Looking for inhibit/enable dds vfo output code for simple cw transmitter

Discussion in 'Homebrew and Kit Projects' started by KH2BR, Feb 8, 2019.

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  1. KD2ACO

    KD2ACO Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    It's possible that the output can be controlled by interrupting the RSET line. This should shut down the DAC without stopping the chip from doing it's thing. No more chirp, no bunches of code flying into the port all the time at every key transition with all of that associated noise.

    The key would want a small cap with schmitt trigger to debounce it, a FET switch like this one to pull RSET low after the resistor, an appropriately sized inductor between the FET and the resistor to shape the on/off transitions for clickless keying of perhaps 8 milliseconds or so.

    Analog switch
    http://www.vishay.com/docs/76432/dg417le.pdf

    If a logic signal was driving the switch, insert that where the debouncer would go.

    ad9850 scheme.jpg

    I like the really nice looking projects on your Zed page! :)
    edit:

    It looks like you could control that line with an op amp with a cap in the feedback loop to control the transition speed, or feed audio to the op amp for AM. All sorts of possibilities here...
     
    Last edited: Feb 10, 2019
  2. WN1MB

    WN1MB Ham Member QRZ Page

    Wasn't trolling, friend. Only made a single, brief, observational sentence pointing out a paradox.
     
    WR2E likes this.
  3. KH2BR

    KH2BR Ham Member QRZ Page

    Ive had those paradoxes before. A time loop paradox where I keep burning out the same part. or frying a 100 pin QNF 3 times. !!
     
  4. AB9LZ

    AB9LZ Ham Member QRZ Page

    Set w32 and w33 in the 40 bit control word to anything but 0, 0 respectively.

    73 m.
     
  5. AB9LZ

    AB9LZ Ham Member QRZ Page

    "interrupting the RSET line. This should shut down the DAC without stopping the chip from doing it's thing. No more chirp, no bunches of code flying into the port all the time at every key transition with all of that associated noise."

    There is no "chirp" with the frequency transitions. You have to send a programming word to the DDS... no getting around that... take adavantage of that, it's a feature no a bug. No need for nonsense external circuits etc.

    73 m.
     
  6. W1GCI

    W1GCI Ham Member QRZ Page

    A while ago, I built a controller box for my DX-60. It started out being just a T/R switch, but then I added a keyer using a PIC and enabled semi-automatic break-in by creating a transmit line on the first key down and release back to receive after a short period of no keying. After that I added the AD9850 VFO and again used the transmit line from the keyer to shift frequency by using that line to enable the RIT on the VFO, where the RIT changed the frequency on receive to out of the band. However, while in transmit mode, the VFO was always on. There is no full auto break-in. It worked pretty good. Then I started looking at enabling VOX for AM and must have seen a shiny object - that's where the project sits.
     
    KH2BR likes this.
  7. AA7EJ

    AA7EJ Ham Member QRZ Page

    Per spec - most "switching / change " times are in nS (nanoseconds).
    It will take much longer to send new frequency data to the device.

    Wound not recommend using RESET , it triggers "power up / down " sequence much longer than few nS.

    Perhaps you could use Rset - which controls the level of DAC output.
    It is NOT same as RESET and in general NOT programmable using software published on web.
    But I may be wrong- I did not look.

    I am not sure how W34 - "power-down" data bit works.
    But it would have same timing "restrictions" while the new data word is passed to the device.
     
  8. AB9LZ

    AB9LZ Ham Member QRZ Page

    Actually it dosen't, and several current designs (KD1JV's tribander is a good example) use this feature to swap from T to R for full break in. I dont have the swap times handy, but in practice with my own little rig, its fast enough to be seamless.
     
  9. AA7EJ

    AA7EJ Ham Member QRZ Page

    " Actually it dosen't, " What ?
     
  10. AB9LZ

    AB9LZ Ham Member QRZ Page

    Sorry, misunderstood that, yes the change times are very quick, and yes it takes longer to write the control word. That said, writing the control word is more than fast enough to to shift the DDS output from an IF RX frequency to the prime TX frequency for full break in.
    DDS's are nice that way, plus with a few lines of code, you get RIT, splits etc.

    73 m.
     

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