Circuit Note

CN-0211

Circuits from the Lab™ reference circuits are engineered and

testedfor quickandeasysystemintegrationtohelpsolvetoday’s

analog, mixed-signal, and RF design challenges. For more

informationand/orsupport, visitwww.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

IF Band-Pass Filter Bank Switching Network for Wireless Infrastructure

Rev.0

CircuitsfromtheLab™circuitsfromAnalogDeviceshavebeendesignedandbuiltbyAnalogDevices

engineers. Standard engineering practices have been employed in the design and construction of

eachcircuit,andtheirfunctionandperformancehavebeentestedandverifiedinalabenvironmentat

room temperature. However, you are solely responsible for testing the circuit and determining its

suitabilityandapplicabilityforyouruseandapplication.Accordingly,innoeventshallAnalogDevices

be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause

whatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage)

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781.329.4700

www.analog.com

Fax: 781.461.3113

©2011 Analog Devices, Inc. All rights reserved.

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.