The Corsair II is an example of a great rig with a poor CW sidetone. A nice sounding sidetone requires a good sine wave and proper shaping of the leading and trailing edges of the keyed waveform. While analog techniques can provide a good sidetone, direct digital synthesis(DDS) has been found to provide a superior sound. That's right, DDS applied to audio frequencies. While it may seem difficult and overkill for audio applications, the implementation at audio frequencies is much simpler than with RF. The only active component (other than a 5 volt regulator), is a single 18-pin 16F88. The hardware is not only simple but inexpensive, yet it provides a predictable, repeatable and stable sidetone. The frequency determining components normally seen in analog sidetone circuits are absent in the DDS circuit. The internal 8 MHz oscillator in the 16F88 provides the timing for the entire operation. The leading and trailing edges are digitally shaped so that popping and thumping is eliminated to the extent not possible in analog circuits. The entire circuit is contained on a PC board measuring 1.2 by 3.0 inches. It is specifically designed for older Ten Tec rigs and interfaces with the audio and keying circuits with 4 connections through which the board receives power and keying and output the sidetone to the audio amplifier in the transceiver. Output level and tone are adjustable with trim pots on the board.
The printed circuit board has been designed so that it can also be used as a Code Practice Oscillator. An audio amplifier is provided which gives ample sound even for demonstrations. Some components positions are left unfilled in either the sidetone or code practice application with only the components required installed.
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