Motorola GM340 Сервісна інформація - Сторінка 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 (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

voltage 9V3 via L4003 and R4002. A switchable 3dB pad (R4066,R4007,R4063, R4064 and

R4070), controlled via line FECTRL_2 and Q4004 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

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 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

The 44.85 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

THEORY OF OPERATION