Receiver Back-End; Automatic Gain Control Circuit - Motorola HT Series Service Manual

6-4
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 transfered 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.
6.2.2

Receiver Back-End

(Refer to VHF Receiver Back End Schematic Diagram on page 9-79).
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 receive 5 volts (R5). The gain controlled
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 the matching for the IF amplifier and
U3220.
The IF signal applied to pin 3 of U3220 is amplified, down-converted, filtered, and 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 will "search" 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 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 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.
6.2.3

Automatic Gain Control Circuit

(Refer to VHF Receiver Front End Schematic Diagram on page 9-78 and VHF Receiver Back End
Schematic Diagram on page 9-79).
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 backend 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.
VHF Theory of Operation: Receiver (for all models except those with PCB 8486473Z04)