Motorola GP series, Detailed Service Manual

Page: 448 / 494

1-4 THEORY OF OPERATION
The IF signal coming out of the mixer is transferred to the crystal filter (FL350) through a resistive
pad and a diplexer (C312 and L306). Matching to the input of the crystal filter is provided by
L353,L354, C377, and C378. The crystal filter provides the necessary selectivity and
intermodulation protection.
3.2 Receiver Back-End
( Refer to Figure 2-2 and the 800 MHz Receiver Back End schematic diagram )
The output of crystal filter FL350 is matched to the input of the dual gate MOSFET IF amplifier
transistor U352 by components L355, R359 and C376. Voltage supply to the IF amplifier is taken from
the receive 5 volts (R5). AGC voltage is applied to the second gate of U352. The IF amplifier provides
a gain of about 11 dB. The amplified IF signal is then coupled into U351(pin 3) via L352, R356 and
C365 which provides the matching for the IF amplifier and U351.
The IF signal applied to pin 3 of U351 is amplified, down-converted, filtered, and demodulated, to
produce the recovered audio at pin 27 of U351. This IF IC is electronically programmable, and the
amount of filtering (which is dependent on the radio channel spacing) is controlled by the
microprocessor. Additional filtering, once externally provided by the conventional ceramic filters, is
replaced by internal filters in the IF module (U351).
The IF IC uses a type of direct conversion process, whereby the externally generated second LO
frequency is divided by two in U351 so that it is very close to the first IF frequency. The IF IC (U351)
synthesizes the second LO and phase-locks the VCO to track the first IF frequency. The second LO is
designed to oscillate at twice the first IF frequency because of the divide-by-two function in the IF IC.
In the absence of an IF signal, the VCO will “search” for a frequency, or its frequency will vary close to
twice the IF frequency. When an IF signal is received, the VCO will lock onto the IF signal. The
second LO/VCO is a Colpitts oscillator built around transistor Q350. The VCO has a varactor diode,
CR350, to adjust the VCO frequency. The control signal for the varactor is derived from a loop filter
consisting of R365, C391, and C392.
The IF IC (U351) also performs several other functions. It provides a received signal-strength
indicator (RSSI) and a squelch output. The RSSI is a dc voltage monitored by the microprocessor,
and used to control the automatic gain control (AGC) circuit in both the front-end and the IF.
The demodulated signal on pin 27 of U351 is also used for squelch control. The signal is routed to
U404 (ASFIC) where squelch signal shaping and detection takes place. The demodulated audio
signal is also routed to U404 for processing before going to the audio amplifier for amplification.