I found the following extremely interesting article about the jamming of pro-Western broadcasts in Estonia during the Soviet period: http://www.okupatsioon.ee/english/mailbox/radio/radio.html" [Broken] One part of the author's description I'm puzzled about: "Although all LW, MW, and SW broadcasts were always amplitude modulated (AM), amplitude modulation was not used for jamming, but rather frequency modulation (FM). And this, on such a narrow strip as is needed in a band for 1 station, that is, 9-10kHz. If the transmission bearing wave is viewed with an oscillograph, the modulation cannot even be observed, as if it didn’t exist." I'm not sure I understand why FM was used to jam AM signals - and what the last sentence means exactly. Which transmission can the modulation not be observed on, the jamming signal or the incoming broadcast?