8571—Technical description
342 NGT Transceiver System Technical Service Manual
300 Hz notch filter
From the Backplane PCB (drawing 04-02691), the Tx audio bus is connected to the
Microprocessor & TCVR Interface PCB (drawing 04-02690) connector P1 pins 32a
and 32b. It continues via an analog changeover switch IC2c selected to a 2-section
digital filter, consisting of IC4, IC3b, IC3a and associated components, to produce a
300 Hz notch filter. It has unity gain at frequencies above 300 Hz, but produces high
attenuation (> –40 dB) at 300 Hz. This removes the 300 Hz control tone from the final
transmitted audio.
IC4 is a dual active filter and is driven by a 30 kHz external clock generated from the
tone generator IC16. The high-pass output (pin 18) and the low-pass output (pin 20) of
the first active filter are phase mixed at the input of amplifier IC3b, resulting in the
required notch response. The output from IC3b is applied to the second active filter and
again the high- and low-pass filter outputs are phase mixed at the input of IC3a. The two
notch filters are slightly offset from each other to produce about a 25 Hz bandwidth
rejection centred at 300 Hz.
Transmit audio to split-site interface controller and transceiver
At the output of the 300 Hz notch filter (IC3b), the transmit audio is connected by a
changeover analog switch (IC2a) to a simple passive LPF consisting of R23 and C8, then
to the primary of isolating transformer T2. From the secondary, the audio is connected
(P1 pins 13a and 15a) via an interface cable to the split-site interface controller transmit
audio input.
Drawings 04-02691 and 04-02690
Drawing 04-02690
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