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Also applied at this point are the DTMF and beep
tones. Muting of the DTMF tones is provided by Q201
which is controlled by the pin 34 output of the micro-
processor. The beep tones are synthesized by 5V and
0V levels from the microprocessor on pin 65.
4.6.4 AUDIO AMPLIFIER (U101, U102)
From mute gate U202, the audio signal is fed
through the keypad/display board to volume control
VR901 which sets the input level to the audio ampli-
fiers. This signal from VR901 is fed back to the
keypad/display board and applied to pin 7 of audio
amplifiers U101 and U102.
With normal operation when the internal speaker
is used, U101 provides the required amplification.
However, when an external speaker is used, U101 is
disabled and U102 provides the required amplifica-
tion. When any load of approximately 1k ohm or less
is connected to pin 10 of the accessory jack, Q106
turns on and Q102 turns off. This removes supply
voltage from U101 and applies it to U102.
The supply voltage to the audio amplifiers is also
controlled by Q103 and Q105. When the audio is
muted such as when no tones are sounding or no
message is being received, the AMUT (pin 37) output
of the microprocessor is low. This turns off both Q103
and Q105 and removes supply voltage from the audio
amplifiers which helps conserve battery power. The
receive audio signal is also muted by gate U202 as
previously described.
4.7 RECEIVE AND TRANSMIT DATA
PROCESSING
4.7.1 RECEIVE DATA FILTER/DETECTOR
(U102A/B, U103A/B)
The receive data (and audio) signal from the
limiter/detector on the RF board is routed to a receive
data filter formed by U102A and U102B. This is a
low-pass filter which attenuates voice and other
harmonic frequencies above the data band. The pass-
band of this filter is controlled by the bandwidth
control signal applied to Q103A. When digital Call
Guard data or a low-frequency Call Guard tone is
being detected, this signal is high which turns on
Q103A. The cut-off frequency of the filter is then
approximately 150 Hz. Then when a high-frequency
Call Guard tone is received, Q103A turns off and the
cut-off frequency is approximately 190 Hz. This
switches additional capacitance into the circuit which
lowers the filter cut-off frequency.
From the filter the data signal is applied to a DC
restoration circuit formed by U103B and U103A.This
circuit converts the data signal from AC floating at
half supply to a digital signal at 0 and 5-volt levels that
can be read by the microprocessor. U103B is a stan-
dard noninverting amplifier with a gain of approxi-
mately 5.6 set by R127 and R126 (R126 is AC
grounded by C120). Diodes D101 and D102 charge
and discharge C120 to establish a DC reference on
pins 2 and 6 of U103 that is the average of the positive
and negative going alternations of the data signal.
The amplified data signal is applied to pin 3 of
U103A. When this level rises above the reference
level on pin 2, the output on pin 1 goes high (5 volts).
Conversely, when the data signal falls below the refer-
ence level, the output goes low (0 volts). In the
transmit mode, Q103B is turned on which effectively
grounds pin 3 of U103A and forces the output on pin 1
low.
4.7.2 TRANSMIT DATA FILTER (U145A/B)
The transmit data signal is produced by the
TXDATA and TXPREC outputs of the micropro-
cessor (pins 111 and 106). A pseudo sine wave is
produced by the four voltage levels that can be
produced by these outputs. This signal is fed to a
transmit data filter formed by U145A/B. This filter is
the same as the receive data filter described in the
preceding section. It attenuates harmonics present in
the waveform produced by the microprocessor.
The input and output signals of the data filter are
fed to gate U141A. This gate is used to bypass the
transmit data circuit during alignment so that a wide
range of harmonic frequencies are passed for use in
setting modulation balance. The control signal to this
gate is the TEST signal from pin 105 of the micropro-
cessor. This signal is low when the transmit data filter
is bypassed.
The signal is then routed to pin 3 on gate U141C
where either it or the SMARTNET
4-9
CIRCUIT DESCRIPTION
™
data signal from
November 2001
Part No. 001-7240-001
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