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From: Jeff Liebermann on 15 Mar 2010 22:23
On Mon, 15 Mar 2010 20:51:52 -0400, tnom(a)mucks.net wrote:

>It's poor filtering that allows an image of the desired signal. That
>image is the intermediate frequency away from the ordinal.

Ordinal as in set theory? Huh?
<http://en.wikipedia.org/wiki/Ordinal_number>
I think you might mean "original".

If the receiver has a 10.7MHz IF and low side LO injection at:
88.5 - 10.7 = 77.8 MHz
the image frequency will be:
77.8MHz - 10.7 = 67.1 MHz

If the receiver uses high side LO injection, the LO will be at:
88.5 + 10.7 = 99.2 MHz
with an image frequency of:
99.2 + 10.7 = 109.9 MHz

Neither 67.1 or 109.9 can be heard on the FM dial.

There are also spurious responses caused by harmonics of the local
oscillator which I can calculate if you want.

I also like the theory that the LO tuning capacitor is at the end of
its range and that dial cord and "slide rule dial" are mis-adjusted.

--
Jeff Liebermann jeffl(a)cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
From: William R. Walsh on 16 Mar 2010 01:36
Hi!

> Well, at 30 years, it sure ain't digitally tuned!

Aw, it sure could be! I'd be in agreement to say that digital tuners weren't
commonplace, but things did have them thirty+ years ago.

My dad's '79 Cadillac has a digitally tuned radio. It's very primitive. Each
segment is a single element, and the colons/decimal point in each were
eschewed in favor of LEDs to indicate those things. It's also got a
casssette player, something that I'm sure was a bit of a novelty then as
well (my guess is that most people had AM/FM in those, possibly with an
8-track).

Every function still works perfectly...

William


From: Sylvia Else on 16 Mar 2010 04:27
On 16/03/2010 8:17 AM, mm wrote:
> I have an clock radio about 30 years old that, when tuning for 88.5
> FM, for example, by turning the dial, will find that station, and then
> a little higher will be another station, and a little higher still
> will be 88.5 again, often a stronger signal than the first one.
>
> I'm sure it's not a second station playing the same stuff.
>
> I know about harmonics on AM MW and Short Wave, but here the tuned
> frequencies are so close, and also I didn't think there was stuff like
> this involving FM. What is going on?

Almost sounds as if it's an AM/FM receiver that's running its MW
hetrodyne oscillator as well as that for VHF. You could then appear to
get two signals about 0.5 MHz apart.

Of course, that would imply two distinct oscillators. Was it ever done
that way?

Sylvia.
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