[TenTec] DDS Dilemma

[Steve Hunt]

A while ago I replaced the PTO in one of my CorsairIIs with the  DDS 
board from N4YG. Once I had modified it for single rail operation I was 
pretty happy with the results; but part of me regretted making the radio 
"non standard", and I missed the "Spot" button functionality. However I 
love the smooth tuning action of the DDS. What to do?

I decided to restore the radio to original condition, and use the DDS as 
a replacement for the PTO in my 263 remote VFO instead. I kept all the 
normal 263 switching functions - including the "split" function - and 
used the DDS as a simple PTO replacement; all I retained of the DDS 
functionality was the twin memory, which I toggle using the 263 
"Reverse" button. Here's a few observations on the mod:

* Very easy to install the DDS board - there's bags of space in the 
remote VFO

* Taking the DDS board out of the radio and feeding it through the LPF 
which is built into the 263 switching board seems to have removed the 
few spurii I had when it was inside the radio

* I had to find some way to disable the DDS output when the 263 is 
switched to "VFO A". A few tests showed that if you don't disable the 
DDS there is only about 40dB isolation; in other words a signal at "VFO 
B" (DDS) frequency would be heard 40dB below a similar strength signal 
at "VFO A" frequency, even though only VFO A was selected. I achieved 
this by shunting to ground the signal from the DDS to its buffer when 
VFO B is not required; I then measured over 80dB of isolation. Split 
operation at high keying speeds showed no ill effects.

* I like that I still have a "standard" Corsair radios but can still 
enjoy the silky smooth tuning of the DDS and can easily toggle between 
the two DDS memories. If I ever thought the Rx capability was being 
compromised by the DDS I can simply switch to VFO A and use the PTO. 
It's now very easy to put the DDS and the PTO on the same frequency and 
switch quickly between them for a comparison - so far I've not found any 
difference in Rx capability on any of the signals I've tried.

* It would have been very easy to retain the DDS "Split" functionality - 
the control signals are all available in the remote VFO - but I thought 
I might get confused having two "layers" of split, and I would have 
needed to add an LED to indicate the DDS split mode had been activated.

Hope that may be of some interest to others.

73,
Steve G3TXQ