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Square wave detector

Discussion in 'Electronic Design' started by pinku, Nov 3, 2007.

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  1. pinku

    pinku Guest

    I need to detect a presence of square wave. If square wave is there I
    need to generate a logic 0 and this square wave is not present I need
    to generate logic 1 or vise versa. Can you suggest me a circuit for
    this functionality.

    Thanks in advance

  2. First, you have to do a lot better than that at defining
    "square wave". You might be surprised at the variety of
    signals that have been called square waves.
  3. Greg Neill

    Greg Neill Guest

    Does your signal line have only a square wave on it,
    or are there other things present when the square
    wave isn't?

    Is the squarewave centered on zero (so it goes positive
    and negative) or does it have zero as its floor? What
    is the magnitude (volts) of the signal?

    What's the period (or frequency) of the square wave?
  4. And, for what period can the square wave be present (absent) before the
    output must correctly signal its status?

    If you've got enough time, you could use a technician, an oscilloscope
    and a toggle switch. Technician examines the 'scope periodically and, if
    the wave status changes, flips the switch to the correct position.

  5. John Larkin

    John Larkin Guest

    I bet a PhD in physical chemistry would work almost as well.

  6. Jerry Avins

    Jerry Avins Guest

    How square is it really? I.E. what's the bandwidth?
    Are there other signals at the same time?

  7. Jerry Avins

    Jerry Avins Guest

    Is the duty cycle 50%? If yes, with what tolerance?

  8. D from BC

    D from BC Guest

    I smell a differentiator somewhere in that circuit.

    D from BC
  9. the question i would have is: detect the presence of the square wave
    against what other alternative? silence or noise?
    we probably won't know that; no common timebase...
    that's an issue of setting a threshold.
    but we should know that. if we do, i would say to cross-correlate
    with, say, two different square waves of the known (or trial)
    frequency that have fundamental 90 degrees apart.

    if we *did* have a common timebase and could do synchronous detection,
    it would be just a single square wave to cross-correlate with.
    hmmm... maybe a matched filter implemented as an FIR with a stretch
    of square wave as its impulse response. that might work for either
    synchronous or async.

    r b-j
  10. msg

    msg Guest

    FWIW, many, many years ago I built an intrusion alarm with a superregen
    receiver that listened for a homemade keyfob's signal; the rcvr
    quieted in the presence of the carrier, noise power dropped and the
    superregeneration frequency spectrum narrowed; filtering as per your
    suggestion above worked well and the system was quite robust.


  11. Art history major?
  12. Close. The local multiversity, UCSC, has a "History of Consciousness"
    major, which should provide an endless supply of graduates capeable of
    detecting a square wave:

    I hate to admit it, but there was a time in the early 1960's when I
    worked on a job that resembled a "square wave detector". In this
    case, I was sentenced to stare at a Tek 520A vector display, for what
    seemed like endless hours, and yell of help if anything changed.
    Unfortunately, things did change so I really did need to pay
    attention. After about 10 consecutive days of this nonsense, I
    simulated a mental breakdown, and was assigned a more creative task.
    That was sweeping the floor from all the crud that was falling from
    the ceiling during construction.

    B.F. Skinner trained pigeons to peck at a CRT to run a guided missile.
    I guess a graduate student can be trained to do the same.
  13. John Fields

    John Fields Guest

    Assuming you mean a square wave looks like this:

    __________ _____
    ___| |__________|

    | |

    You could use a couple of counters, a clock, a magnitude comparator
    and some glue logic to do it in hardware and determine whether
    clocks accumulated during time t1 were equal to the number
    accumulated during t2. If they are, then a square wave is present.

    Use one counter to accumulate clocks during t1, the other to
    accumulate clocks during t2, the edges to do the comparisons and
    then to clear the counters on the fly. That way your worst latency
    will be 1/2 cycle. You'll also need to do a zero detect so you
    won't detect 0 counts = 0 counts as a square wave.
  14. John Larkin

    John Larkin Guest

    Or an HC123?

    Or just reset a counter every input edge, and clock it from something
    else. When it reaches some terminal count, it hangs, and that's the
    timeout. We do that in FPGAs as signal-loss detectors.

  15. pinku

    pinku Guest

    Thanks in tones ..for all the idea.
    The signal is not 50% duty cycle. This signal is coming from other
    card. I need to glow a LED if square wave is present. Voltage level is
    digital i.e 0V and +5V. Basically if there is toggle in the line I
    need to drive low and if no toggling drive high.
    If this detection can be done using simply digital circuit along with
    capacitor and Diode will be great.

    Thanks in advance
  16. Jerry Avins

    Jerry Avins Guest

    As long as the major interest isn't abstract expressionism.

  17. The basic TTL spec defines a LO by <1/3 Vcc,
    and the HI by >2/3 Vcc, every gate works to
    that standard, (approximately), so a TTL spec
    gate becomes a detector.
    Take that TTL spec'd O/P threw a series cap,
    and the power transferred should lite-up your
  18. John Fields

    John Fields Guest

    I took: "I need to detect a presence of square wave." to mean that
    he wanted to determine whether the signal was 50% duty cycle (a
    square wave) or not; not something one can easily detect with a

    As it turns out all he wants to know is, "Is the signal there?" so
    sure, a 123 would work, but why waste 1/2 a chip?
  19. Jamie

    Jamie Guest

    30 years ago?, that puts you up in the old duffer category!

    And here, I thought you were a young gentleman. At least you
    still have your mind fully intact which is more than I can
    say for a couple of others here! :)

    It'll be aprox 29 years for me employed in the field
    while doing school mean while at the start which was rough on
    me. There were few weekends we got to enjoy our self.
  20. John Larkin

    John Larkin Guest

    Saw it in half, and save a few cents?

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