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The AM signal generator method kind of relies on old TV/radio service tube era signal generators with crude dirty modulators (at least if you want the detector to have output to hear and measure). Basically the tube era modulators distort the carrier enough to create a form of FM modulation along with the AM modulation.* Modern SS generators often don't have that useful distortion, and even some of the high end laboratory grade stuff from the tube era lacked it.
Easy way to check for it is to take an analog FM radio and feed it 10.7MHz (nearly universal FM IF frequency) or something in the FM band if the generator goes that high and see if the radio produces the modulation tone.
* In some cases video buzz in the sound of vintage sets is created from a similar effect where over modulation of the AM video carrier creates FM noise the TV can demodulate...I've described this in-depth in other threads before and don't feel like doing it again right now ( after midnight).
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