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Various signal questions

  1. Oct 18, 2005 #1
    I've got to design a signal conditioning circuit that takes in a few different signals and converts them all to a standard signal. The overall purpose is to determine the frequency of the individual signals.
    Any ideas on how to make a variety of signals into one standard signal? Preferably a square wave?
     
  2. jcsd
  3. Oct 18, 2005 #2

    berkeman

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    Staff: Mentor

    Sounds like a simple zero-crossing detector would do what you want. Or mid-supply crossing detector, depending on what the average voltage is of your incoming signals. You could capacitively couple the incoming signals if you want to get rid of the DC component. You can make a zero-crossing detector with a comparator, for example. Be sure to add some positive feedback to give you hysteresis, to avoid buzzing the output of the comparator when the input signal is near the zero-crossing threshold.
     
  4. Oct 20, 2005 #3
    Here are a few examples of some of the signals I will be dealing with:
    sine wave ranging from 0-50 V up to 5KHz
    square wave with DC offset
    sqaure wave with extended 'high' times looks like this """""""""|____|""""""""""""""""""""""|____|"""""""""""""""""""""""""""""
    the frequency at which the low signals appear is related to the frequency of the signal

    I researched the crossing detectors you suggested, but it seems to me as though they make each signal into 1 frequency? Maybe I missed something.

    I need each of these signals to retain their original frequency and transform them into a square wave so I can count the edges to determine the signal frequency.
     
  5. Oct 20, 2005 #4

    berkeman

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    Staff: Mentor

    I think that's a typo, Joe. Or else a true-ism.
    It sounds like you want to convert the various signals into a digital rectangular wave. You need to constrain the frequency range a bit, so that you can pick a DC blocking capacitor value on the low frequency end, and pick the speed of your components on the high frequency end. You also need to have specified what the settling time has to be from the start of the signal coming into your circuit to when the circuit's output digital waveform has settled into a valid representation of the incoming signal (to within some number of %).
    Then build the circuit that I mentioned earlier, with a single-supply comparator that has a reference input at half of the supply, and the + input is connected to the signal source via a DC blocking capacitor. Put a voltage divider between the output of the comparator and its + input, to provide biasing to mid-rail plus some hysteresis. You will want to connect your signal source to the DC blocking cap via some resistance, and put diode clamps to the rails between the isolating resistance and the DC blocking cap. That way you can still have this circuit work with a 50V input waveform, and use a 5V comparator, for example.
    The values of the resistors and capacitor are left as an exercise for the reader. Just keep in mind all the specs that I was asking about above as you calculate the time constants, hysteresis voltage, offsets, etc. Have fun! -Mike-
     
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