Receiver Back-End; Automatic Gain Control (Agc) - Motorola Professional Radio Service Manual

2-8
The IF signal coming out of the mixer is transferred to the crystal filter (FL301) through a resistor pad
and a diplexer (C322 and L310). Matching to the input of the crystal filter is provided by C324 and
L311. The crystal filter provides the necessary selectivity and intermodulation protection.
2.6.2

Receiver Back-End

The output of crystal filter FL301 is coupled via R351 and C325 to the input of IF amplifier transistor
Q302. Voltage supply to the IF amplifier is taken from the receiver 5 volts (R5). The IF amplifier
provides a gain of about 7dB. The amplified IF signal is then coupled into U301(pin 3) via C330, C338
and L330 which provides a high-pass T-match for the IF amplifier and U301.
The IF signal applied to U301 (pin 3) is amplified, down-converted, filtered, and demodulated, to
produce recovered audio at U301(pin 27). This IF IC (U301) is electronically programmable, and the
amount of filtering, which is dependent on the radio channel spacing, is controlled by the
microprocessor. Additional filtering, once externally provided by the conventional ceramic filters, is
replaced by internal filters in IF IC (U301).
The IF IC uses a type of direct conversion process, whereby the externally generated second LO
frequency is divided by two in U301 so that it is very close to the first IF frequency. The IF IC
synthesizes the second LO and phase-locks the VCO to track the first IF frequency. The second LO is
designed to oscillate at twice the first IF frequency because of the divide-by-two function in the IF IC.
In the absence of an IF signal, the VCO searches for a frequency, or its frequency will vary close to
twice the IF frequency. When an IF signal is received, the VCO locks onto the IF signal. The second
LO/VCO is a Colpitts oscillator built around transistor Q320. The VCO has a varactor diode, CR310,
to adjust the VCO frequency. The control signal for the varactor is derived from a loop filter consisting
of components C362, C363, C364, R320, and R321.
The IF IC also performs several other functions. It provides a received signal-strength indicator
(RSSI) and a squelch output. The RSSI is a dc voltage monitored by the microprocessor, and used as
a peak indicator during the bench tuning of the receiver front-end varactor filter. The RSSI voltage is
also used to control the automatic gain control (AGC) circuit at the front-end.
The demodulated signal on U301(pin 27) is also used for squelch control. The signal is routed to
U404 (ASFIC) where squelch signal shaping and detection takes place. The demodulated audio
signal is also routed to U404 for processing before being supplied to the audio amplifier.
2.6.3

Automatic Gain Control (AGC)

The front end automatic gain control circuit provides automatic reduction of gain for the front end RF
amplifier via feedback. This prevents overloading of backend circuits by drawing some of the output
power from the RF amplifier output. At high radio frequencies, capacitor C331 provides a low
impedance path to ground for this purpose. CR308 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 Q315
provides this current where, upon saturation, current will flow via R347, PIN Diode, collector and
emitter of Q315 and R319 before going to ground. Q315 is an NPN transistor used for switching here.
Maximum current flowing through the PIN is mainly limited by the resistor R319.
The Radio Signal Strength Indicator, RSSI, a voltage signal, is used to drive Q315 into saturation,
hence turning it on. RSSI is produced by U301 and is proportional to the gain of the amplifier and the
input RF signal power to the radio.
The resistor network at the input to the base of the Q315 is scaled to turn on Q315, hence activating
the AGC at certain RSSI levels. In order to turn on Q315, the voltage across the transistors base to
ground must be greater or equal to the voltage across R319, plus the base-emitter voltage (Vbe)
present at Q315. The resistor network with thermistor RT300 is capable of providing temperature
compensation to the AGC circuit, as RSSI generated by U301 is lower at cold temperatures