Monday, June 16, 2014

DDS VFO purchase

I recently purchased an old crystal controlled Codan lunchbox radio and want to put it on 40 and 80m so I've found a rather nice DDS VFO on ebay for AU$45. This one has two PIC CPUs on the board, a nice backlit display and comes with a tuning knob but no info on wiring it up.

The same unit is in other ebay auctions and one of them has this "schematic" which will be a help, if it's correct.


My board came with some cables with sockets to match the connectors on the board and the rotary encoder. Here's other info, for future reference. (It all seems a little mysterious to me but hopefully will be clear when I play around with it).

Note that the wiring of the rotary encoder in the sketch above (from an eBay ad) is shown from the rear of the encoder. I wired it up in reverse and it works but the tuning goes clockwise for up in frequency.

My thanks to Stephen VK2BLQ for access to his workshop, here's my VFO all boxed up.


One criticism of these boards is the lack of mounting holes. I've stuffed the box with cloth to press it against the front and that works fine but is not ideal.

I've now added a 9V three terminal regulator so that the unit can run from my 12V bench supply.

All up this is a terrific VFO, pushing while turning the knob changes the digit that will be adjusted so it's quick to get around. Thanks to commenter Yiannis, the documentation and software source has been traced to Ham Radio India.

The objective of this device for me was to use it as a VFO for my Codan 6924 radio. There's a handy page outlining how to do this. This weekend I ran a cable to an empty crystal spot.


This makes the old radio look very fancy.


As I tune around I notice carriers sweeping by at some frequency ranges but not others. I took the DDS over to VK2BLQ's lab and it seems that for many frequency ranges the output drops dramatically. I'm not sure what's going on here but my guess is that it's designed to cover only certain bands and not others.

2 comments:

Yiannis Kotsis-Giannarakis said...

Hi, I am Yiannis sv1mne, from Athens Greece. The DDS VFO you display is an implementation of a project published in Ham Radio India (Schematic and source code available at http://www.hamradio.in/circuits/dds.php). It can use a numeric keypad and has a separate PIC for band change control. I have built many of them and modified the source code for 4x4 keypad. (Re: DDS 4.0 source code http://www.hamradio.in/forum/viewtopic.php?p=10409#p10409)

Have fun!

73

Yiannis

Peter Marks said...

Thank you Yiannis,

Great information, I very much appreciate your comment.

73

Peter