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sine generatore redux

Discussion in 'Electronic Design' started by Spehro Pefhany, Dec 23, 2012.

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  1. (wrong ng first.. 8-( )

    Tried a 32MHz PIC24 as a DDS using a 16usec PWM cycle (8 bit), running
    from 3.3V. Three 32-bit phase accumulators. I used the Bell 202

    With a simple RC filter (5K6/10n -> 10K/10n).. no calculation, just
    guesstimated, I get pretty decent behavior (2nd harmonic 35 dB down):-

    I have four simultaneous outputs PWM'd at 62.5kHz (right now, running
    from the internal calibrated oscillator, no crystal)

    1. 1200Hz sine wave
    2. 2200Hz sine wave
    3. 1200/2200Hz continuous phase switched by input
    4. 1200/2200Hz coherent phase "

    (Re) discovered a nasty errata on the PWMs .. even worse than
    documented, I think. Avoid 0x0000 as a duty cycle, so I regenerated
    the sine table to avoid that.

    Only problem I see is that (because of the filter) the two frequencies
    have quite different amplitudes.. 1.2V and 1.8V p-p roughly. Could be
    solved by a better filter, or do the switching externally, or use sine
    tables with different amplitudes.

    It would be easy enough to use a crystal and/or to trim the
    frequencies with ADC inputs, but the internal oscillator gets pretty
    close. Changing the frequencies in the source code is trivial:

    Phaseinc1 = 1200.0 * 16E-6 * 256*256*256*256 + 0.5;
    Phaseinc2 = 2200.0 * 16E-6 * 256*256*256*256 + 0.5;

    Not including the filter, the entire circuit is just the chip itself
    and bypass caps (2 x 0.1uF and 1 10uF are the official
    recommendations- the latter for the internal 2.5V core regulator

    Best regards,
    Spehro Pefhany
  2. Rocky

    Rocky Guest

    The 2nd harmonic distortion is mostly cause by the phase modulation of the signal by the PWM. Reducing the modulation will reduce this.
  3. Rocky

    Rocky Guest

    On Sunday, December 23, 2012 8:37:18 PM UTC+2, Rocky wrote:
    Reducing the modulation will reduce this.
    Should be reducing modulation INDEX will reduce this
  4. Reduce the depth of PWM modulation?

    I could reduce it to 6 bits, or I could keep the modulation at 100%
    over 6 bits and crank up the PWM to 250kHz from 62.5kHz.

    Best regards,
    Spehro Pefhany
  5. Rocky

    Rocky Guest

    Just for interest I tried it with an PIC16F628 using a 4 bit resistor DAC (5K, 10K, 20K & 40K) with a sampling rate of 20kHz and got the highest spurious signal at about 34dB down using a 1st order filter with cutoff at 3kHz.The highest spurious signal was at 17.8kHz
    Excluding the sin table a total of about 20 lines code.

    Tempting to do the demodulator.

    @John Larkin - What baud rate?
  6. Guest

    as soon as you start making more than a few hand fulls it makes sense
    to make a test fixture with pogo pins, even if it is analog there's
    always a few voltages etc. that needs checking so programming as just
    one more step it the test

    or you could go for something like:

  7. That's kind of a neat solution for smaller quantities. No fixture.
    I wonder what connector John is using.. the usual JTAG 2x10 shrouded
    header is cheap, but pretty big.

    Of course the PICs need fewer pins for programming and debug.. I
    usually use a 2x3 header. That gives you Vdd, ground, /reset, the two
    (data and clock) programming/debug pins and a spare pin.

    The particular PIC24 I used (PIC24FJ16GA002) $1.86/100 also has a JTAG

    Since it has 5 PWM outputs I was thinking maybe you could combine PWM
    outputs with a R/2R ladder to do even better, but that's just crazy

    Best regards,
    Spehro Pefhany
  8. Rocky

    Rocky Guest

    I think programming has its challenges too. In this sort of application the benefit is repeatability without trim-pots.
  9. The 6p6c RJ plug is a Microchip thing.. they put that on their
    in-circuit programmers and debuggers (non-JTAG).

    Not a very practical choice at the user end-- the RJ female is
    undesirably large and tall for most products, and it doesn't even fit
    a proto board. Most of us use a 1 x 6 or 2 x 3 header and an adapter
    cable for in-circuit programming and debugging. At least the
    programmer end is easy to unplug and replace when the cable dies.

    Best regards,
    Spehro Pefhany
  10. I think if you can do it digitally for less then it's better to go
    that way.. repeatable, fewer worries about temperature, unit-to-unit
    variations and so on. No trimpots unless you really want them. I used
    to make PID controllers with "I" time constants in the scores of
    minutes with analog parts, don't miss that one bit, but the experience
    is still useful for when things really need to be that way. You can
    also readily compensate for the shortcomings of analog bits once you
    get things ino the digital domain- pre-distort, apply customized 2D
    transfer functions for temperature compensation, etc. etc.
    More fun to get something working, but still a fair bit of work to
    make it guaranteed to work under all conditions and with parts at all
    tolerance extremes.

    Best regards,
    Spehro Pefhany
  11. Nico Coesel

    Nico Coesel Guest

    If the customer wants something different its nice to have things
    under software control. A while ago I designed a high power function
    generator. Not high frequency but interesting enough due to the
    voltages and power output required. Somewhere halfway thru the project
    it turned out it needed to be current controlled instead of voltage
    controlled. Because the whole thing is controlled by a microcontroller
    and not dedicated PWM chips the change was made within 30 minutes.
    Same story when it turned out the production batch was more prone to
    core saturation than the prototype.
  12. SoothSayer

    SoothSayer Guest

    Samtec makes some pretty nice smaller scale items.

    We still use the 0.1" stuff for interconnects in in-house test gear,
    simple 2 layer PCBs, etc.
    But all of our product level circuit assemblies use the smaller scale
    stuff these days. We are even phasing out a lot of older, legacy gear.
    Funny to see thousands of dollars worth of working gear hitting the scrap
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