Automatic Gain Control Circuit - Motorola Professional Radio Service Manual

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LO Clamp Disable
HCI Disable
LO Clamp Disable
*RX1:receive voice with carrier squelch, PL or DPL (Flutter Fighter can be on or off).
**RX2:refers to receive mode with all other data HST/MDC/DTMF (Flutter Fighter must be off).
2.22.3.1 Receive Path for Radios with Hear Clear
The audio signal enters Hear Clear controller from DEMOD_OUT signal on DISC. The detected audio
"DISC" enters the Hear Clear Flutter Fighter through C857 and C859. C857 connects the signal to FF
IN (U851-E4). C859 is a beginning of a noise sampling circuit consisting of components – C859,
R853, C860, R854, C861, R855 and C862; and Hear Clear Ports Ref, Noise Filter In, and Noise Filter
Out, Noise Hold.
After exiting Hear Clear at the "FF OUT" (U851-F4), the signal enters ASFIC at DISC (U404-2). Within
the ASFIC, the signal passes through a low pass filter and high pass filter limiting the audio bandwidth
to 300Hz-3KHz. It then goes through de-emphasis and exits the ASFIC at AUDIO (U404-41). The
audio is then routed to the Audio PA in the same manner as the standard receive audio.
The purpose of the Flutter Fighter is to sample the amount of Noise in the receive audio between 10-
20KHz using the Noise Filter (U851-B5), Noise Filter Out (U851-C6), and Noise Hold (U851-D5). In
addition, it monitors the rate of change of RSSI (Receive Signal Strength In) (U303-1). The detected
audio DISC enters into the Hear Clear IC at "FF IN" (U851-E4). The circuit then reduces the amount
of popping Noise associated with fading. The improved audio exits the IC at "FF OUT" (U851-F4).
2.22.3.2 Hear Clear Routing of Data/Signaling
While receiving, sub-audible signals PL/DPL go through the Flutter Fighter along with the audio, and
is unaffected by the Flutter Fighter operation. On entering the ASFIC, the sub-audible signaling is
separated from the voice and decoded.
While receiving other signals HST/MDC (not sub-audible), the Flutter Fighter is set to the "pass
through mode". In this mode, the Flutter Fighter is routed from "FF IN" to "FF OUT" without any
processing.
2.22.4

Automatic Gain Control Circuit

The automatic gain control circuit provides automatic gain reduction of both the low noise amplifier in
the receiver front end and the IF amplifier in the receiver backend. This action is necessary to prevent
overloading of the backend IF IC.
The IF automatic gain control circuit provides approximately 50 dB of attenuation range. The signal
strength indicator (RSSI) output of the IF IC produces a voltage that is proportional to the RF level at
the IF input to the IF IC. This voltage is inverted by U350, R351, R353, R352, R354 and C355 and it
determines the RF level at which the backend end AGC is activated as well as the slope of the
voltage at the output of U350 vs. the strength of the incoming RF at the antenna. The inverted output
of U350 is applied to the second gate of the IF amplifier U352 via R355. As the RF signal into the IF
IC increases the following occurs:
• the RSSI voltage increases,
• the output of inverter U350 decreases, and
• the voltage applied to the second gate of the FET is reduced thus reducing the gain of the IF
amplifier.
Table 2-8 Hear Clear Logic and IC Status
U851-C2 GND
U851-B6 SWB+
U851-D1 GND
0 0
1 1
0 0