Motorola HT Series Detailed Service Manual page 84

7-22
Only the Flutter Fighter section of this IC is used by this radio. The Compressor and the Expander
are included in the ASFIC. There are six enable/control lines on the Hear Clear IC which determine
the ICs mode of operation. The Flutter Fighter Enable line (U851-E3) is controlled by ASFIC DACRX
line (U404-4). The logic control and the IC status is summarized in
Name
IC Enable
Flutter Fighter Enable
LO Clamp Disable
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).
7.11.4.1 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 300 Hz–3 kHz. 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
10–20 kHz 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).
7.11.4.2 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.
Low Band, 800 MHz, PassPort & 900 MHz Theory of Operation: 900 MHz Receiver
Table 7-2. Hear Clear Logic and IC Status
Ref. Des Set By
U851-C4 SWB+
U851-E3 DACRX
U851-A5 SWB+
U851-C2 GND
U851-B6 SWB+
U851-D1 GND
Table 7-2 on page
RX1* RX2**
1
1
1
0
1
0
7-22.
1
0
1
0
1
0