For several years, I’ve wanted to build my own DDS (Direct Digital Synthesis) VFO (Variable Frequency Oscillator).

Analog Devices makes one of the most popular ranges of DDS chips which digitally generate precise sine waves covering frequencies from practically "DC to daylight"; well, up to many hundreds of MHz.

Some time ago I managed to obtain several samples but, for some time afterwards, that was as far as things went. I was just too busy with work and family to devote any time to the project.

Fig.1: the promise of things to come? The DDS VFO with its cellular phone "readout" mounted inside an HF transceiver the author is currently working on...

In addition, I couldn’t locate a suitable design to build. Practically all existing designs use one of the PIC microprocessor family. Others use several PIC microprocessors; yet others use a further large bunch of ICs to interface displays and keypad functions. With my microprocessor development tools all focused on the 8051 family – and being fundamentally of a contrary nature – I was determined to use an 8051 chip in my DDS VFO rather than mess about gearing up for another microprocessor, and keep the chip count minimal.

Underlying this was a feeling that if I wrote my own software, I could customise it to suit my precise requirements and be better placed to develop one or two other DDS-based projects I have in mind. Of course, that naively assumes I’ll find the time to complete those new designs.