DDS IQ VFO
This is an excellent Direct Digital Synthesis (DDS) Variable Frequency Oscillator (VFO), using the AD9854 chip which is an upgrade on the 10-bit AD9851 that I used previously. I obtained the bare boards with AD9854 and pre-programmed PIC micros from Craig Johnson's site, and built up the rest of the circuit using details from the QEX article. The PICs supplied provide 0 - 30 MHz coverage with 1 Hz resolution and a number of facilities for transciever operation that I don't need. This time I have used a proper optical rotary encoder (128 steps per resolution) and the tuning operation is nice and smooth. I installed the toroids and 2N3904/6 amplifiers and tested clean output with a maximum of about 8 V peak-to-peak. The output of the AD9854 is absolutely flat from 0 - 30 MHz, but with the LPF and amplifiers there is a slight ripple noticable above about 20 MHz and a small mis-match in gain between the two outputs at the upper end. This won't trouble me as the analog output will be squared off to drive a Tayloe detector.
For the front panel I printed the pushbutton numbers onto photo quality (140 gsm) paper, aligning it exactly to match the 6 x 4 array of pushbuttons which protrude ever so slightly through the front panel. Operation is reliable.
My aim is to do some experimental work with AM and SSB direct conversion receivers using the Tayloe detector. Details of these experiments will follow.
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