FM on SW

[Electronic M]

Also the IF has a limited bandwidth. If you tune an FM signal such that it varies in frequency along the slope of the IF you will actually convert the signal in the IF system from FM to AM which can then be detected by a standard AM detector stage.

They used to use that trick to make 441line AM sound pre-war TV sets work after 525lline FM sound became the standard in mid 1941.

[jr_tech]

Originally Posted by Electronic M

Also the IF has a limited bandwidth. If you tune an FM signal such that it varies in frequency along the slope of the IF you will actually convert the signal in the IF system from FM to AM which can then be detected by a standard AM detector stage.

Yes ! I should have mentioned "slope detection" which worked fairly well on the old S-41G (sorta mushy, but understandable).
Just tried it on a more modern receiver (Icom 8500) and it did not work so well. I found an FM station that did not have IBOC, RBDS, or SCS carriers hanging off the sides of the main FM carrier, switched to "AM mode" and tuned around, using several different bandwidths. Sounded terrible... Possibly the steepness of the "skirts" of the IF response of the 8500 are too steep to allow "slope detection" to work very well.
Another factor may be that modern FM transmitters are quite clean, with very little "AMing" of the FM carrier present.

jr