Robin...I assume that your term "tremulant stop" pertains to a classic organ, the type that has been in used for hundreds of years. It does not necessarily have anything to do with the current definition of tremolo.

I am a scientist but not an engineer, and would ask that you consult one and get a more authoratative answer if you need one. I will point out that radio is either "amplitude modulated" or "frequency modulated", which are analogs to tremolo and vibrato.

adios...pdq