DDS VFO

This is a DDS VFO based on Curtis Preuss, WB2V design from the July '97 Issue of QEX.
See http://www.qsl.net/yo5ofh/projects/dds_vfo_by_n2apb/dds_vfo.htm

I wanted a stable signal source for measuring quartz crystals for use in a filter circuit. I have an old 'Boatancher' valve Advance instruments signal generator but drifts too much, certainly inadequate for measuring crystals.

The circuit board was purchased from Far circuits. http://www.farcircuits.net. I have found that this site supplies a vast amount of PCB for most of the popular QRP circuits from magazines and club Journals such as QST, QEX.
W1FB's notebook etc.

The PCB were reasonably priced, and the P+P to DX locations was good too.

At the heart of the DDS VFO is an Analogue Devices AD9850, a 28 lead SSOP surface mount chip, with the frequency being controlled from a 16f628 PIC IC. An Etched PCB is highly recommended to aid soldering. In fact this was my first attempt at surface mount soldering. To solder the AD9850 it was best to align the IC on to the PCB tracks and first tack the corner pins down with a bit of solder, this allows fine alignment to be made before the IC is soldered permanently. Then when you are happy with the alignment solder all the pins on each side. Don't worry about solder bridges because these can be removed with some solder wick.

It was the first time I used solder wick and was quite impressed that in some applications is better than a de-Soldering pump. but for surface mount applications is a must.

The version I made used the Siggen 3C software developed by Bruce AA0ED and Craig AA0ZZ. The software can be downloaded from the files section in the DDS VFO Yahoo group website.
http://groups.yahoo.com/group/dds-vfo/

This version has.
Variable rate tuning based on the speed on which the shaft encoder is turned.
Band Memories to cycle around the ham bands.
Calibrate Mode if an external button is pressed on power up.
The popular 16f628 PIC IC, earlier versions had the 16f84 PIC IC.

I found that optical shaft encoder were very expensive to buy so I decided to homebrew my own using a modified potentiometer and the IR transmitters and detectors from an old ball type computer mouse. This worked extremely well, all the encoder parts are mounted 'ugly style' on a piece of copper clad board on the back of the modified potentiometer.
Follow this link for details of the optical encoder I used:
http://users.on.net/~merrifield/opto/index.html





The DDS VFO as it stands gives an output of 250mV. I added an amplifier board to give about 1Vpk-pk. The amplifier board was copied from the IQ-PRO circuit from the DDS VFO website link given above.