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.
