On Mon, 17 Mar 2014 13:15:16 -0700, Jon Danniken
wrote:

Hi all, I picked up an SDR radio dongle, and have been playing around
with receiving. As I am playing around with it today, I am noticing
something odd (to me).

We have a college station locally that broadcasts at 88.1MHz FM. As
with many college radio stations, it has a very weak signal, but I can
just pull it in with rabbit ear antennas.

As I was poking around at ~24MHz, and I was able to pull this same
station in at 23.645MHz. Even more odd, I was getting a much better
signal at 23.645MHz than at the 88.1MHz "official" frequency.

Additionally, as I scanned around, I also found all of the other
stations doing this as well; 91.9MHz, which is too weak to tune in, I
can hear perfectly at 28.035MHz. I can also tune in to 96.1MHz at
30.240MHz, 105.5MHz at 30.325, and so forth.

So here's my question, is this something that is "normal" in radio, or
does this instead point to some peculiarity with my radio/software setup.

Thanks for any suggestions,

Jon

The 'front' end of many relatively low cost, wide range receivers has
very poor selectivity. The output of the Mixer stage is the SUM and
the DIFFERENCE between the local oscillator frequency and the incoming
signal. The result is when you tune a frequency, you are really tuning
to two different frequencies. The difference between one signal and
the IF and the Sum of another signal and the IF, Usually there is some
sort of pre-selector, bandpass filter or semi-tuned RF amplier that
effectively blocks the unwanted frequency, so there is no energy in
one of the two possibly tuned frequencies

For example if you have a conventional FM receiver, traditionally the
Intermediate frequency is 10.7 Mhz. So if you tuned the receiver

Another strategy is to 'up convert' instead of down convert. This puts
much more spectrum between the sum and difference frequency.

All that your experience says is that SDR probably has poor front end
selectivity.