Motorola GP series Service Information - Page 11

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1. Technical Specifications

Lowband Receiver

3.0

Lowband Receiver

(Refer to Figure 2-2 and the Receiver Front End and Receiver Back End schematic diagrams)

The Lowband receiver consists of a front end, back end, and automatic gain control circuits.

Detailed descriptions of these stages are contained in the paragraphs that follow.

Antenna

Lowpass

Jack

Filter

3.1

Receiver Front-End

(Refer to the Receiver Front End, Receiver Back end and Transmitter schematic diagrams)

The RF signal received by the antenna is routed through the transmitter lowpass filter and antenna

switch. These circuits are described in the transmitter section.The signal next passes through a

highpass filter consisting of L501, L502, C538, C533 and C504. This filter serves to reject below

band signals and has a 3 dB corner frequency of 27 MHz.

The output of the highpass filter is connected to an RF amp consisting of Q509 and associated

biasing components. This is a BJT amplifier powered off 5 volts and has 13 dB of gain. The amplifier

drives a lowpass filter consisting of L503, L504, L507, C534, C535, C536, C537 and C515. This

filter is a pole zero design that filters off harmonic components from the RF amp. The 3 dB corner of

this filter is at 56 MHz.

The output of the lowpass filter is connected to the passive double balanced mixer consisting of

components T501, T502 and D501. After mixing with the first local oscillator up-converted to a

109.65 MHz IF signal.

The IF signal coming out of the mixer is transferred to the crystal filter (FL301) through a resistor pad

(R507, R508 and R509) and a diplexer (C516 and L508). Matching to the input of the crystal filter is

provided by L301, L302, C301 and C302. The 3 pole crystal filter provides the necessary selectivity

and intermodulation protection.

Antenna

Highpass

RF Amp

Switch

Filter

First LO

from FGU

Recovered Audio

Figure 2-2 Lowband Receiver Block Diagram.

Lowpass

Mixer

Filter

Squelch

RSSI

17.0 MHz

Reference Clock

Crystal

IF Amp

Filter

AGC

Processing

Demodulator

IF IC

U303

Synthesizer

SPI Bus

2-3

Second

LO VCO