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Klarinet Archive - Posting 000350.txt from 2001/02

From: George Kidder <gkidder@-----.org>
Subj: Re: [kl] Combination tones......
Date: Sat, 10 Feb 2001 15:41:59 -0500

Grant et al,

Since Tony P. and others have dealt with the math of the situation, and
since I agree with them, I will let that alone. (Don't know where my
Helmholtz is.) I do have some experimental data to contribute:

At 17:28 2/8/01 -0700, Grant wrote:
>
>I can't say I know that much about radio broadcasting, but I suspect
>that (a) the FCC assigns carrier frequencies to avoid having two that
>heterodyne at the same frequency as a third, and (b) radio receivers
>are designed to reject spurious carriers.

Sorry, they don't, nor do they have to. (For those outside the USA, "FCC"
is the Federal Communications Commission responsible for assigning radio
transmitting frequencies of all sorts in this country.) Good radio
receivers are indeed designed to be highly selective - receiving only the
frequency desired, and not accepting other frequencies. No receiver can
tell the difference between a "desired" and a "spurious" carrier on the
same frequency, as long as the "spurious" signal is really present.

>Come to think of it, the
>FCC never assigns AM 700 (AFAIK), or any other multiple of 20 KHz:
>they're all 510, 530, 570, etc., which means that any difference
>frequencies will be some multiple of 20 KHz (e.g., 1470 KHz - 550 KHz
>@-----.
>
Unfortunately for your case, a few minutes observation with the digital
radio in the kitchen (no external antenna) shows 16 AM broadcast stations
strong enough to be detected around noon of a sunny clear day. Of these, 7
(almost half) violate your "rule". Listening at night would probably show
a station at every 10 KHz point, including 920. We should know that
already, since the manufacturers of these radios are unlikely to have
possible tuning points where there cannot be a station.

George Kidder
Bar Harbor, ME

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