Motorola PRO Series, Подробное руководство по обслуживанию

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Section 8: 2-2 VHF (136–174 MHz) 25–45 W Bipolar Receiver Front-End
6881091C63-F
There are two 2-pole 44.85 MHz crystal filters in the 1st IF section and 2 pairs of 455 kHz ceramic
filters in the 2nd IF section to 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 (C3001, C3002, D3001 and associated components). The 2-pole
pre-selector filter tuned by the dual varactor diode D3001 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 (U3501) in the
Transmitter section. A dual hot carrier diode (D3003) limits any inband signal to 0 dBm to prevent
damage to the pre-amplifier.
The RF pre-amplifier is a surface mount device (SMD) Q3001 with collector-base feedback to
stabilize gain, impedance, and intermodulation. Transistor Q3002 compares the voltage drop across
resistor R3002 with a fixed base voltage from divider R3011, R3000 and R3012, and adjusts the
base current of Q3001 as necessary to maintain its collector current constant at approximately
15-20 mA. Operating voltage is from the regulated 9.3V supply (9V3). During transmit, 9.1 volts
(K9V1) turns off both transistors Q3002 and Q3001. This protects the RF pre-amplifier from
excessive dissipation during transmit mode. A following 3dB pad (R3006 – R3008 and R3016 –
R3018) stabilizes the output impedance and intermodulation performance.
A second 2-pole varactor tuned bandpass filter provides additional filtering of the amplified signal.
The dual varactor diode D3004 is controlled by the same signal FECTRL_1, which controls the pre-
selector filter.
2.3 First Mixer and 1st Intermediate Frequency (IF)
The signal coming from the front-end is converted to the 1st IF frequency of 44.85 MHz using a
passive double-balanced mixer (D3031). Its ports are matched for incoming RF signal conversion to
the 44.85 MHz IF using high side injection. The high-side injection signal (RXINJ) from the
frequency synthesizer circuit has a level of approxi13 dBm and is injected via matching
transformer T3002.
The IF output signal (IF) from transformer T3001 pin 2 is fed to the first 2-pole crystal filter FL3101.
The filter output in turn is matched to IF amplifier Q3101 which 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 2-pole crystal filter FL3102. The signal is further
amplified by a pre-amplifier (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.
2.4 2nd Intermediate Frequency (IF) and Receiver Back-End
The 44.85 MHz 1st 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 (2nd LO) to
produce the 2nd IF signal at 455 kHz. The 2nd LO frequency is determined by crystal Y3101. The
2nd IF signal is amplified and filtered by an external pair of 455 kHz ceramic filters FL3112, FL3114
for 20/25 kHz channel spacing or FL3111, FL3113/F3115 for 12.5 kHz channel spacing. These pairs