PWM Demodulation

Discussion in 'The Projects Forum' started by rueffy, Sep 9, 2013.

  1. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Hello Again,

    I'm in the process of implementing the following circuit provided by Thomas Adamson from Electronic Communications. The diagram only shows solutions at block level, the blocks are fairly simple to implement with the exception of the ramp generator. I've come across a few ramp generator circuits online, but can't seem to find one that has ON / OFF functionality. Most appear to be uncontrolled (oscillating) or just have ON trigger function then after a predetermined time turn off. Is anyone aware of a circuit that can perform this function?

    Also of interest is the way this circuit performs, many online circuit diagrams involving PWM Demodulation involve converting to PAM - this involves summing the ramp waveform with a synchronous pulse and then clipping out the ramp - just leaving the PAM. This circuit appears to skip that step and just filter the ramp waveform - can anyone foresee any problems with this?

    Thanks for your help as always.

    [​IMG]
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    The ramp generator can be a latch circuit driving an op amp integrator. The ON signal causes the op amp to integrate and the OFF signal resets the integrator.
     
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  3. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Thanks crutschow, that sounds like a good approach.
     
  4. bance

    Member

    Aug 11, 2012
    315
    34
    Here's a circuit that does what you want...

    HTH Steve
     
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  5. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Thanks bance, will look into it once my prototyping board is repaired.
     
  6. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    The parts are on the way from maxima, I found I was able to eliminate the need for a differentiator, negative clipper and positive clipper with this ramp generator as it operates on steady state pulses rather then edges, all I needed was an inverting op amp to make the pulses negative as the ramp generator is negative pulse driven.

    A couple of other questions, my integrator or LPF - this should have a corner frequency or upper limit the same as the highest frequency of my sampled signal yes? In this case I am decoding voice (up to 3500kHz) so I've set the corner frequency there using a passive filter.

    I'm also going to try and fed my filtered signal directly into a simple electronics speaker...not sure if this will work without amplification though.

    Here is my completed circuit

    [​IMG]
     
  7. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Hi Bance,

    My MAX4490 and MAX6018 samples came in the post today, I was surprised when I opened them to find they are almost microscopic in size, far too small for me to work with - I guess I wasn't familiar with package sizes. After some investigation I can't find a maxim integrated equivalent that is of a larger size, do you think I could substitute a similar IC in?
     
  8. rueffy

    Thread Starter Member

    Jun 29, 2013
    39
    3
    Thank you to all those that helped contribute to the development of my circuit. The project is now fully functional and I've made a video as part of my presentation to a class, the video shows the system in operation with a full description of signal flow etc.

    http://www.youtube.com/watch?v=CCAkpJ-jG6A

    rueffy.
     
    Last edited: Nov 25, 2013
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