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Quadrature Square Waves

Discussion in 'Electronic Design' started by Jim Weir, Nov 15, 2003.

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  1. Jim Weir

    Jim Weir Guest

    I know I've got this technique around here somewhere, but I've just spent two
    hours going through the files and another two hours on the 'net and have come up

    Seems to me I remember a pretty nifty way (with xor gates, as I recall) of
    making two square waves in quadrature with another. Seems that you took a
    binary multiple of the frequency you wanted and gated/flipflopped your way down
    to the desired fundamental.

    That is, if you want two 1 kHz. square waves in quadrature, you start off with
    either a 2 kHz. or 4 kHz. square wave and mogrify those.

    Anybody got a clue where I can start looking for this circuit?

  2. John Larkin

    John Larkin Guest

    Given clock at 2f, and two d-flops. Set up the first flop to divide by
    2; second flop follows the output of first one, but clcok it on the
    opposite clock edge.


  3. The normal way, is to feed the signal into one input of one XOR gate, and
    both inputs of the other. The output of this gate is then fed to the other
    input of the second gate. The output of this gate becomes a 'signal changed'
    pulse. This can then be fed into flip-flops to give the required square
    I don't think you can do it with just two XOR gates, you need the flip-flop
    as well.

    Best Wishes
  4. Jim Weir

    Jim Weir Guest

    I'm a slow student. I don't understand "follow". And do you mean clock the
    second flop with a phase inverted clock?

    Thanks for responding.


    John Larkin <>
    shared these priceless pearls of wisdom:

    ->Given clock at 2f, and two d-flops. Set up the first flop to divide by
    ->2; second flop follows the output of first one, but clcok it on the
    ->opposite clock edge.
  5. Don Taylor

    Don Taylor Guest

    I REALLY hope this isn't a homework problem.

    Re-invent the wheel:

    ___| |
    _| |__

    4 states -> 2 bit counter.

    arbitrarily assign count<->states, hope for the best.

    counter states
    0 0 0 0
    0 1 0 1
    1 0 1 1
    1 1 1 0

    K map it. <details omitted> Reduce it <details omitted>

    Result, one of the bits of your 2 bit counter is one of your
    quadrature signals, the xor of the two counter bits is the
    other quadrature signal.

    It has been WAY too many years since I've done this,
    please check the results carefully. But actually, once the
    answer has been presented I think you can look at the
    list of counter<->states and see that works.
    A skill sort of like working crossword puzzles.
  6. John Larkin

    John Larkin Guest

    clk---------ck \q-----+ \clk------ck \q
    +--d q --- | --------+--------d q----------- out2
    | | |
    +--------------+ +------------------------- out1

    So out1 is a square wave at frequency clk/2. Out2 follows out1 but is
    delayed 1/2 of the clk time = 1/4 of out1 period = 90 degrees.

    Make \clk by inverting clk, or by using a third ff to divide a 4x
    clock into two phases.

  7. John Fields

    John Fields Guest


    | +-----+ | +-----+ |
    +--|D Q|--+--|D Q|-------->Q
    | | | | |
    4f>---+--|> |--+--|> | |
    | | | | | Q-|O--+
    | +-----+ | +-----+

    _ _ _ _ _ _ _
    4f __| |_| |_| |_| |_| |_| |_| |_|

    _______ _______
    I __| |_______| |____

    _______ ________
    Q ______| |_______|
  8. Jim Weir

    Jim Weir Guest

    Jim Weir <>
    shared these priceless pearls of wisdom:

    I know you all mean well, but I truly hope that none of you are teachers.
    Thanks for all your well-intentioned help, but I'll figure it out for myself or
    find the ancient notes.

    ASCII undecipherable diagrams take an immense amount of your time to generate
    and I'm sorry for wasting your time.

  9. James Meyer

    James Meyer Guest

    Sorry you feel that way Jim. The ASCII diagrams are quicker to do than
    to write a word description that contains as much information. I truly feel bad
    for anyone trying to teach you anything.

    None of the diagrams were undecipherable here. You aren't blind, are
    you? If you had a text-to-speach utility for "reading" the messages, I could
    see the "undecipherable" part. I saw a message once that told a blind guy to
    "RTFM". 8-(

    For every original requester who goes away unfulfilled, there are at
    least 10 lurkers who get a great deal out of reading the answers. So your
    question and the answers are valuable even if you don't think so.

  10. They aren't indecipherable if you use Courier font. Consider that no-one
    would use them if they were as useless as you claim.
  11. Hello Jim,
    there is a small type error in John Fields circuit. I have
    corrected it for you. This circuit has been successfully tested
    with a simulator(LTSPICE) by myself. If you can't clearly see
    the schematic, then your news reader isn't set to a fixed font.
    There should be a setting like "text only" or "x plain text".
    I can send you the schematic as picture(.jpg), if you are interested.
    The advantage of this circuit is that the outputs I and Q are
    exactly 90 degree out of phase regardless of the duty cycle of the
    incoming clock. The incoming clock clock frequency has to be four
    times the output frequency with this circuit.
    The two D-Flipflops are available in one 74HC74, 74HCT74 package
    or something equivalent from other logic families like AC, ACT, ALV ... .
    Better thank John for taking the time to draw a clear schematic.
    We like to say: "One picture shows(tells) more than thousand words".

    There are two ASCII schematic editors around too. They are helpful
    especially for larger circuits. AACircuit V1.24 pyASCII

    Best Regards

    John Fields corrected circuit.

    | +-----+ | +-----+ |
    +--|D Q|--+--|D Q|-------->Q
    | | | | |
    4f>---+--|> | +--|> | |
    | | | | | Q-|O--+
    | +-----+ | +-----+

    _ _ _ _ _ _ _
    4f __| |_| |_| |_| |_| |_| |_| |_|

    _______ _______
    I __| |_______| |____

    _______ ________
    Q ______| |_______|
  12. JeffM

    JeffM Guest

    ASCII undecipherable diagrams
    Should you decide against best advice
    and chose not set up your newsreader to use Courier font,
    another way to view ASCII art is to cut the text
    and paste it into a monospaced text editor (Notepad).
    (I'm assuming you use Windoze.
    Forte Agent is available for Windows and Linux.)
  13. Jim,

    I'm not a teacher anymore but when I still was one I'd say: "Jimmy, JImmy.
    Do a little thinking before you complain. Use that little grey cells hidden
    behind your eyes. You've got them to use them. Those man that wrote this
    replies are skilled techs and engineers. Do you really think they use
    undecipherable ASCII's? The answers they give you are the ancient solutions
    of an similar old problem. The only thing you have to do is using a fixed
    font to read them."

  14. generate

    Not if you use the right tool:
  15. John Fields

    John Fields Guest

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