I now have a N3ZI DDS unit (still waiting for the LCD coming from China) which has alotta features including RIT and Dual VFO options. One thing I need is a remote VFO and readout for rigs like the Heathkit SB-10X series and Swan 350/500CX/700CX.
First I am tackling the Swan. I checked out how they did the remote VFOs for them and what they do is switch the VFO Frequency for each ham band. The 350 and 500/700 are different, so that is a problem. For example, on a Swan 350, at 3.800 Mhz, the VFO freq is 8.973Mhz. On a 500/700 it is 9.300 Mhz. And so on for the rest of the bands. For some reason, they don't list the VFO freq for 20 Meters on the 350, so it must use the same as one of the other bands (I don't know which one!).
But here is the list for the 500/700
Dial Freq VFO Freq
For some reason, the Swans use a VFO power voltage of -10V, which is made by using a Zener dropping it down from the -110VDC Bias voltage. Also, they drop the +215VDC down to key the relay for VFO Switching. THis is per the Swan 508 Remote VFO.
I don't like this switching scheme one bit, as if there is EVER some kind of failure, imagine what 215V would do to a 5 volt device! UP IN SMOKE!!!!!!!!! In the Swan VFO, all they have to lose is some 2N706 transistors!
Meanwhile, the Heathkit Remote VFO system uses a 5 to 5.500 MHZ signal that stays the same for all bands. They do switch in an Offset for LSB operation.
I think Kenwood and Yaseu do the same thing, only they have 2 SSB filters so they don't need a mode offset.
Consequently, You have to switch heterodyne crystals for each band. That signal can be found on the output panel of the Heathkits; labeled "HFO". The VFO is labeled "LMO" and the 3rd signal is "BFO" which is around the IF freq of 3395. These 3 signals are mixed to produce the desired operation frequency, and also mixed in the SB-650 Digital readout to provide a count.
So this is the very initial groundbreaking of a design for a Universal remote VFO. By programming in the kind of radio to be used, the VFO will allow full transceive or split freq operation on the DDS VFO. Wonder if anyone has done this before, or if it is even possible?