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Sine to square wave conversion

Discussion in 'Electronic Design' started by Sandeep, Nov 4, 2005.

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  1. Sandeep

    Sandeep Guest

    Hi all,
    I want to know how to convert a 1KHZ square wave into a stable sine
    wave .Please can anybody help me out
    Many Thanks
  2. Sandeep

    Sandeep Guest

    Sorry the subject is Square to sine wave conversion.
    Many thanks
  3. Leon

    Leon Guest

    A filter?

  4. Jon

    Jon Guest

    A few methods:
    Use a phase locked loop chip that has sine wave output capability.
    Lock the PLL to the input square wave.
    Run the square wave through a sharp low pass filter to remove the
    harmonics. In determining the filter requirements you must decide how
    much distortion you can tolerate. A square wave has only odd
    harmonics, so the 1st harmonic that is present is the 3rd. The
    harmonic amplitudes are in the ratio 4/Pi (Fundamental) 4/(3Pi) (3rd
    harmonic) , 4/(5Pi), etc.
    If the output sine wave must be in phase with the fundamental component
    of the square wave, then use a bandpass filter with a center frequency
    of 1 KHz. The phase lags and leads of this filter will cancel at the
    center frequency. For a given distortion, the required order of the
    bandpass filter will be twice the required order of the lowpass filter
  5. Brian

    Brian Guest

    Here are some schematics of a Low-pass and a Band-pass filter, for 1 KHz.

  6. John Larkin

    John Larkin Guest


    A simple high-Q L-C, or equivalent 2nd order bandpass, can easily
    filter better than a 10-pole lowpass.

    | |
    | |
    L C
    | |
    | |
    | |
    gnd gnd

  7. Guest

    All depends on what you mean by "better".

    Q? then you might be right.

    But if you mean steepness in the transition band, no, you are very
    wrong. You get 20 dB/decade (power) slope per pole or zero. If you
    want it steeper, you need more poles and zeros. Since a bandpass has
    two transition bands, the overall order needs to be twice that of a
    highpass or lowpass with only one transition band in order to have the
    same slopes.
  8. Jim Thompson

    Jim Thompson Guest

    Eh? A little knowledge is a dangerous thing. The 20dB/decade is
    related to bandwidth, so a very narrow (high Q) BP filter will fall
    much faster than an LP.

    ...Jim Thompson
  9. Guest

    We're talking about attenuating overtones at 3x, 5x, 7x, etc the center
    frequency, not the sharpness of the response around the center
    frequency itself.
  10. John  Larkin

    John Larkin Guest

    If the square is perfect and has no 2nd harmonic, my "10-pole"
    statement may be a bit of an exaggeration... 3^10 is a pretty big
    number. But a reasonably high-Q single-L-C bandpass will massively
    out-filter a 2nd order LPF. As you note, the dropoff of a bpf is
    relative to its bandwidth, not its cf. That makes sense, as a bandpass
    is classically synthesized by *shifting* a lowpass filter.

    A 10 Hz wide LC bandpass, centered at 1KHz, Q=100, sure has a steeper
    slope than 20 dB/decade! Actually, it approaches 20 dB/decade far out
    from the peak, but the fun's over by then.

    Of course, you could make a "bandpass" by cascading a lowpass with a
    highpass, in which particular case cs is right, but that would be a
    silly thing to do here.

  11. John  Larkin

    John Larkin Guest

    Exactly. And the narrower the bandpass, the better we attenuate those
    harmonics. The attenuation slope past 3f won't be high, but the ratio
    of cf amplitude to 3f+ amplitude increases with Q, without limit.

    Just graph the amplitude response of a simple R-L-C bandpass of Q=100.

  12. Jon

    Jon Guest

    You are right in that the initial transition slope of a bpf will be
    steeper than for the same type (Butterworth, Chebyshev, etc) lpf.
    However, the ultimate slope of a bpf wil be equal to n*20/2 db/decade
    for a bpf, and n*20 db/decade for a low pass.
  13. Guest

    Okay, you've convinced me of a possibility I hadn't looked into
    carefully enough.

    If the frequency is accurate and stable enough and the components
    accurate and temperature stable enough and then this is probably the
    way to go.
    If 1khz was meant more approximately, or environemental conditions or
    design for manufacturing makes precise tuning undesirable, might it be
  14. John  Larkin

    John Larkin Guest

    Right, a 2-pole high-Q bandpass can get tricky. But you can design a
    higher-order bp filter, 4 pole maybe, that's reasonably flat on top,
    to allow for modest source frequency and parts tolerances, and still
    get the attenuation advantages of a true bandpass.

    Of course, it doesn't make sense to build a bandpass by cascading hp
    and lp sections... we don't need a highpass section here, just a
    lowpass that will pass the fundamental and whack the 3rd and up.

    It gets interesting to make a sine from a square wave over a wide
    frequency range. A pll or a tracking filter come to mind.
    Switched-capacitor filters, either bp or lp, are compact and easily
    tuned, but need some modest anti-aliasing passive filters before and
    after to avoid complications.

    Or a pll multiplier feeding a dds synthesizer!

  15. I read in that wrote (in
    You don't NEED a band-pass filter, because you are trying to eliminate
    only frequencies higher than the one you want. So a high-Q low-pass
    filter, peaked at 1 kHz, would be good. That gives you 'gain' at the
    frequency you want, while it attenuates the harmonics in proportion to
    their order.
  16. Jim Thompson

    Jim Thompson Guest

    The way I tuned my sonar BP (gm-C) filters was to insert a square wave
    at the desired center frequency and then vary gm with a DAC until the
    phase flipped from +90° to -90° (center frequency).

    I wonder if, in similar fashion, one might implement a tracking filter
    that automatically follows the input square wave frequency ??

    ...Jim Thompson
  17. Joerg

    Joerg Guest

    Hello Jim,
    Why would you want to vary the excitation frequency on a sonar (the
    civilian versions, of course)? If it was a long range job where return
    signals become mushy in the distance you'd need a tracking filter that
    keeps the low pass limit constant but reduces the high cut-off with depth.

    Regards, Joerg
  18. John  Larkin

    John Larkin Guest

    I did a filter once, connected to a multiplier as a quadrature phase
    detector, so it would have zero volts out at 90 degrees phase shift.
    That was used to compare the filter's input to output and tune a
    varicap for zero multiplier output. The filter inherently produced 90
    degrees shift at its center frequency, so ta-daah!

  19. John  Larkin

    John Larkin Guest

    There is some grounds for debating whether such a filter, a lowpass
    with a big gain peak just below Fc, is indeed a "bandpass" filter.



    can be a Butterworth (or worse), or can have a huge gain peak just
    before it falls off. It does have a 90 degree phase lead at the peak,
    which makes it handy for servo tracking the input frequency. A true
    2-pole RLC bandpass has zero phase shift at Fc.

  20. Joerg

    Joerg Guest

    Hello John,
    That would be like debating whether a seal is more of a land animal or a
    sea animal ;-)

    Regards, Joerg
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