Table 7-3. Hear Clear Enable Lines Configuration - Motorola MCS 2000 Service Instructions Manual

7-26
Hear Clear IC
(Refer to schematic page 10-26 for reference)
The Hear Clear (HC) is typically used for 900 MHz radios. The HC has 3 main circuit blocks within
the IC which are used by this radio; 1) Compressor, 2) Flutter Fighter, and 3) Expander circuits.
There are 6 enable lines on the Hear Clear IC which determine its mode of operation. The IC ENAB
line U0250-C4 is tied to SW B+, so whenever the IC is placed it is always active. The remaining 5
lines are controlled by the ASFIC General Control Bit lines, GCB0, GCB1, GCB3, GCB4, and GCB5.
The table below summarizes their logic states.
Ic Enable
Flutter Fighter Enable
LO Clamp Disable
Hi Clamp Enable
HCI Disable
Compander Enable
TX1: transmit mode with carrier squelch, PL or DPL.
RX1: receive voice with carrier squelch, PL or DPL.
TX2: transmit mode with all other data HST/MDC/MPT/DTMF etc.
RX2: refers to receive mode with all other data HST/MDC/MPT/DTMF
Logic State "X" means either 1 or a 0, i.e. "don't care".
Transmit Path for Radios with Hear Clear
For transmit, the signal comes from the appropriate microphone and enters the ASFIC at U0200-A7
or U0200-B6 as would standard TX audio. After entering the ASFIC, the signal is internally routed to
U0200-A6 ASFIC MIC AMP OUT, where it leaves the ASFIC and enters the Hear Clear compressor
at U0250-D3. The signal then exits the compressor at U0250-F3, where it is routed back to the
ASFIC (U0200-C7). C0261 provides AC coupling. Inside the ASFIC the signal goes through an LPF
and HPF which band limit the signal between 300 - 3 kHz. The signal is then pre-emphasized and
exits the ASFIC at U0250-C8, passes through a coupling cap and enters the ASFIC at U0200-E8.
Again inside the ASFIC the signal goes through a limiter, splatter filter, and a pair of attenuators
which set the amplitude (deviation level) of the signal.
The Compressor is used in transmit mode. The purpose of this circuit is twofold; 1) improve S/N ratio
for low level audio, and 2) maintain the same dynamic range of a 12.5 kHz bandwidth channel as is
obtained in a 25 kHz bandwidth channel.
The compressor raises low level signals and lowers high level signals. The compressor circuit
produces a signal whose output voltage (U0250-F3) is based on the input voltage level (U0200-A6)
of the signal. It is NOT a function of frequency (as is Preemphasis). The voltage transfer function is:
COMPOUT == SQRT[ 80*ASFICMICAUDOUT ]
Notice that 80 mV in yields 80 mV out. Some example levels are:
• 20 mV input ==
• 80 mV input ==
• 150 mV input == 110 mV output
December 6, 2004
Controller Section Theory of Operation: Audio and Signalling Circuits

Table 7-3. Hear Clear Enable Lines Configuration

Name Ref. Des
U0250-C4
U0200-B5
U0250-A5
U0250-C2
U0250-B6
U0250-D1
40 mV output
80 mV output
Logic State
Set By TX1 RX1
SW B+ 1 1
U0200-B5 X 1
U0200-B3 1 1
U0200-C4 0 0
U0200-A3 1 1
U0200-A2 1 1
TX2 RX2
X 1
X 0
1 X
X 0
X 1
X 0
68P81083C20-D