CN-0211 Datasheet, PDF (1/6 Pages) Analog Devices – IF Band-Pass Filter Bank Switching Network for Wireless Infrastructure

Circuit Note
CN-0211
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tested for quick and easy system integration to help solve today’s
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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
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.
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
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.
Rev.0
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