Automatic Gain Control (Agc); Receiver Back-End - Motorola PRO3150 Service Manual

VHF Receiver
The output of the post-RF amplifier filter is connected to the passive double balanced mixer which
consists of T3301, T3302 and CR3301. Matching of the filter to the mixer is provided by C3317,
C3318 and L3308. After mixing with the first LO signal from the voltage controlled oscillator (VCO)
using high side injection, the RF signal is down-converted to the 45.1 MHz IF signal.
The IF signal coming out of the mixer is transferred to the crystal filter (Y3200) through a resistor pad
(R3321 - R3323) and a diplexer (C3320 and L3309). Matching to the input of the crystal filter is
provided by C3200 and L3200. The crystal filter provides the necessary selectivity and
intermodulation protection.

2.10.2 Receiver Back-End

The output of crystal filter Y3200 is matched to the input of IF amplifier transistor Q3200 by capacitor
C3203. Voltage supply to the IF amplifier is taken from the receiver 5 volts (R5). The controlled gain IF
amplifer provides a maximum gain of about 10dB. The amplified IF signal is then coupled into U3220
pin 3 via L3202, C3207, and C3230 which provides impedance matching for the IF amplifier and
U3220.
The IF signal applied to pin 3 of U3220 is amplified, down-converted, filtered, and then demodulated,
to produce the recovered audio at pin 27 of U3220. This IF IC 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 the IF module (U3220).
The IF IC uses a type of direct conversion process, whereby the externally generated second LO
frequency is divided by two in U3220 so that it is very close to the first IF frequency. The IF IC (U3220)
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 will lock onto the IF signal. The second
LO/VCO is a Colpitts oscillator built around transistor Q3270. The VCO has a varactor diode, D3270,
to adjust the VCO frequency. The control signal for the varactor is derived from a loop filter consisting
of C3278 to C3280, R3274 and R3275.
The IF IC (U3220) also provides a received signal-strength indicator (RSSI) and a squelch output.
The RSSI is a dc voltage monitored by the microprocessor, and is 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 pin 27 of U3220 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 going to the audio amplifier for amplification.

2.10.3 Automatic Gain Control (AGC)

The front end automatic gain control circuit provides automatic reduction of gain of the front end RF
amplifier via feedback. This prevents overloading of backend circuits and 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 make up a voltage divider designed to turn on Q3301 at certain RSSI
levels. 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
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