Front-End Bandpass Filters And Pre-Amplifier - Motorola CDM Series Detailed Service Manual

Section 10: 2-2
The two 2-pole 44.85 MHz crystal filters in the high-IF section and two pairs of 455 kHz ceramic
filters in the low-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
second 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 Bandpass Filters and 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 (C5001, C5002, D5001, and related components). The 2-pole pre-
selector filter, tuned by the varactor diodes D5001 and D5002, pre-select the incoming signal (RXIN)
from the antenna switch to reduce spurious effects to the stages that follow. The tuning voltage
(FECTRL_1), ranging from 2 volts to 8 volts, is controlled by pin 20 of PCIC (U5501) in the
transmitter section. A dual hot carrier diode (D5003) limits any inband signal to 0 dBm to prevent
damage to the pre-amplifier.
The RF pre-amplifier is an SMD device (Q5003) 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 L5003 and R5002. A 3dB pad (R5006, R5007, R5011, and R5008 - R5010)
stabilizes the output impedance and intermodulation performance.
A second 2-pole varactor tuned bandpass filter provides additional filtering of the amplified signal.
Varactor diodes D5004 and D5005 are controlled by the same signal, FECTRL_1, which controls
the pre-selector filter. A following 1 dB pad (R5013 – R5015) stabilizes the output impedance and
intermodulation performance.
2.3 First Mixer and High Intermediate Frequency (IF)
The signal from the front-end is converted to the first IF (44.85 MHz) using passive double-balanced
mixer D5051. Its ports are matched for incoming RF signal conversion to the 44.85 MHz IF using
low side injection via matching transformers T5051 and T5052. The injection signal (RXINJ) coming
from the RX VCO buffer (Q5332) is filtered by the low-pass filter consisting of L5053, L5054, and
C5053 - C5055 followed by matching transformer T5052 which has a level of approximately 15dBm.
The mixer IF output signal from transformer T5501, pin 2 is fed to the first two-pole crystal filter
(FL3101). The filter output in turn is matched to IF amplifier Q3101 which is actively biased using
collector-base feedback resistors R3101and R3106 to provide a current drain of approximately 5
mA drawn from the 5 volt supply. The output impedance of this device is matched to the second
two-pole crystal filter FL3102. The signal is further amplified by a pre-amplifier Q3102 before going
to 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.
2.4 Low Intermediate Frequency (IF) and Receiver Back-End
The 44.85 MHz first IF signal from the second IF amplifier feeds the IF IC (U3101) at pin1. Within the
IF IC the 44.85 MHz high IF signal mixes with the 44.395 MHz second local oscillator to produce a
low IF signal of 455 kHz. The second LO frequency is determined by crystal Y3101. The second IF
signal is amplified and filtered by an external pair of 455 kHz ceramic filters (FL3112 and FL3114) for
20/25 kHz channel spacing, or FL3111 and FL3113/F3115 for 12.5 kHz channel spacing. These
6881091C63-F
UHF Band 2 (450–512/520 MHz) 25–40 W Bipolar Receiver Front-End