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Table 4-1 Microprocessor U208 Pin Descriptions (Continued)
Pin No.
Port Name
107
MICMUTE
108
SQUELCH
109
BWC
110
RXDATA
111
TXDATA
112
SN_RX
113
MD0
114
MD1
115
BOOT1
116
DPOTST
117
-
118
-
119
EXT_DC
120
-
4.6 RECEIVE AUDIO PROCESSING
4.6.1 INTRODUCTION
The receive audio and data signal from the
limiter/detector module on the RF board is applied to
the following circuits:
•
Receive Audio Processing - Pin 2 of U101A (see
following information)
•
Receive Data Processing - Pin 3 of U102A (see
Section 4.7)
•
™
SMARTNET
Data Processing - Pin 2 of U141B
(see Section 4.9)
4.6.2 BANDPASS FILTER (U101)
U101A and U101B form a bandpass filter which
passes frequencies from 300-3000 Hz. This attenuates
low frequency Call Guard (CTCSS/DCS) signaling
and high frequency harmonics. This filter also
provides de-emphasis of the audio signal.
On the input of U101A, Q102 provides wide and
narrow band compensation. The base of Q102 is high
in the wideband mode and low in the narrow band
mode. Therefore, R103 is grounded in the wideband
mode which lowers the input level to U101A. This
In/Out
Output
Mutes microphone audio signal
Input
Squelch control
Output
Transmit/receive data filter bandwidth signal output
Input
Receive data input
Output
Subaudible tone/main audio signal output
Input
SMARTNET receive data input
-
External pull-up
-
External pull-up
Input
Flash ROM data import control
Output
Digi-pot strobe signal
-
-
-
-
Input
Option signal input
-
-
Descriptions
maintains a constant audio output level since the
higher deviation in the wideband mode results in a
higher input level.
Also on the input of U101A, Q101 provides
NPSPAC compensation. The base of Q101 is high
when a NPSPAC channel is selected and low at other
times. This grounds R102 in the NPSPAC mode to
lower the input level since NPSPAC channel devia-
tion is higher than narrow band deviation.
4.6.3 EXPANDER (U204)
From U101A/B the signal is fed to pin 7 of the
expander in U204. When companding is enabled by a
low input on the PT input (pin 8) of U204, this stage
provides 1:2 expansion of the audio signal to remove
the 2:1 compression that occurred when it was trans-
mitted. This compression and then expansion reduces
the amount of noise present in the audio signal. When
the PT input is high, companding is disabled and the
signal if fed through U204 without expansion.
The audio signal is fed out of U208 on pin 15 and
then fed to audio mute gate U202. This gate is
controlled by the RMUTE output (pin 36) of the
microprocessor. The audio is also muted by the audio
amplifier as described in the information which
follows.
4-8
CIRCUIT DESCRIPTION
Active
Low
-
High
-
-
-
-
-
Low
-
-
-
-
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November 2001
Part No. 001-7240-001
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