3012—Technical description
NGT Transceiver System Technical Service Manual 305
As the DSP provides very little in terms of useable I/O, IC107 also acts as a peripheral
device to provide required I/O functions, including:
• control interfacing to the transceiver (SCAN, PTT, BUSY and QUIET line
functions)
• serial interfacing for the non-volatile memory (IC205)
• driving the LED indicators via shift registers IC206 and IC207
IC107 also connects the digi switch (S101) to the processor. This switch is used to select
the operating mode, including special factory test modes.
Throughput indicator
Serial shift registers IC206 and IC207 control the modem’s Throughput indicator and
LED. The Throughput indicator on the DSP Modem PCB displays the current data
throughput of the data modem. Each LED in the indicator represents approximately
100 bps of data.
Power supply
In order to reduce overall power consumption, a high-efficiency 260 kHz switching
regulator generates the main +5 V supply rail.
The switching regulator (IC103) works as a buck converter, together with an inductor
(L102), a Schottky diode (D102) and low-ESR tantalum capacitors (C111 and C112) to
create the supply. This is further filtered by L103 and C113 to provide a quiet analog
+5 V supply to feed the CoDec (IC108).
For PCB issue 06 and earlier, the CoDec (IC108) also requires a negative supply rail,
which is generated by a switched-capacitor generator (IC104). The resulting output at
pin 5 is regulated at approximately –7 V. This rail has is post-regulated by a conventional
3-terminal linear regulator (IC105 (LM79L05)), providing a smooth –5 V DC rail to feed
the CoDec.
Drawings 04-03366 (sheet 1) and 04-03366 (sheet 2)
NOTE
If a modulation method other than 16 channels is selected, the
Throughput indicator shows the capacity of that modulation method.
Drawings 04-03366 (sheet 1) and 04-03366 (sheet 2)
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