Video Lectures created by Tim Fiegenbaum at North Seattle Community College.

We are in multi-sim and we're looking at the simulation of a simple AM detector circuit.  This is in the circuits that came with your text.  It is titled “Figure 09_26.MSM.”  I encourage you to run this simulation.  First of all, let's see, we have an AM modulated signal here in the previous screen we had discussed how an AM transmitter and receiver, the basic blocks of how that works.  What we want to do is connect channel A to the originating signal.  Then channel B we want to be able to look at the recovered signal and we're going to connect a ground in here and this will serve as a ground for A and B.

Let's turn this circuit on and look at the output.  Then we will talk about it.  Here's our O-scope, I need to turn the circuit on.  Then here we have our waveforms.  The first waveform here, this is coming from our AM signal source.  This would be in essence the transmitted signal coming from our AM signal source.  Then over here we're looking at, in orange here, this is the recovered signal.  You can see the relationship that the recovered signal is the same shape as the outside of the AM envelope.  Let's look at what's actually happening in this circuit.

First of all, we come in with our, let's see I don't know if you can see, okay now you can see that, we come in without AM modulated signal, the first thing that it hits is this diode.  The diode, in this case, serves as rectification.  Since this is the bias here it would be positive and negative.  This would pass the part of this wave shape that's above zero volts.  We would essentially pass the positive half of this wave envelope.  Then we would go over here and C1 and C2 are going to act as filters.  Remember when we looked at the rectifiers with DC power supplies we had capacitors in there.  This smoothed out the AC.  Now the trick in setting these up is that we want to smooth out the AC in the RF portion of this, but we do not want to do it so much that we smooth out the signal that is our recovered signal.

The time consoles for the two stage filter network must be short relative to the original modulating signal, but long relative to the high-frequency carrier signal.  That is the simulation of a simple AM detector.  We have our modulated signal coming in and we have our recovered signal coming out.  This particular signal we would feed it into an amplifier and the speakers and then we would be able to listen to the voice, music, data, or whatever that was transmitted from the AM transmitter.