Rx Sub-Audible Decode Circuit; Tx Audio; Tx Sub-Audible Encode Circuit; Microprocessor - Motorola MAG ONE Manual

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4.2

RX Sub-Audible Decode Circuit

The filter U202 is a 260Hz 8 order LPF to pass CTCSS, DCS signals and to block any other signals.
The signals that pass the LPF filter changes this sub-tone to logic signals in the Q208 pass
comparator (U202-D) so that the microprocessor can decode the CTCSS and DCS correctly.
4.3

TX Audio

When the radio user pushes the PTT button, the microprocessor activates the entire transmitter
section, including the microphone. The audio signal from the microphone passes through the 3
order 300Hz high pass filter and through the limiting amplifier (U104-C, A) that has a 6db/oct pre-
emphasis function. The pre-emphasized audio signal is adjusted to a proper level by R159 and
passes through a 6 order 3KHz low pass filter (U104-D,U105-C) before being modulated. Wide and
narrow channel spacing is controlled by the N/S switch (Q108).
4.4

TX Sub-Audible Encode Circuit

The encode circuit of CTCSS and CDCSS mixes signals from four ports of the microprocessor. This
mixed signal passes through 3 order 300Hz LPF (U105-B) to obtain clear CTCSS and DCS signals.
4.5

Microprocessor

U101 controls all functions of the radio as a Microprocessor. It is operated by an external
7.3728MHz crystal. It detects external activities such as the pressing of the PTT, muting/un-muting
conditions, changing of channels, and then making the radio operate accordingly.
Audio and Control Circuit