ADG904-R,
U2.
Similarly, the second SAW filter (Triquint 856684) is connected
to U1 and U2. In this case, the filter is connected to Pin 17
(RF2) of U1 and Pin 4 (RF1) of U2. The 856684 filter has a
center frequency of 140 MHz and a typical 1 dB bandwidth of
16.18 MHz.
The third SAW filter (Triquint 856656) is connected to Pin 7
(RF3) of U1 and Pin 14 (RF4) of U2. The 856656 filter has a
centre frequency of 140 MHz and a typical 1 dB bandwidth of
11.82 MHz
It is important to use the PCB land layout pattern recommended
by the manufacturer of the SAW filters. To achieve the
maximum isolation from the input port to the output port,
a plated slot was positioned under the filter. Improving this
isolation enables the full out-of-band attenuation to be achieved
and minimizes the in-band ripple. Each filter is matched to 50 Ω
on the input port and output port using 0603 size inductor and
capacitor lumped elements. This is an important aspect for
achieving good performance from the circuit evaluation board.
To further improve isolation, the 50 Ω transmission lines that
connect the filters to the switches and to the RF edge
connectors are designed as coplanar waveguides.
The standalone insertion loss vs. frequency for the 11.82 MHz
bandwidth filter is shown in Figure 3. This data was measured
with the filter on a separate circuit evaluation PCB with the
same transmission line design and matching components, but
with no switches in the RF path. This response is used as
reference data. The insertion loss at 140 MHz was −9.17 dB,
which is consistent with the data sheet value of −9.2 dB typical.
CIRCUIT DESCRIPTION
The circuit consists of two
ADG904-R
CMOS RF switches and
three IF SAW filters connected as shown in Figure 1. The
switches have a supply voltage range of 1.65 V to 2.75 V. In this
evaluation, a nominal value of 2.5 V is used. The RF input is
connected to Pin 10 (RFC) of the
ADG904-R
switch, U1. The
ADG904-R
switch is a SP4T switch where the RFC pin is the
common connection to all four switches (RF1, RF2, RF3 and
RF4). Pin 4 of U1 (RF1) is connected to the input of the
Rev.0
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each circuit, and their function and performance have been tested and verified in a lab environment at
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©2011 Analog Devices, Inc. All rights reserved.
Circuit Note
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
CN-0211
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.
–7
–8
–9
0
–10
–20
MAGNITUDE (dB)
MAGNITUDE (dB)
–30
–40
–50
–60
–70
–80
–90
105
–10
–11
–12
–13
–14
130
115
125
135
145
155
165
175
132
134
136
FREQUENCY (MHz)
138 140 142 144
FREQUENCY (MHz)
146
148
150
Figure 3. Standalone 140 MHz (Center Frequency), 11.82 MHz (1 dB BW), 856656 SAW Filter Frequency Response, No Switches
0
–10
–20
SAW FILTER + SW
SAW FILTER
–7
–8
SAW FILTER
–9
MAGNITUDE (dB)
MAGNITUDE (dB)
–30
–40
–50
–60
–70
–80
–90
105
–10
–11
SAW FILTER + SW
–12
–13
–14
130
115
125
135
145
155
165
175
132
134
136
FREQUENCY (MHz)
138 140 142 144
FREQUENCY (MHz)
146
148
150
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
Rev. 0 | Page 3 of 6
09853-006
09853-005
CN-0211
0
–10
–9
–20
–8
Circuit Note
MAGNITUDE (dB)
–40
–50
–60
–70
MAGNITUDE (dB)
–30
–10
–11
–12
–13
–80
09853-009
09853-010
–90
105
115
125
135
145
155
165
175
–14
125
130
FREQUENCY (MHz)
135
140
145
FREQUENCY (MHz)
150
155
Figure 5. 140 MHz (Center Frequency) 20.7 MHz (1 dB BW) 856592 SAW Filter Frequency Response Measured on Evaluation Board with Switches
0
–10
–20
–9.0
–9.5
–10.0
–10.5
MAGNITUDE (dB)
MAGNITUDE (dB)
115
125
135
145
155
165
175
–30
–40
–50
–60
–70
–80
–90
105
–11.0
–11.5
–12.0
–12.5
–13.0
–13.5
–14.0
130
135
FREQUENCY (MHz)
140
FREQUENCY (MHz)
145
150
Figure 6. 140 MHz (Center Frequency) 16.18 MHz (1 dB BW) 856684 SAW Filter Frequency Response Measured on Evaluation Board with Switches
COMMON VARIATIONS
The absorptive version of the
ADG904
switch can also be used
in this filter bank switching circuit. The absorptive version
contains 50 Ω shunt resistors on the RFx ports. This results in a
good 50 Ω match when the switch is in the off state. This is
important in some applications where reflected signals in the
off channels need to be absorbed.
The circuit can be easily scaled to include more or fewer filter
channels. Depending on the number of channels, other switches
from the
ADG9xx
family may be better to use and offer more
flexibility with layout. The
ADG936
is a dual SPDT switch, the
ADG918
is an SPDT switch, and the
ADG901
is an SPST
switch. All of these switches are available in both absorptive and
reflective configurations.
CIRCUIT EVALUATION AND TEST
The circuit is designed to be connected to a vector network
analyzer (VNA) via SMA type connectors. The SMA connectors
on the board are female. To switch between each filter channel
there are jumper terminals on the board that can be manually
changed. Note that one of the switch throws in each ADG904-R
switch is not used. Therefore, when these unused switches are
selected, the RF input and RF output ports are isolated, i.e., the
switches are not connected to a filter.
Rev. 0 | Page 4 of 6
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