...turns your Motorola "MC micro" into a fully-fledged amateur radio transceiver.

DAC-Mod (optimizing the digital-analog-converter)

The audio DAC of the trunked (EVA5) version was originally designed to support data transmission only. The converter is build from discrete components consisting of a resistor network (R812 - R815) followed by an active low-pass filter (U801A, C804, C805, R821). The network is driven directly by two CPU pins (30 and 31).

The following tools are required for this mod:

  • Soldering iron
  • Solder wick, solder (if needed)

Original circuit

Location in the schematic diagram
Location on the board
Unmodified DAC circuit
Unmodified PCB, arrows indicating the required modifications
Circuit with modifications in red
Modified PCB

Modifying the circuit

The modification is simple:

Reposition R811 to connect 5V and node R812-R815.

Swap R815 and R814.

There are still only two pins of the CPU being used, but additionally to the output states "1" and "0" the high-impedance input state is also employed.

The re-ordered resistor network now allows 9 distinct states/voltages to be set (the symmetric nature of the original network (where both CPU pins had a series resistor of 200 kOhm) would only yield 6 distinct states/voltages).

The effective resolution of the DAC is thereby raised to an equivalent of 3.17 bit. The converter is now able to produce a signal (at full scale level) with more than 19 dB SNR.