GSM 3 V Transceiver IF Subsystem

The AD6432 IF IC provides the complete transmit and receive

IF signal processing, including I/Q modulation and demodulation,

necessary to implement a digital wireless transceiver such

as a GSM handset. The AD6432 may also be used for other

wireless TDMA standards using I/Q modulation.

The AD6432's receive signal path is based on the proven architecture

of the AD607 and the AD6459. It consists of a mixer,

gain-controlled amplifiers, integrated roofing filter and I/Q

demodulators based on a PLL. The low noise, high-intercept

variable-gain mixer is a doubly-balanced Gilbert-cell type. It has

a nominal -13 dBm input-referred 1 dB compression point and

a 0 dBm input-referred third-order intercept.

The gain-control input accepts an external control voltage input

from an external AGC detector or a DAC. It provides an 80 dB

gain range with 27.5 mV/dB gain scaling, where the mixer and

the IF gains vary together.

The I and Q demodulators provide inphase and quadrature

baseband outputs to interface with Analog Devices' AD7015

and AD6421 (GSM, DCS1800, PCS1900) baseband converters.

An onboard quadrature VCO, externally phase-locked to

the IF signal, drives the I and Q demodulators. The quadrature

phase-locked oscillator (QPLO) requires no external components

for frequency control or quadrature generation, and demodulates

signals at standard GSM system IFs of 13 MHz, or

26 MHz with a reference input frequency of 13 MHz; or, in

general, 1X or 2X the reference frequency. Maximum reference

frequency is 25 MHz.

This reference signal is normally provided by an external

VCTCXO under the control of the radio's digital signal

processor. The transmit path consists of an I/Q modulator

and buffer amplifier, suitable for carrier frequencies up to

300 MHz and provides an output power of -17.5 dBm in

a 50 Ohm system. The quadrature LO signals driving the

I and Q modulator are generated internally by dividing by

two the frequency of the signal presented at the differential

LO port of the AD6432. In both the transmit and receive

paths, onboard filters provide 30 dB of stopband attenuation.

The AD6432 comes in a 44-lead plastic thin quad flatpack

(TQFP) surface mount package.