Automatic Gain Control Circuit - Motorola GP328 Plus Service Manual

5A-8
5.3

Automatic Gain Control Circuit

(Refer to VHF Receiver Front End Schematic Diagram on page 5A-22 and VHF Receiver Back End
Schematic Diagram on page 5A-23)
The front end automatic gain control circuit provides automatic reduction of gain, of the front end RF
amplifier via feedback. This action is necessary to prevent overloading of back end circuits. This is
achieved by drawing some of the output power from the RF amplifier output. At high radio
frequencies, capacitor C3327 provides the low impedance path to ground for this purpose. CR3302 is
a PIN diode used for switching the path on or off. A certain amount of forward biasing current is
needed to turn the PIN diode on. Transistor Q3301 provides this current.
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 -30dB up to +10dB.
Receiver