CIRCUIT DESCRIPTION
4-10
November 2001
Part No. 001-7240-001
U143A is selected. The data output signal on U141C,
pin 4 is then applied to pin 24 of U147 which is the
input of the digitally controlled potentiometer 1 in that
device. The data modulation level is set during align-
ment and controlled by the logic. The output of this
potentiometer on pins 22 and 23 is then fed to U148A
where it is combined with the transmit audio signal
(see Section 4.8.3).
4.8 TRANSMIT AUDIO PROCESSING
4.8.1 GATE (U203), HIGH-PASS FILTER (U151)
The internal microphone audio signal is applied
to pin 6 of gate U201B on the keypad/display board,
and the external microphone signal from pin 2 of the
accessory connector is applied to pin 2 of gate U201A.
This gate is controlled by the Mic Sense signal from
pin 7 of the accessory connector. When no external
microphone is connected, this input is pulled high by
R202 and U201B is enabled and U201A disabled.
Q202 is controlled by the T5C signal and therefore is
on in the transmit mode and off in the receive mode.
The microphone audio signal is then applied to
mic audio mute gate U203B on the logic board. This
gate is controlled by the signal on pin 107 of the
microprocessor (low = mute).
The transmit DTMF tones from pin 5 of U201 are
combined with the beep tones fed through R204 and
then applied through C204 and R203 to pin 2 of gate
U203A. This gate and Q201 are controlled by pin 34
of the microprocessor (low = mute).
The microphone audio signal from U203B and
the DTMF tones from U203A are then combined and
applied to pin 3 of the compander in U204. When
companding is enabled by a low input on the PT input
(pin 8) of U204, this stage provides 2:1 compression
of the audio signal. This compression and then re-
expansion when it is received reduces the amount of
noise present in the audio signal. When the PT input of
U204 is high, companding is disabled and the signal is
fed through U204 without compression.
The transmit audio/DTMF signal is fed out of
U204 on pin 2 and applied to a high-pass filter formed
by U151B. This filter has a –3dB cutoff frequency of
approximately 300 Hz to attenuate frequencies that
could cause interference with the Call Guard signals.
4.8.2 LIMITER (U151A)
Pre-emphasis of the DTMF tone and microphone
audio signals is provided by C160/R168. U151A is an
amplifier functioning as a limiter (limiting occurs
when it saturates). Limiting prevents overmodulation
caused by high input levels from the internal or
external microphone. The bias voltage to this stage
and also to U151B is produced by voltage divider
R170/R171.
4.8.3 LOW-PASS FILTER (U146A/B)
U146A and U146B from a low-pass splatter filter
which attenuates frequencies above approximately 3
kHz. This prevents adjacent channel interference
caused by the harmonic frequencies which that result
from amplitude limiting.
The output signal from this filter is applied to
digital potentiometer 3 on pin 20 of U147 which sets
the transmit audio/DTMF deviation level. This level is
set via the control logic when the transceiver is
aligned.
The transmit audio/DTMF signal is then
combined with the transmit data signal by U148A. The
output signal on pin 1 of U148A is fed to the TCXO
on the RF board and also to digital potentiometer 4 on
pin 7 of U147. The output signal from this potentiom-
eter on pin 8 is applied to the VCO on the RF board.
The level of this signal is set via the control logic
during alignment to balance the TCXO and VCO
modulation signals.
4.9 SMARTNET DATA PROCESSING
NOTE: The SMARTNET
™
/SmartZone
®
data
processing circuit is not currently used.
A filter formed by U142A/B and U143A provides
filtering of both the transmit and receive SMARTNET/
SmartZone data signals. Switching of these signals is
provided by gate U141B which is controlled by pin 62
of the microprocessor. When this signal is low, the