If the sidetone in your transceiver has a sound that is unpleasant or fails to meet your expectations, the N4YG DDS Sidetone/Code Practice Oscillator board could very well be the solution you have been looking for. Or you may need a code practice oscillator with plenty of volume and a good sound. This little board, measuring 1.2 by 3.0 inches, provides a very nice crisp sound without pops or clicks.

So how does this single chip do all this? It does it the same way any other DDS system does it. First of all, a stable frequency source is required, in this case it is an 8 MHz oscillator which is an internal feature of the 16F88 processor. Using this reference, firmware within the processor provides a 8-bit digital output word representing the amplitude of a sine wave. This digital word is updated at 28 microsecond intervals. This means that for a 700 Hz tone output, approximately 51 samples are provided for every cycle of the sidetone. These digital words are converted into an analog level by the R,2R,4R,8R,16R,32R,64R,128R,256R resistor network connected to the output port of the processor. After being filtered, this signal, the sidetone signal is input to the audio amplifier of the transceiver. All this occurs only when the key signal is applied to the processor. When keyed, the processor applies weighting to the leading and trailing edges of the keyed envelope to remove clicks which would otherwise be very annoying. This is done logarithmically (the way the ear perceives sound), changing the amplitude in seven 3 dB steps. This cannot be achieved in an analog circuit. Furthermore, whereas analog circuits usually have random initial and ending phase, DDS keying begins and ends at zero phase. This further reduces keying clicks. To hear what it sounds like click below.

Click To Hear Sidetone Audio

The DDS Sidetone board can be integrated into almost any rig. Click on the link below to see a very instructive video by Brian, W3ATT, regarding his installation in the Ten Tec OMNI-C.

OMNI-C Intallation Video

The two images above show the Sidetone implementation on the left and the Code Practice Oscillator on the right. The primary differences are the inclusion of an audio power amplifier, U2, in the Code Practice version. Also note that the VOLUME and TONE controls are on-board in the sidetone version, but off-board (panel mounted pots) in the code practice version.

. For detailed instructions for DDS Sidetone Installation in the Ten Tec Corsair II and the Argosy II click here.

The kit includes:

- PC Board

- All Board Parts (Resistors, Trimmer Pots, Capacitors, 5 Volt Regulator, Programmed 16F88, I/O Connector)

- An I/O Cable

For detailed instructions for DDS Code Practice Oscillator Construction click here.

The kit includes:

- PC Board

- All Board Parts (Resistors, Capacitors, Programmed 16F88, I/O Connector)

- An Board I/O Cable

- Two Panel Mount Potentiometers (Volume & Tone)