In article , Paul Burridge
writes:

ISTR.......
(through the mists of time) that the excessive bandwidth of an AM
transmission signal can interfere with the reception of a NBFM signal,
though the converse is not true. Can anyone confirm this (or else
flatly deny it/affirm it)?

Paul, you need to be a bit more specific on the above.

First, if the AM carrier is not close to the FM center frequency, then
the only energy from the AM signal is from the sideband content of
the AM.

Second, if the AM sideband energy is below the FM carrier level,
the FM receiver limiter is going to do its suppression thing and
(in the receiver) lower the AM sideband level. The amount of AM
suppression depends on the relative no-modulation signal levels of
the two.

Alternate second: If the AM signal, any part, is stronger than the
FM signal's level, then the AM signal is going to "take over" and
that limiter will suppress the FM signal.

If the FM signal's swing happens to intrude on the passband of the
AM receiver - and its level is high enough - then some distorted
modulation from the FM signal will come through. The higher the
relative FM signal is to the AM level, the worse the interference from
distorted FM content. Conversely, if the AM signal is stronger than
the FM, then the distorted interference from the FM signal is less.

Very basic RFI stuff. You can confirm this yourself with a receiver
capable of demodulating AM and FM (make it narrowband if you
want, doesn't matter much here) and reasonably-calibrated AM
and FM signal sources. The interference of one against the other
will be due almost entirely to relative signal levels and the carrier
(no modulation) frequency differences versus the receiver selectivity
bandwidth.

About the only thing different between FM and NBFM in the receiver
is the final selectivity bandwidth. There will be some sort of limiting
in there even with simple detector types.

Len Anderson
retired (from regular hours) electronic engineer person