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From: crs@lanl.ARPA
Newsgroups: net.analog
Subject: Re: Does anybody know why Auto AM radios tune with variable inductors?
Message-ID: <36468@lanl.ARPA>
Date: Sun, 12-Jan-86 17:24:54 EST
Article-I.D.: lanl.36468
Posted: Sun Jan 12 17:24:54 1986
Date-Received: Tue, 14-Jan-86 06:08:22 EST
References: <2859@watvlsi.UUCP> <924@mit-eddie.UUCP>
Distribution: net
Organization: Los Alamos National Laboratory
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> >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.

This is just a guess but...  The variable capacitors used for tuning radios
have plates whose shape and position on the shaft are quite eccentric.  I
would guess that with such a configuration, the constant vibration would
cause the thing to rotate thus changing the setting.

-- 
The opinions expressed are not necessarily those of my employer,
the government or your favorite deity.

Charlie Sorsby
...!{cmcl2,ihnp4,...}!lanl!crs
crs@lanl.arpa