AM Demodulator / Detector

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In summary, the conversation is about using Pspice to design an RF amplifier with a modulated input signal and an amplified output signal. The output is then connected to a simple envelope detector circuit, where the user has noticed that the output signal is distorted and not reaching the desired peak of 700mV. After some troubleshooting, the user adds a 2Megohm resistor between the detector and capacitor, which appears to improve the signal but still does not reach the desired amplitude.
  • #1

extremepilot

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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 I am 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.
 
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  • #2
C16 will charge up through D7 and there is no discharge path.
 
  • #3
Carl Pugh said:
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, I am 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:

1. How does an AM demodulator work?

An AM demodulator works by converting the amplitude-modulated signal back into its original form. This is done by using a diode and a capacitor to remove the carrier frequency and extract the modulating signal.

2. What is the purpose of a detector in AM demodulation?

The purpose of a detector in AM demodulation is to extract the modulating signal from the carrier signal, which is necessary for the transmission of information. It also removes the carrier frequency, allowing the original signal to be reconstructed.

3. What are the different types of AM detectors?

The three main types of AM detectors are envelope detector, synchronous detector, and product detector. Each type has its own advantages and disadvantages, and the choice depends on the specific application.

4. What are the benefits of using an AM demodulator?

An AM demodulator allows for the transmission of information over long distances, as the amplitude modulation technique is less affected by noise and interference compared to other modulation techniques. It also allows for the use of a single frequency for multiple transmissions.

5. What are some common issues with AM demodulators?

Some common issues with AM demodulators include distortion, which can result in the loss of information during demodulation, and noise, which can affect the quality of the reconstructed signal. Other issues may include interference from nearby signals and frequency drift. These issues can be mitigated through proper circuit design and signal processing techniques.

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