Homebrew DDS VFO for Boat Anchor rigs

Discussion in 'Homebrew and Kit Projects' started by WA6MHZ, Mar 22, 2012.

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  1. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    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
    3.800 9.300
    7.000 12.500
    14.000 8.500
    21.000 15.500
    28.500 23.000

    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?
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    What frequency do you plan to Operate on ?

    Maybe I missed it.
  3. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    This is for 80-10M SSB transcievers like the Swan 500CX and Heathkit SB-101. The TRICK is to make it universal so it will work with multiple transceivers. It would be easy to make it for a dedicated rig.
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    I thought the N3ZI DDS unit had programmable IF.

    Sorry, I guess I don't understand your problem.
  5. AB9LZ

    AB9LZ Ham Member QRZ Page

    Nothing groundbreaking about it, DDS's are very frequency agile, it only takes a few lines of code to provide instant IF shift for t/r purposes, splits, and even rit (splits and rit are the same thing, i just use a seperate encoder for my rit).

    One could go through the hassle of loading all of the IF shift maps for every boat anchor into the eeprom of the controller and provide a menu selection, but the better path is to just allow the user to load his own shift via a serial connection of some sort.

    A neat trick would be to mount a gray code optical encoder on the original BA vfo shaft, and somehow "float" the display in the vfo window with a partially silvered mirror to complete the package.

    73 m/4
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    I agree.

    If it is connected to a computer, just write a program and let it do all the work. (May be done already)
  7. K9STH

    K9STH Ham Member QRZ Page

    Most of the Swan units use the 80-meter VFO for 20-meters.

    The fact that most Swan units use different VFO frequencies for each band (the VFO frequency gets higher as the band goes higher) and don't use a heterodyne oscillator and a single frequency range VFO, is one of the major reasons that they drift so bad. The higher frequency the VFO the harder it is to get it stable.

    Glen, K9STH
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    Yes, and it is a drill to follow a Swan around the frequency.

    Got to love them Swan's , they fly all over the place.

    But they do make some transmit power, controlling it is a different story. (Heat and Frequency, Hard to control both at the same time)
  9. K5UOS

    K5UOS Ham Member QRZ Page

    I think it has been done on an individual basis. I have seen TS-520’s with digital VFO’s and even homebrew tube transceivers with DDS.

    But a single Universal DDS VFO for use with boat anchors is a decent perhaps original idea. Though, like AB9LZ says, the concept is not new.

    I just don’t see serious BA owners modifying valuable rigs this way.

    The joy of vintage radio (or current homebrew) is in part the clunky analog LC VFO’s. At least it is for me.

  10. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    right now, the idea is just to plug it into the back of the Swan 500CX, through the 9 pin octal type plug making for no modification to the transciever. Think I will use the 12VAC supplied out that jack and go the scheme of the Swan 508 remote VFO. The holdup right now is coming up with one of those 9 pin plugs which are really an odd duck. Since the N3ZI DDS has 2 VFOS in it, I could, in theory, completely eliminate the use of the stock SWAN drifting VFO and use the A/B DDS for split freq. This also will give me a Digital readout of the Swan frequency by entering the Swan IF into the N3ZI unit. I have a 700CX in the back room, think I will have a look at it. Nothing happening though til I get the LCD coming from China.
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