Well, Andy, since you wanted to stir it up...
I'm not an avionics or electrical engineer, nor have I ever been. I haven't studied any electronics since I left school, except for my ATPL exams. But my understanding, which is a combination of A-Level physics and Bristol's notes, is as follows:
In the old-fashioned style of VOR, the "FM" signal really was frequency modulated, and transmitted in all directions. But in a Doppler VOR, it's more complicated. An unmodulated, relatively high frequency signal is transmitted through a number of discreet transmitters which are arranged in a circle. Initially it is trasmitted through the first transmitter, then the next, then the next, then the next. So, depending where you are located in relation to it, at any point in time the source will be either moving nearer to you or further away from you. This gives rise to a Doppler shift, and you see a slightly different frequency to that which was broadcast, with the frequency constantly varying as the source moves first towards you, then away from you. Thus it appears that you are receiving a frequency-modulated signal. The phase of this signal will depend on your QDM from the VOR.
As for the AM signal, in the Doppler VOR this is a simple transmitter. In the old-fashioned VOR, it is a combination loop/omnidirectional aerial, very similar to "traditional" direction finding equipment (of which the ADF is a derivative). This transmits a "limacon"-shaped signal, with some areas receiving a stronger signal than others. Again, the signal which is transmitted is unmodulated, relatively high frequency. The aerials are rotated, and as the rotate, the receiver will pick up first a high-amplitude signal, then a low-amplitude signal, as the limacon rotates and the receiver passes through different parts of the signal. Thus, it appears to the receiver as if an amplitude-modulated signal is being received. Again, the phase of this signal will depend on your QDM.
And that is about the sum of my knowledge, so I doubt I'll be able to add much more to this thread!
FFF
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