Great explanation Doug.
Thanks
Lenny


"Doug Smith W9WI" wrote in message
...
(quoting in the wrong ordergrin

Lenny wrote:
Is this normal?

Yes.

Can SSB only tune an AM station on it's exact frequency?

Yes.

I have an R-75 with all the Kiwa mods.
Here's the scenario:
I'll tune to an international broadcast station on AM.
Then, if I switch to either USB or LSB and tune off the frequency either
way,
I'll hear a whine which gets higher in pitch as I move further away from
the center frequency.

The "whine" is known as a "heterodyne" or "beat note". There are a couple
of ways of explaining it. I'll go with this one:

A regular AM broadcast has three components. There's the "upper
sideband", the "lower sideband", and the "carrier".

The upper sideband is essentially the program audio, increased in
frequency by the quoted frequency of the station. (so if the station is
announcing it's on 11800KHz, the upper sideband is the program audio, with
every component increased in frequency by 11800KHz.)

The lower sideband is the mirror image of the upper.

And the carrier is a "dead air" signal *on* the station's quoted
frequency. It is necessary for the receiver to retrieve the program
audio - the carrier is basically mixed with one or both sidebands to
retrieve the audio.

You can hear what happens when the carrier is missing by tuning in a USB
or LSB signal with the receiver in AM mode. (during the day there should
be plenty of USB signals between 14150-14350KHz, and at night plenty of
LSB signals between 3700-4000KHz)

However, that carrier is a big waste of electrical power. (and since the
sidebands are mirror images of each other, it's really only necessary to
transmit one of them) In "single-sideband mode" the carrier is not
transmitted. But since the carrier is necessary for intelligible audio...
an "artificial carrier" is introduced in the receiver.

When you switch your receiver back to USB/LSB mode, the carrier is
provided, and the USB/LSB signals near 14200 or 3850KHz become
intelligible.

When you try to tune an AM signal - that already has a carrier - in USB or
LSB mode - you now have *two* carriers. One transmitted by the station,
one generated artificially in your receiver.

The two carriers will interfere with each other, generating a "beat
frequency" or "heterodyne". And that's the whine you're hearing.

If you had a way of measuring the pitch of the whine, you'd find it was
equal to the difference between the frequencies of the carrier transmitted
by the station, and that generated in the receiver.

(it might also be equal to the difference between the frequency displayed
on your receiver's display, and that announced by the station. However,
sometimes software considerations can cause that to not be the case.)

=======================

It can be helpful sometimes to use USB/LSB mode for regular AM signals on
purpose. Sometimes, "selective fading" causes some frequencies to fade
more than others. If this causes the carrier to fade out while the
sidebands are still present, you get the same distortion you get listening
to a USB signal in AM mode.

By selecting USB (or LSB) mode for a regular AM signal, if the signal's
own carrier fades out, your receiver provides a substitute carrier,
keeping the signal intelligible.

Also, you can select which sideband to listen to - so if one is suffering
from interference and the other isn't, you can choose to listen to the
interference-free sideband.

You do (as you've observed) have to be sure the receiver is set
*precisely* to the station's transmitting frequency. Sometimes, DXers use
this method to measure the frequency of a station believed to be
off-channel...

--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

Ham stuff for sale:
http://www.w9wi.com/articles/4sale.htm