AM Demodulator / Detector

  • #1

Main Question or Discussion Point

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:

Answers and Replies

  • #2
384
3
C16 will charge up through D7 and there is no discharge path.
 
  • #3
C16 will charge up through D7 and there is no discharge path.
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?
 
  • #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:

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