From: vjh21@cas.org (Vince Herried ext 2877)
Subject: Re: commercial AM receivers
Keywords: AM receivers
Organization: CAS
Date: Wed, 16 Dec 1992 23:08:03 GMT

In article <1992Dec16.201833.26699@nstn.ns.ca>, klank@fox.nstn.ns.ca (Karen Lank) writes:
|> Hello,
|> 
|> I am looking for some information on commercial AM receivers to aid in a
|> research project currently underway at Applied Microelectronics Institute.
|> 
|> Any information on where I might obtain a description of commercial AM 
|> demodulation algorithms or, ideally,  some emulation software would be greatly 
|> appreciated.  Automatic gain control modelling would be of particular
|> interest. 
|> 
|> Thanks,
|> Karen Lank.
|> 

Karen, abt 20 years ago I did electronics stuff for a living.  I had formal
training in the subject also.

by commercial AM demodulation do you mean demodulation of the 'commercial' AM
band from 550 KHz to 1600KHz?  Or do you mean in commerical radio equipment
meant for commerical or industrial service?

The vast majority of AM radio receivers
demodulate the signal with a simple diode and a capacitor:

	\---/
	  \/
	   |
	___________
	| RF sect  |
	|(optional)|		
	____________
	     |		
	____________       _____________
	| mixer     |------| oscilator |
	_____________      _____________
	     |
	_____________
	|  if amp    |
	______________
	     |
	_______________
	| demodulator |
	_______________
	     |
	___________________
	| audio amplifier |	
	___________________
	     |
	__________________
	| speaker system  |


The demodulator for an AM radio circuit looks like:


			resister		
			|	    capacitor	
							
------------------/\/\/\/\/\/\/---| |------------------------------------>  output to audio amp
				        |                          |
					|                          |
				     ------		 	 -----  		
				     \   /                       _____
				       \/       diode detector     |
				     ------                        |
					|			ground
					|
					ground	(or common)

		
	  

The resister capacitor diode arangement rectifys the signal and the
2nd capacitor on the right filters out the RF signal.  


Automatic gain control is usually obtained (my memory is fading)
by taping some of the signal where it goes to the audio
section and sending it through a low pass filter that rejects
frequencys above several cycles per minute (we are talking low frequency)
This signal is then used to reduce the gain of the IF section.

If a strong signal is received, the signal on the top of the capacitor
on the right (above) is increased in amplitude.  The AGC circuit
removes the audio and only detects the slow (several seconds) increase
in voltage and sends it back to the IF section where most the
signal amplification takes place.

Depending on the radio, some don't even use an AGC circuit,
some include the options to change the charactists of the low pass filter
values depending on what type of AM signal is being received.

May I suggest that you refer to the 'Amateur Radio Handbook'
published by the Amateur Radio Relay League (ARRL).
It is published 'new' every year and goes for around $30.00.
You should be able to find one at a good public library
or an amateur radio store.  This book is written
for the layman (person with out a electronics engineer degree)
that wishes to understand RADIO.  It has been a usefull reference
to me over the years.  But then, I'm an Amateur Radio Operator (HAM).




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