[TenTec] DDS Dilemma

[Karl Fischer, DJ5IL]

Steve and all readers,

I was tired of rebuilding the PTO of my Corsair II every two years
or so. A few weeks ago I replaced it with a DDS-VFO and I am very
happy now. My solution is based on the DDS-60 daughtercard
( http://midnightdesignsolutions.com/dds60/index.htm ). I built a small
controller-board based on the PIC 16F88 and wrote a control
program for the DDS-60. The tiny controller-board together with
the daughtercard are installed above the noise blanker board inside
the CII. I discarded all parts of the old PTO except for the aluminium
chassis which I used to support a simple homebrew cardanic bearing
for a 1024 ppr magnetic shaft encoder. The OFFSET pot remains, its
setting is acquired by AD-conversion.

My software works like this: a short push on the SPOT button enters
the dual-VFO mode, sets VFO B = VFO A and switches to VFO B.
Every subsequent short push swaps VFOs. Tuning acts on the active
VFO only which is used for RX, during TX the inactive VFO is used
and displayed. The OFFSET is active in RX mode only and is
applied to both VFOs. A long (> 0.5 sec.) push on the SPOT button
exits the dual VFO mode and at the same time engages the original
SPOT function. So the SPOT function is not available in split mode
but in the normal mode it works like before. The tuning rate switches
between low and high depending on the shaft rotation speed.

As soon as I find time I will publish deatails and make the software
freely available on my website http://cq-cq.eu

73
Karl, DJ5IL 

-----Original Message-----
From: tentec-bounces at contesting.com [mailto:tentec-bounces at contesting.com]
On Behalf Of Steve Hunt
Sent: Tuesday, August 02, 2011 5:55 PM
To: Discussion of Ten-Tec Equipment
Subject: [TenTec] DDS Dilemma

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
_______________________________________________
TenTec mailing list
TenTec at contesting.com
http://lists.contesting.com/mailman/listinfo/tentec