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AM Demodulator / Detector

  1. Nov 16, 2011 #1
    Im working in Pspice, I have an RF amp with a modulated input signal coming in on ~600kHz carrier wave and an amplified signal coming out, here is what the output looks like 1Megaohm load after a blocking capacitor to isolate AC:
    http://i.imgur.com/56NSI.png

    I am trying to use a simple http://en.wikipedia.org/wiki/File:C_Simple_envelope_detector.gif" [Broken] circuit after the RF amp without a load resistor. using t = 1/2*pi*f = RC with f=400hz and C=0.01uF I solved for R (4k). Here is the circuit:
    http://imgur.com/09Dpk
    and here is what the output out of the detector looks like:
    http://imgur.com/TZSXA
    Looks distorted if im not mistaken and no where near the ~700mV output peak. What did I design wrong in the detector?
    Edit: noticed I probed at ground node of detector but output signal looks the same at the top node of detector output.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Nov 16, 2011 #2
    C16 will charge up through D7 and there is no discharge path.
     
  4. Nov 16, 2011 #3
    I don't know how it can charge up through D7 unless it's leakage current, Im guessing that's not causing the main issue , but how do I relieve it? High resistance to ground between C16 and D7?
     
  5. Nov 16, 2011 #4
    I added a 2Megohm resistor between D7 and C16 and it looks like it took care of the slant type distortion. Not sure if this is right but I redesigned the detector with R as my 1Megohm load and solved for C.
    The output signal is around 110mV peak now:
    http://i.imgur.com/5oDbd.png
    Can I get the amplitude back up to 700mV peak ?
     
    Last edited: Nov 16, 2011
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