Motorola GM328 Service Manual page 418

2-2
The 2-pole 44.85 MHz crystal filters in the 1st IF section and two pairs of 455 kHz ceramic filters in
the 2nd IF section provide the required adjacent channel selectivity. The correct pair of ceramic
filters for 12.5 or 25 kHz channel spacing is selected via control line BWSELECT. The 2nd IF at 455
kHz is mixed, amplified, and demodulated in the IF IC. The processing of the demodulated audio
signal is performed by an audio processing IC located in the controller section.
2.2 Front-End Band-Pass Filters & Pre-Amplifier
The received signal from the radio's antenna connector is first routed through the harmonic filter and
antenna switch, which are part of the RF power amplifier circuits, before being applied to the receiver
pre-selector filter (C4001, C4002, D4001 and associated components). The 2-pole pre-selector filter
tuned by the varactor diodes D4001 and D4002 pre-selects the incoming signal (RXIN) from the
antenna switch to reduce spurious effects to following stages. The tuning voltage (FECTRL_1)
ranging from 2 volts to 8 volts is controlled by pin 20 of PCIC (U4501) in the Transmitter section. A
dual hot carrier diode (D4003) limits any inband signal to 0 dBm to prevent damage to the pre-
amplifier.
The RF pre-amplifier is an SMD device (Q4003) with collector base feedback to stabilize gain,
impedance, and intermodulation. The collector current of approximately 11-16 mA is drawn from the
9.3 volt supply via L4003 and R4002. A 3dB pad (R4006,R4007,R4011 and R4008 - R4010)
stabilizes the output impedance and intermodulation performance.
A second 2-pole varactor tuned bandpass filter provides additional filtering of the amplified signal.
The varactor diodes D4004 and D4005 are controlled by the same signal FECTRL_1, which
controls the pre-selector filter. A following 1 dB pad (R4013 - R4015) stabilizes the output
impedance and intermodulation performance.
2.3 First Mixer and 1st Intermediate Frequency (IF)
The signal coming from the front-end is converted to the first IF (44.85 MHz) using a cross over quad
diode mixer (D4051). Its ports are matched for incoming RF signal conversion to the 44.85 MHz IF
using low side injection via matching transformers T4051 and T4052. The injection signal (RXINJ)
coming from the RX VCO buffer (Q4332) is filtered by the lowpass filter consisting of (L4053, L4054,
C4053 - C4055) followed by a matching transformer T4052 and has a level of approximately 15dBm.
The mixer IF output signal (IF) from transformer T4501pin 2 is fed to the first two pole crystal filter
FL3101. The filter output in turn is matched to the following IF amplifier.
The IF amplifier Q3101 is actively biased by a collector base feedback (R3101, R3106) to a current
drain of approximately 5 mA drawn from the 5 volt supply. Its output impedance is matched to the
second two pole crystal filter FL3102. The signal is further amplified by a preamplifier (Q3102) before
going into pin 1 of IFIC (U3101).
A dual hot carrier diode (D3101) limits the filter output voltage swing to reduce overdrive effects at
RF input levels above -27 dBm.
UHF (403-470 MHz) Receiver