On Wed, 04 Aug 2004 15:11:48 +0200, Ruud Poeze wrote:

Giovanni Landman schreef:

On Tue, 03 Aug 2004 00:47:27 +0200, Ruud Poeze wrote:


FM interference on SW bands also happens to cheap one IF radioos.
Could it be that in the front end sum and difference signals are
produced between the various stations?
PE 100.7-98.9 = 11.8 This is in the 25 meter band.
ruud

Ruud, you have to include the FM LO in your story.

You certainly know that for an FM receiver with IF stage, the LO is
generally tuned 10.7 MHz higher than the the frequency of the station
you want to listen to (and then the 10.7 MHz internally contains the
desired signal, which can be detected further in the IF stage).
Also, you already know about FM images due to overloading the HF stage
of cheap radios; in this case 1.8 M below the lower (97.1) and 1.8 M
above the upper (102.5).

But the other strong signal (in your example 98.9, so 1.8 MHz below the
"desired" frequency), also mixes with the LO, which will again produce a
signal, around 12.5 MHz (10,7 + the difference).

The unwanted signal will not be audible on that cheap radio directly;
the 10.7 MHz IF filters (whatever cheap quality they are) will stop it.
Around that cheap radio however, FM-like signals around 12.5 MHz (in
between the 25 and 22 meter bands) can be received.

At another location (e.g. Rotterdam) the unwanted signals will be around
11.8 MHz (in the middle of the 25 m band), due to the very strong
transmitters at 102.7 (538) and 103.8 (Yorin), and when tuned to the
latter. And of course, there are more frequencies popping up, due to
combinations with the other very strong Tx @ 104.6 (RTL FM).



I was discussing possible interference of FM stations nearby with the
radio in the SW mode.

You should have said earlier ;-)

Then the FM LO is not operational, only the SW Local Oscillator, which
will be typical 455 kHz higher then the tuned SW frequency.

Of course.

Since cheap radios hardly have any antenne filter, strong FM signals
might produce all sort of mixes, many of them are in a SW broadcasting
band.

Still pretty strange then. Following your example, from 1.8 MHz
difference to the middle of the 25 mb ... that's really a lot of
harmonics further away ;-)
Then again, there are other candidate Tx which can cause the
overloading.

--

mvg,
Giovanni.