Receiver (For Models With Pcb 8486473Z04); Figure 6-3. Vhf Receiver Block Diagram - Motorola HT Series Service Manual

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Table of Contents

VHF Theory of Operation: Receiver (for models with PCB 8486473Z04)

Radio signal strength indicator, RSSI, a voltage signal, is used to drive Q3301 to saturation i.e.

turned on. RSSI is produced by U3220 and is proportional to the gain of the RF amplifier and the

input power to the radio.

Resistors R3304 and R3305 are voltage dividers designed to turn on Q3301 at certain RSSI levels.

In order to turn on Q3301 the voltage across R3305 must be greater or equal to the voltage across

R3324, plus the base-emitter voltage (Vbe) present at Q3301. Capacitor C3209 is used to dampen

any instability while the AGC is turning on. The current flowing into the collector of Q3301, a high

current gain NPN transistor, will be drawn through the PIN diode to turn it on. Maximum current

flowing through the PIN is limited by the resistors R3316, R3313, R3306 and R3324. C3326 is a

feedback capacitor used to provide some stability to this high gain stage.

An additional gain control circuit is formed by Q3201 and its associated circuitry. Resistors R3206

and R3207 are voltage dividers designed to turn on Q3201 at a significantly higher RSSI level than

the level required to turn on PIN diode control transistor Q3301. In order to turn on Q3201 the

voltage across R3207 must be greater or equal to the voltage across R3208, plus the base-emitter

voltage (Vbe) present at Q3201. As current starts flowing into the collector of Q3201, it reduces the

bias voltage at the base of IF amplifier transistor Q3200 and in turn, the gain of the IF amplifier. The

gain can be controlled in a range of -30 dB up to +10 dB.

6.3