#am detection

So I touched mostly on different types of modulators and the circuits that could be part of the actual modulation.  I've decided I'd be cheating myself if I didn't at least attempt to cover demodulators.  As briefly, briefly, mentioned before, demodulators separate out the data/voice signal from the carrier signal.

AM Demodulation/Detection

For AM signals, the simplest kind is an envelope detector that produces an output voltage that reproduces the modulated signal’s envelope.

So if we think back to AM modulation, the modulating signal (data or voice) is like a sandwich smusher that changes the amplitude of the carrier wave.  That sandwich smusher is known as the signal's "envelope" (Red line below).

An envelope detector detects the envelope of the incoming signal by using a rectifier and capacitors.  (I know, I used the name in its own definition - bad form!)

Background reminder: Rectifiers are diodes capable of handling large currents.  Diodes in turn only allow current in one direction.  Because of this, an AC signal that comes through will show up as a DC signal made up of pulses.The simplest way to demodulate AM is a diode detector. 

AM waves are the same above and below the y axis, so with a detector, it doesn't matter whether you use the positive side of the envelope or the negative side. 

So in come AM radio frequencies, into this circuit where it encounters a rectifying diode which is placed in parallel with a capacitor and resistor. 

While the amplitude of the the input signal is positive, the diode is forward-biased meaning that charge is flowing in the proper direction.  The capacitor (C1) charges up to the max value of the input signal. 

Then when the amplitude of the input signal is negative, the diode becomes reversed biased meaning that due to the properties of the diode, no current is able to run through.  The capacitor then starts discharging through the resistor (R1) it's coupled with.

If I'm not mistaken, that smooths out the voltage, because before the capacitor's able to completely drain, the input signal's amplitude is positive again.  This leaves you with a relatively smooth data signal!

The main limitation with envelope detectors is that it can only be effectively used with DSB (no distortion) and VSB (very low distortion) signals.  For a more indepth look at the reasons:

http://www.radio-electronics.com/info/receivers/am_demod/diode_am_demod.php

http://sound.westhost.com/articles/am-radio.htm

Next - SSB/CW demodulation via product detectors which I will cover in my next post!