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From: alw@mit-eddie.UUCP (Alan Wu)
Newsgroups: net.analog
Subject: Re: Does anybody know why Auto AM radios tune with variable inductors?
Message-ID: <924@mit-eddie.UUCP>
Date: Fri, 10-Jan-86 17:51:24 EST
Article-I.D.: mit-eddi.924
Posted: Fri Jan 10 17:51:24 1986
Date-Received: Tue, 14-Jan-86 03:22:09 EST
References: <2859@watvlsi.UUCP>
Reply-To: alw@mit-eddie.UUCP (Alan Wu)
Followup-To: net.analog
Distribution: net
Organization: MIT, Cambridge, MA
Lines: 16
Keywords: automotive radios

In article <2859@watvlsi.UUCP> ksbszabo@watvlsi.UUCP (Kevin Szabo) writes:
>I have found that 99% of auto AM radios use a bank of variable
>inductors for tuning; whereas all other portable AM's use
>variable capacitors.  Anybody out there know why?  My only
>guess is that the variable capacitors might not be rugged
>enough, but it seems unlikely.

I remember reading some 10-15 years ago (in a book about vacuum tubes)
that the reason variable inductors were used in automotive radios was
their superior resistance to vibration and better stability in spite
of extreme temperature changes.  This was in comparison to air-dielectric
capacitors (it was an old book even when I read it).  I'm not sure why
modern variable capacitors aren't used today -- perhaps it's tradition
or perhaps there's still a temperature drift problem.
-- 
--Alan Wu    UUCP: alw@mit-eddie.uucp    ARPA: alw@eddie.mit.edu.arpa