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In article <600@imtec.co.uk>, peter@imtec.co.uk (Peter Rushworth) writes:

	I think we were discussing means of FM modulation...

> >It was explained to me that the phantastron tube was basically set up
> >so that an electron beam travelled from cathode to anode as in most
> >tubes, but that there was a large magnet clamped around the tube to
> >force the beam into a helical path to make it longer.  Apparently
> >there were a set of electrodes that stretched or compressed the length
> >of the helix, thus changing the time it took for the electron beam to
> >traverse the tube, thus phase modulating the signal.
> >
> What you describe sounds similar to what I think is called the "serrodyne"
> effect (spelling from memory). I once worked on a system which used
> a travelling wave tube to modulate a (GHz) signal like this. I don't recall
> magnets, but there was a sawtooth input (<10Khz) to the helix, as you
> increased the amplitude, the signal freqency reduced.

	You may be referring to a "serrasoid" modulator, which is a circuit
and not a specific device.  In this circuit, a reference frequency in a
sawtooth waveform is fed to one input of a comparator; the modulating
voltage is fed to a voltage-to-quadratic function converter whose output
is fed to the other input of the comparator.  The output of the comparator
goes through a filter, and a frequency multiplier if necessary.
	The circuit works because the output of the comparator generates
short pulses whenever the quadratic modulating function crosses the
reference sawtooth with a positive slope.  The spacing of these pulses
varies with time, and when _filtered_ form a linear FM waveform.  The degree
of frequency variation is limited with this basic circuit, which is why a
frequency multiplier is often employed after the filter to achieve the
desired FM deviation.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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