Motorola GM Series Service Information - Page 12

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2-2

There are two 2-pole 44.85 MHz crystal ﬁlters in the high-IF section and 2 pairs of 455 kHz ceramic

ﬁlters in the low-IF section to provide the required adjacent channel selectivity. The correct pair of

ceramic ﬁlters for 12.5 or 25kHz 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 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 circuitry, 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 (FECNTL_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 an SMD device (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 switchable 3dB pad (R3022, R3024, R3016 and R3018) controlled via Line

FECNTL_2 and Q3021 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 FECNTL_1, which controls the pre-

selector ﬁlter.

2.3

First Mixer and High Intermediate Frequency (IF)

The signal coming from the front-end is converted to the high-IF frequency of 44.85 MHz using a

cross over quad diode 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 circuitry has a level of approximately 13 dBm and is injected via matching transformer

T3002.

The mixer IF output signal (IF) from transformer T3001 pin 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 voltage 5V. 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.

2.4

Low Intermediate Frequency (IF) and Receiver Back End

The 44.85 MHz high-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 low-IF signal at 455 kHz. The 2nd LO frequency is determined by crystal Y3101. The

THEORY OF OPERATION