Re: A Simple Radio Circuit

[jriley@lib.drury.edu (Jim Riley)]

The signal that comes from the radio station is radio-frequency, i.e. of the
order of kilo or megacycles.  The information that you want from the signal,
the audio-frequency sound is imposed on the r-f as a modulation of the
amplitude of the r-f, (at least in a-m radio).  The useful signal is thus
the envelope of the r-f signal.  The ear phone itself cannot possibly
respond fast enough to follow the r-f signal and would thus merely move with
the time averaged value which is zero without the diode.  By placing the
diode in the circuit only one half of the r-f signal is allowed through.
The earphone responds to the time average (over many cycles) of the
rectified r-f which yields an audible frequency in the ear phone.  

The phenomenon is much easier to understand if you draw a graph of an r-f
signal whose amplitude varies from max to min with an audio frequency of say
440 hz.  Note that the average is zero.  Now erase the negative half of the
wave (this is essentially what the diode would do).  Note that the average
is now a non-zero, audio frequency wave.

The reception of radio signals by fillings in the teeth may or may not be an
urban myth.  The posibility exists because the amalgam is a conductor
separated from the nerve endings in the tooth by an interface of some sort.
It is possible that the interface acts as a diode and the amalgam of the
filling acts as an antenna.  Possible, but I would like to see it documented
somewhere.

> In a simple radio circuit, there is a diode that links the signal to the 
> piezeoelectric crystal, the ear phone.  Could someone please let me know 
> why this is required  Also, is it true that people have supposedly heard 
> radio stations through their fillings and, again, how might this be 
> achieved?  Thanks for the info.
>
> ----------
> Tom McCarthy   Email:mcca6300@spacelink.msfc.nasa.gov
Jim Riley, Department of Physics
Drury College
Springfield, MO 65802
e-mail: jriley@lib.drury.edu
Phone:  (407) 873 7233