Sampling High frequency periodic signal

Discussion in 'The Projects Forum' started by DreamCatch16, Mar 13, 2014.

  1. DreamCatch16

    Thread Starter New Member

    Feb 10, 2014
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    I want to sample a periodic sinusoidal signal ranging from 40Hz up to 5Mhz,
    I need to sample at least 10 samples per cycle so the sampling frequency should be at least 50Mhz if doing a continuous sampling.
    For you information I am using the PIC18f4550 which is a slow microcontroller and I want to prevent from changing it, also I can have a reference signal that is at the same frequency of the sampled signal.
    My question is: Is there any techniques for sampling a high frequency periodic signal?
    Or is there any high speed ADCs (10bits up to 16bits) with a programmable sampling frequency and an internal buffer memory that I can read it after taking the samples?
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    What you do is have a bank of high speed flash converters that drop their data into a deep FIFO memory under control of a multiphase clock. The microcontroller can unload the FIFO at it's leisure when the sampling snapshot is done. Assuming you can find a suitable A/D converter and a suitable memory this should be a straightforward FPGA to design and implement.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
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    Or sample with a short duration aperture at a lower rate, pass it through a low pass filter, and measure the beat frequency that results.
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Are you trying to measure frequency or amplitude?
     
  5. DreamCatch16

    Thread Starter New Member

    Feb 10, 2014
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    I am trying to measure the phase shift and the amplitude of the signal by taking sample and then reconstruct the signal and getting the phase shift according to the reference signal.
     
  6. DickCappels

    Moderator

    Aug 21, 2008
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    Would just detecting the zero crossings give you sufficient information?
     
  7. DreamCatch16

    Thread Starter New Member

    Feb 10, 2014
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    by chance (and multiple research on RS components website) I found this ADC THS1206 which is 12bit ADC and 6Msps with 16 word internal FIFO memory, I can not use it for continuous samping but I think I can use it with the ETS technique, what do you think? is it a good choice or I have too look for something else?
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    In order to accurately measure amplitude and phase of a 5MHz sine wave you will need some sophisticated techniques.

    Firstly, in order to measure phase you will need a reference signal. This does not have to be a sine wave signal but could be a simple digital trigger or sync signal.

    There are two approaches I can think of:

    1) Instead of using a high speed ADC, you can use zero-crossing to measure phase. Use timing circuits to measure the time interval between your reference signal and the first zero-crossing of your signal to be measured.

    To measure the amplitude, use a precision rectifier and integrator. Then you can use a slow ADC to measure signal amplitude.

    2) If you want to go the high speed ADC route, you can sample 10 or 20 times the signal frequency, that is, sample at 50 or 100Msps for a 5MHz signal and use math algorithms to determine the amplitude, frequency and phase. There are programs available which will perform a least-squares fit to a sine wave.

    You will have to adjust the sampling rate to suit the input signal.
     
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