40MHz peak Detector

Discussion in 'The Projects Forum' started by quantum1, Aug 6, 2014.

  1. quantum1

    Thread Starter New Member

    Aug 6, 2014
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    Hello!
    Can you please advise me on the way forward. I have a project to determine the seperation between 40MHz bursts of signals. Zero crossing detection would have been ideal but the signal is quite noisy. I therefore am trying to build a circuit that detects the occurrence of a peak and generates a pulse upon said detection. These pulses are then used to measure the time.

    So this is different from the standard peak detector in that I do not want to know and do not care what the peak value is. I want to know the time of this peak. The result is that the peak detector has to track the signal and pulse once it starts decreasing. I am stumped
     
  2. to3metalcan

    Member

    Jul 20, 2014
    228
    23
    Sounds like you want to use the output of a peak detector to trigger a monostable? After that, your description is confusing.
     
  3. to3metalcan

    Member

    Jul 20, 2014
    228
    23
    Actually, it sounds more like you want a flip-flop bistable...one pulse when the peak detector goes high, and another when it goes low.
     
  4. quantum1

    Thread Starter New Member

    Aug 6, 2014
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    My Signal is the one in blue, while the output I am hoping to get is the one in red. I had been thinking of a comparator across the diode of the peak detector, as the peak is distinguished by the transition from charging the capacitor to not charging the capacitor. I have never seen a circuit like this however. And also the reverse recovery time of the diode is a problem. I will post my ideas of what the circuit I have in mind will look like and you can advise me.
     
  5. quantum1

    Thread Starter New Member

    Aug 6, 2014
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    I have uploaded my idea of a circuit to do the job. Please give your thoughts
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    So the signal is just one cycle of the 40MHz?
     
  8. quantum1

    Thread Starter New Member

    Aug 6, 2014
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    Signal is a train of four consecutive cycles with a gaussian envelope. Thanks
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    Your diagram shows a short pulse from the first cycle. Is that want you want (just one short pulse), or do you want to pulse width to be equal to the 4 cycle period (or does it matter)?
     
  10. quantum1

    Thread Starter New Member

    Aug 6, 2014
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    It doesn't really matter. I just want an indicator for when a turning point is encountered.
     
  11. DickCappels

    Moderator

    Aug 21, 2008
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    632
    If it is your intent to measure the time between peaks of the 40 MHz carrier, that would be the circuit you uploaded.

    From the waveforms, it appears that you want to measure the frequency of the sine wave, but you stated that you want to measure the time between the peaks of the bursts -just want confirmation of which one you are trying to do.

    If you want to measure the timing between the bursts, and the clusters of bursts are frequent, you can pass your 40 MHz signal through the fast diode detector, then using an opamp peak detector with a long time constant, catch the peak signal and take a fraction of it (perhaps 80%) to one input of a comparator and connect the other input of the comparator to the output of the first peak detector.

    [​IMG]

    The RC on the output of the first peak detector is large enough to smooth the individual 40 Mhz peaks but fast enough to respond to the burst frequency. The second peak detector stores the peak of the bursts from the first detector. You might have to do some amplification between the fast peak detector and the rest of the circuit.

    I'm glad to see that you thought of compensating the diode drop with another diode. For the temperature dependent voltage drops of the two diodes to track they need to operate at the same current.
     
  12. DickCappels

    Moderator

    Aug 21, 2008
    2,651
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    In case anybody is still on this thread, if the signal is large enough to get a couple hundred millivolts out of the dtector (and less than about 4 volts), the circuit below will produce pulses at the peaks of the output. The base resistor for the second transistor should be very large so that it provides a relative long time constant with the capacitor between the transistors. If the signal stops completely, the output will go low. This is a very reliable "peak detector". In reality, it was used as a sync stripper in countless composite video amplifier designs in the 1970's through the late 1980's.






    [​IMG]
     
  13. quantum1

    Thread Starter New Member

    Aug 6, 2014
    6
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    Thanks, I will prototype it and let you know how I get on.
     
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