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Freq. Independent Phase Shifter

Discussion in 'Electronic Design' started by Bill Murphy, May 9, 2010.

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  1. Bill Murphy

    Bill Murphy Guest

    I am using a commercial stereo amp to output continuous wave test
    signals in the low audio range, up to about 2KHz. However, I need a
    third channel with a 120 degree phase shift. Is there a circuit that
    will do this evenly across this entire frequency range?

    Is it possible to do same using an off-the-shelf transformer and
    current subtraction?

    Any advice would be appreciated.

    Bill Murphy
  2. Winston

    Winston Guest

    I would approach this using DDS (Direct Digital Synthesis).

    A binary counter generates an address for two different memory
    devices. The devices are programmed with a binary sine wave.
    The second device is exactly like the first except the data are
    displaced an equivalent of 120 degrees in the address space.
    Each data stream drives a separate DAC. The output of the second
    DAC is filtered and amplified to drive your 3rd amplifier which
    has exactly the same phase characteristics as your first two

    Vary the clock speed into the binary counter to vary
    the output frequency.


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  3. Winston

    Winston Guest

    That'll be what I learned today.

    Sounds like this approach could be coded into a $2.00 PIC
    using PWM outputs fairly painlessly.

    Thanks, John!

  4. Jamie

    Jamie Guest

    Fancy name "DDS" for techniques that have has been around for years. :)

    How about selecting a Uc (programmable processor) that has 3 DAC's on
    it. and some input so that you can select the freq you need and generate
    the output for your 3 phase signal you're creating..

    Or, 3 DDS function generators, with their external triggers dasy
    chained, or 1 Dual unit like mine, and one extra with the external
    trigger put in play to keep all three at 120 apart.

    hell, I did a replacement 3 phase generator for an obsolete chip using
    Buffered PWM output..

    Yawn, to many possibilities.
  5. Phil Allison

    Phil Allison Guest

    "Bill Murphy"
    ** How even is "evenly" ??

    How accurate does the 120 degree shift need to be ??

    Are the test signals all sine waves or not ??

    Is the shifted output level flatness critical or not ??

    ** Yep - try posting a question that explains what you REALLY want to do.

    ..... Phil
  6. Sjouke Burry

    Sjouke Burry Guest

    Use a microprocessor with 3X DAO

  7. Since the OP only needed frequencies up to 2 kHz and was using COTS
    amplifiers, a cheap dedicated computer running a multiple (4-6)
    channel sound card running at 8-48 kHz sampling frequency would do the

    Two separate stereo sound cards can not be used, since the cards
    generate their own sampling frequency from their own crystal, which
    are not synchronized.
  8. If you only need the 90 degree phase shift without fast frequency
    swings, the PLL can be used, since the phase detector produces zero
    output (after averaging), when the VCO is 90 degree out of phase
    relative to the reference.
  9. Joerg

    Joerg Guest

    Paul's suggestion with multi-channel sound cards is a good one. But keep
    in mind that phase shifts in the very low range (tens of Hertzes) can be
    iffy on some cards, output cap tolerances and all that. Unless you want
    to go in there with a solder iron.

    A Hilbert shifter works well, depends on the precision and how many
    octaves you want. Also, you'd need to get hold of 0.5% or better film
    capacitors which is not easy anymore these days. That was different in
    the 80's.
  10. Joerg

    Joerg Guest

    It all depends. When you do this for a living you need to weigh the time
    it takes to whip this up against the cost and installation time of a
    multi-channel sound card. When you are retired, different thing. But
    many of us aren't there yet.

  11. Some manufacturers make 4-6 channel codecs suitable for sound cards
    and home theater systems. These are typically specifies for
    frequencies above 3 Hz.

    How low does a typical COTS stereo amplifier go ?

    While the amplitude response may be bad, why would the phase shift
    characteristics suffer from this ?
    The cap tolerances would indeed cause problems.
    For low production run devices, it would be perfectly feasible to use
    production measurements and software to compensate for the capacitor

    In discussing, which is the "best" way of implementing something, you
    always must consider the expected size of the production run. It is
    quite different to design a one off product or design a product for
    100, 10,000 or 1,000,000 unit production / year, each requiring a
    different approach.
  12. Joerg

    Joerg Guest

    Hint: One can load revolving wave tables into a sound card :)
  13. Joerg

    Joerg Guest

    The one I use for testing small power supplies for international power
    standards or 400Hz aircraft supplies goes to under 10Hz. My sound card
    on the lab bench goes to 3Hz where it's 6dB down.

    Yup, that's what messes up the phase down there.

    It's temperature and often drive voltage dependent, for example if they
    used a Z5U cheapie in there. You can't tell even when looking at it.

    Sure, that's why cost thinking has to go all the way, not just parts and
  14. Bill Murphy

    Bill Murphy Guest

    What about a single card with 4.1 or 5.1 outputs?

    Either way, how would I generate the three 120 degree offset channels
    in software?

    Thank you for your reply.

    Bill Murphy
  15. Perfectly OK, if the channels are truly discrete (no psychoacoustic
    matrix surround generation) and transparent (no band limiting in the
    ..1 channel etc.).
    You can precalculate a few (thousand) future samples into memory
    buffers (queue) and then let the sound card output the buffer at the
    speed specified by the sample clock. The buffers need to be updated,
    before the sound card has consumed all previous samples.

    The actual sample generation is done in the same way as in DDS with a
    numerically controlled oscillator (NCO).

    You need a 32 bit integer variable "phase accumulator" which is
    updated with a specific value at each iteration of a program loop,
    which defines the frequency. Take the high bits from the phase
    accumulator and use it to index a sine table. The value from the sine
    table is inserted into the queue going into the sound card (or written
    e.g. to a .WAV file).

    To generate signals with a fixed phase relative to the master signal,
    take the current phase accumulator value, add a constant (the phase
    shift) and using the upper bits, access sine look up table and insert
    result into the queue for a different audio channel.

    If the sample values are written into a .WAV file, the data can be
    replayed using any audio player.

    The sine instruction is surprisingly fast on some x86 processors, so
    it could replace the sine look-up table. However, the phase
    accumulator must be an integer register, which overflows in a
    predictable way. A floating point register can not be used as a phase
    accumulator, since after long time, the least significant bits are
    lost and the sine function returns a constant value.
  16. Bill Murphy

    Bill Murphy Guest

    Same signal, all spearated by 120 degrees. Done in CoolEdit.
    Can you give me a hint how, or where to find out please?

    Bill Murphy
  17. Phil Allison

    Phil Allison Guest


    ** The use of 0.5% tolerance caps implies a phase ripple or error of better
    than 1 degree max.

    Using 1% tolerance or 1% values selected from 5% stock, the max phase error
    is not more than 2 degrees.

    There is no problem designing a Hilbert phase shift network that covers from
    22 Hz to 20 kHz using only 10nF and 1nF polystyrene caps of nominal 1%
    tolerance, 1% MF resistors and a few FET op-amps.

    ..... Phil
  18. Phil Allison

    Phil Allison Guest

    "Bill Murphy"

    ** Just by summing them in various ratios !!

    Call one " 0 " and the other " 90" - then summing equally gives 45
    degrees relative to " 0" .

    Invert the " 0 " signal - then summing in various ratios with " 90 "
    gives all the angles between 90 and 180 degrees.

    Draw a phasor diagram and you will easily see it.

    ..... Phil
  19. JosephKK

    JosephKK Guest

    You should have said you wanted a tunable 3-phase supply (exciter).
  20. 0.1% resistors are cheap these days. Where can you buy PS caps of any
    tolerance ? NPO ceramic parts are available, but $$$$.

    Best regards,
    Spehro Pefhany
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