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Need DDS module, 2nd round

Discussion in 'Electronic Design' started by Joerg, Mar 2, 2013.

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  1. Joerg

    Joerg Guest

    Hi Folks,

    Long story short, none of the DDS modules I've found so far has the
    ability to be sweeped or FM-modulated. This one has a signal input
    terminal block "SIN" but I received a response from the manufacturer
    that it connects to nowhere, it has no function:

    http://imall.iteadstudio.com/tools-and-equipment/im120723003.html

    What I need in my case is DC-100Hz of sweep or FM bandwidth. Operating
    frequency under 10kHz and sweep range up to 2kHz. Of course I can whip
    up an analog solution but it won't be as precise and most of all not
    really programmable.

    Is there anything better? Cost is not a big issue but should be somewhat
    small and a display would be nice.
     
  2. Nico Coesel

    Nico Coesel Guest

    If by 'operating frequency under 10kHz' you mean the output frequency
    doesn't exceed 10kHz a fast microcontroller and a DAC (codec) should
    be more than sufficient. It doesn't sound like its rocket science to
    me...
     
  3. Joerg

    Joerg Guest

    But it is to me, I am an analog guy who knows barely enough C and
    assembler to be dangerous. Plus there ain't much time, I need some
    module that I can simply plop in and connect. The one in the link would
    be ideal if the modulator input weren't be dead.
     
  4. Nico Coesel

    Nico Coesel Guest

    I guess they thought they could re-program the DDS fast enough / on
    the fly to get a glitch-free output. How much time do you have?
     
  5. Bill Sloman

    Bill Sloman Guest

    DDS outputs are intrinsicly glitch-free - what you change is the
    number added to the phase accumulator on each step, and the stair-case
    approximation to the sine wave remains continuous, though the cycle -
    you are just changing the spacing of the treads..

    Joerg should be able to sweep his sine wave pretty smoothly by
    incrementing/decrementing the last digit of his frequency-setting
    number frequently. Whether he can do it frequently enough is another
    question. He might have to program a PAL to do it for him.
     
  6. Guest

    it should be possible, I just looked at the code it is just a single
    file (arduino Sketch)
    and a library for the lcd. The SPI to the DDS is bit banged a 16MHz
    should
    be able to do that quite fast

    The schematic shows JP4 connected to ADC0 ac coupled with 4.7u cap and
    2x10k

    so everything needed is there I think, all that is need is a timer
    interrupt to
    read the adc and update the DDS

    -Lasse




    -Lasse
     
  7. Nico Coesel

    Nico Coesel Guest

    That depends on whether the DDS is reset or not when the frequency
    setting is updated. There has to be a reason the designers at
    iteadstudio didn't implement the modulation. If it where simple they
    would have implemented it.
     
  8. Joerg

    Joerg Guest

    As I said, about 100Hz is the max sweep cycle frequency. So a few kHz
    update rate should do. Normally that isn't a problem with a DDS chip.
     
  9. Joerg

    Joerg Guest

    If necessary I could do it that way. But I am not really a good
    programmer. Ideall would be a module that has FM modulation or sweep. It
    doesn't have to be DDS, just stable and not too large. Ripping it out of
    a function generator is ok as well, as long as it's not much more than
    two cigarette packs or so in size.
     
  10. Joerg

    Joerg Guest

    Haven't found anything useful for this application.

    Yup. Something that can be plopped into my RF board and control two
    frequencies (so two of those modules are needed). SPI or RS232, power
    supply, output, modulation input.
     
  11. Joerg

    Joerg Guest

    Yep, that sure would be overkill :)

    That's what I want to avoid, I've got enough on my plate as it is. If I
    really can't find a module I'll buy function generators and hack them
    apart on arrival, prying their main boards out. That would make my
    circuit board the size of a large family pizza, but man's gotta do what
    man's gotta do.
     
  12. Guest

    if you don't need external input it is even easier, if I had the HW
    I'd give it a try

    do you really need the display and the controls or could you do with
    few preprogrammed
    waveforms/sweeps/frequencies on an easy to get dev-board with a few
    buttons added?


    -Lasse



    -Lasse
     
  13. Guest

    use the a sound card, possible external usb?

    -Lasse
     
  14. Joerg

    Joerg Guest

    I need the input because it'll be used in a loop. It will actually
    become a glorified dipmeter. The phase response at the input of an LC
    lowpass filter will be measured and then this becomes the error signal
    in a PID loop. The PID loops adjusts the frequency of the DDS (or
    whatever other sine wave generator) so the phase remains constant even
    when the lowpass components change their values.

    A display would be nice but I could also roach on a frequency counter.
    The output has to be sine wave, can't easily filter that because the
    thing has to work in frequency ranges I won't know up front. Being able
    to set the base frequency locally would be very helpful.
     
  15. Joerg

    Joerg Guest

    Theoretically, yes. But since this whole thing will sit inside a PID
    loop I'd then have to process the error signal (a phase information
    measured in my box) in the PC. Meaning lots of code writing and that's
    what I am trying to avoid as much as possible.
     
  16. Guest

    at 10kHz almost any mcu with a dac would do

    -Lasse
     
  17. Guest

    if you want it to be standalone what would spi/rs232 be for?


    -Lasse
     
  18. Jamie

    Jamie Guest

    The AVr's handle jobs like that very easy and they are easy to code.

    You can get one witha ADC input and code a SPI output.

    Jamie
     
  19. Joerg

    Joerg Guest

    To set the base frequency. Otherwise it won't know where to center the
    FM signal upon start-up.
     
  20. Joerg

    Joerg Guest

    In case someone else is in a similar situation, I've just discovered two
    interesting alternatives if no suitable DDS module can be found. The 1st
    is ye olde XR2206 which I thought had gone unobtanium:

    http://electronics-diy.com/product_details.php?pid=537&name=1Hz - 2MHz XR2206 Function Generator Kit

    The 2nd solution is rather ugly. I could take these DDS modules and
    replace the oscillator with a LTC resistor-set silicon oscillator. Those
    can be current steered. Of course then the digital frequency display of
    the module would become fairly meaningless but could still sort of
    signal if my PID loop has locked or not.
     
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