Motorola MCS 2000 Service Instructions Manual page 109

Controller Section Theory of Operation: Audio and Signalling Circuits
Receive Path for Radios with Hear Clear
The audio signal enters the controller from the ZIF on DISC. The discriminated audio DISC enters
the Hear Clear Flutter Fighter through C0200 and C0267. C0200 connects the signal to FF IN
(U0250-E4). C0267 is the beginning of a noise sampling circuit consisting of components C0267,
R0256, R0253, C0264, C0263, R0254, R0255, R0257, and C0265; and Hear Clear ports Clip Ref,
Noise Filter In, and Noise Filter Out, Noise Hold.
After exiting the HC at FF OUT (U0250-F4), the signal enters the ASFIC at RX IN (U0200-H6).
Within the ASFIC the signal passes through a low pass filter and a high pass filter limiting the audio
band width to 300-3 kHz. It then goes through deemphasis and exits the ASFIC at U0200-H5
FILTERED AUDIO.
Upon exiting the ASFIC at FILTERED AUDIO, the signal passes through capacitor C0250, which
provides AC coupling. The signal then enters the Hear Clear at EXP IN (U0250-C1) and exits the
Hear Clear Expander at EXP OUT (U0250-A2). The normalized signal is the routed back to the
ASFIC through C0260 for volume adjustment, entering at U0200-J5 and exiting the ASFIC at U0200-
J4 as RX AUDIO. The audio is then routed to the Audio PA in the same manner as standard receive
audio.
The Flutter Fighter is for receive only. It samples the amount of Noise in the receive audio between
10 kHz and 20 kHz, using the Noise filter in (U0250-B5), Noise filter out (U0250-C6), and Noise hold
(U0250-D5) ports. In addition, it monitors the rate of change of RSSI (Receive Signal Strength In)
(U0250-F5). The discriminated audio DISC enters the HC at FF IN (U0250-E4) and the circuit then
reduces the amount of popping Noise associated with fading. The improved audio exits the IC at FF
OUT (U0250-F4).
The Expander is used after deemphasis but before the ASFIC volume attenuator.
The purpose of the expander is to transpose compressed audio back to "normal" audio. As with the
compressor circuit, the expander circuit adjusts the amplitude of a signal based upon its input
amplitude, NOT its frequency.
The voltage transfer function is: EXPOUT= 0.41*(EXPIN/0.28)2.
The importance here is to notice that an input signal of 191 mV will exit as 191 mV. A smaller signal
will be made even smaller and a signal larger than 191 mV will exit EXP OUT even larger.
Some example levels are:
• 100 mV EXP IN == 52 mV EXP OUT
• 191 mV EXP IN == 191 mV EXP OUT
• 250 mV EXP IN == 327 mV EXP OUT
Again this operation is NOT A FUNCTION OF FREQUENCY between 300 Hz and 3 kHz.
Hear Clear Routing of Data/Signalling
All signalling for transmit enters the transmit stream after the Hear Clear Compressor and therefore
does not pass through the compressor.
In receive, subaudible signalling PL/DPL goes through the Flutter Fighter along with audio, and is
unaffected by the Flutter Fighter operation. After the Flutter Fighter, upon entering the ASFIC, the
sub-audible signalling is separated from the audio and decoded. Subaudible signalling never passes
through the expander.
In receive, for all other signalling HST/MDC/MPT etc. (not sub-audible), the Flutter Fighter is set to
"Pass Through Mode". In this mode the Flutter Fighter passes signals from FF IN to FF OUT without
any adjustment.
68P81083C20-D
7-27
December 6, 2004