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From: dsi@unccvax.UUCP
Newsgroups: net.analog
Subject: Re: Does anybody know why Auto AM radios tune with variable inductors?
Message-ID: <416@unccvax.unccvax.UUCP>
Date: Sat, 11-Jan-86 09:21:07 EST
Article-I.D.: unccvax.416
Posted: Sat Jan 11 09:21:07 1986
Date-Received: Tue, 14-Jan-86 04:45:39 EST
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     Aside from ruggedness considerations, one  benefit of inductor
tuned AM radio front ends is their compactness.  I have seen complete
inductor tuned front ends, including RF amplifier, mixer, and oscillator
done in about the space of 2 28 pin DIP's stacked on top of each other 

     It is also easier to wind the coils with variable pitch (so that the
displacement of the slug in the inductor represents a constant frequency v.
distance relationship on the dial) but evidently many manufacturers forget
this. In fact, Ford Motor Co (variously also known as Aeroneutronic Ford
and Ford Refrigeration and Entertainment Division) doesn't stop with the
AM section.  There is a "fourth" slug tuned circuit which is 
an oscillator.  However, this oscillator drives a frequency
discriminator providing the FM tuning diode voltage.
Here the same bennies apply, such as constant dial displacement
versus frequency. At FM, you get the added bonus of not having micro-
phonics in the audio due to rattling tuning capacitors.

     An inductor tuned front end also provides a better impedance match
to the type of antenna found in the car (very short monopole) which 
means a better S+N ratio.

David Anthony
DataSpan, Inc