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Thread: Schmitt Trigger

  1. #1

    Default Schmitt Trigger

    Hello,
    I need to buy a schmitt trigger for a prop project, to convert a sine wave to a square wave. I tried various op amps (lm358, lf411, 741) with no success. The frequency I need is about 430 khz. Can someone suggest any ICs?
    Thanks,
    Curtis

  2. #2

    Default Re: Schmitt Trigger

    Try a 74HC14. That's a hex inverting schmitt trigger. Propogation delay for HC version is 13nS so should be plenty fast enough.

  3. #3

    Default Re: Schmitt Trigger

    Curtis,

    As I stated in your other thread, a Schmitt trigger will introduce a phase shift. You will not get an accurate phase measurement from a Schmitt-triggered waveform.

    -Phil

  4. #4
    frank freedman's Avatar
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    Default Re: Schmitt Trigger

    Curtis and Phil,

    I have been following this (i think it relates to the ultrasound thing Curtis is working on?) for a bit, and if the phase shift is known or measurable, it can probably be monitored and applied as a constant offset possibly in real time. It may be that the unit may need to determine that value for cal purposes at run time anyway. What I am most curious is just what is the range of measurement that Curtis is trying to measure.

    Random thoughts of an enquiring mind,
    Frank

  5. #5

    Default Re: Schmitt Trigger

    Unfortunately, the phase shift won't be constant, but a function of amplitude.

    -Phil

  6. #6

    Default Re: Schmitt Trigger

    Thanks everyone,
    My research showed that the schmitt trigger may do the job, but I am not as well versed as you all in this field. Thanks for the honest dialog. My purpose is to make an instrument with one single ultrasonic transducer and bring it to the surface of cultural materials (artifacts) in order to view and analyze defects. I would like to start with a 40 khz sensor and move forward to higher frequencies. Does there exist any other method for converting sinewaves to square waves without introducing an additional phase shift other that what is changing because of the defects in the material tested? In my last project, I used eddy currents to analyze defects in metals and also recover polished off engravings. I did this by measuring the voltage (amplitude) changes as the coil (probe) passed over a material. I never could measure phase shift which would have given me more data.
    Thanks,
    Curtis

  7. #7

    Default Re: Schmitt Trigger

    The method (I/Q detector) I already suggested in your other thread is the way I would do it. That way you're not burdened with having to convert the sine waves to square waves in the first place, and the phase measurement will be more accurate, since it occurs over multiple cycles. BTW, the phase difference between your return signal and the I signal will be ATAN2(Q, I).

    -Phil

  8. #8

    Default Re: Schmitt Trigger

    Thanks Phil and everyone,
    I'll try the method phil suggested. I printed the thread (I/Q) detector and will read it more carefully and learn it.
    Thanks,
    Curtis

  9. #9

    Default Re: Schmitt Trigger

    An opamp isn't great for zero-crossing detection, a comparator is the correct chip for that (far faster switching, tolerant of + and - inputs being very different). A schmidt trigger is for handling noisey waveforms. A comparator can have a controlled amount of positive feedback added to provide schmidt trigger function at the expense of threshhold accuracy.

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