
CIRCUIT DESCRIPTION
From the filter the data signal is applied to a DC restoration circuit formed by IC107B and IC107A. This circuit converts the data signal from AC floating at half supply to a digital signal at 0- and
The amplified data signal is applied to pin 3 of IC107A. 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, Q104 is turned on which effectively grounds pin 3 of IC107A and forces the output on pin 1 low.
4.7.2TRANSMIT DATA FILTER (IC206B/ IC206A)
The transmit data signal is produced by the TXDATA and TXPREC outputs of the micropro- cessor (pins 27 and 34, see Table
The output and input of the data filter is fed to gate IC203. 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 35 of the microprocessor. This signal is low when the transmit data filter is bypassed.
The signal is then routed to pin 7 or gate IC210 which selects either it or the SMARTNET data signal
from IC113B. The data output signal on IC210, pin 1 is then applied to pin 24 of IC209 which is the input to one of the digitally controlled potentiometers in that device. The data modulation level is set during align- ment and controlled by the logic. The output on pins 22 and 23 is then fed to IC201 where it is combined with the transmit audio signal (see Section 4.8.3).
4.8 TRANSMIT AUDIO PROCESSING
4.8.1GATE (IC204), HIGH-PASS FILTER (IC202)
The microphone audio signal is applied to pin 2 of one of the gates in IC204. This signal may come from the internal microphone or an external micro- phone connected to pin 8 of the accessory connector. This gate is controlled by the MMUTE signal from pin 33 or the microprocessor. When this signal is low, the microphone audio signal is muted.
The transmit DTMF tone is applied to pin 6 of IC204 which is the input of the other gate in that device. This gate is controlled by the DTMFMT signal from pin 82 of the microprocessor, and when that signal is low, the DTMF tone is muted.
The transmit audio and DTMF signals are then combined and applied to pin 3 of compandor IC208. When companding is enabled by a low input on the PT input of IC208, this stage provides 2:1 compression of the audio signal. This compression and then
The transmit audio/DTMF signal is fed out of IC208 on pin 2 and applied to a
4.8.2 LIMITER (IC202B)
| November 1999 |
Part No. |