A widely used WW-II aircraft radio altimeter used a triangular waveform to
FM modulate a 400 MHz oscillator, employing a mechanical variable capacitor
constructed similar to a permanent-magnet loudspeaker. To get the
capacitor's diaphragm to reverse accurately, at the positive peak of the
trianglular waveform, required a sharp, negative-going impulse to be added to
the
peak of the triangle, creating a sharp notch in the waveform about 30% deep.
This makes me wonder about the limitations of speaker cones attempting to
reproduce complex waveforms. If they had overall feedback for positional
correction, the spectrum of the resulting driving waveform might contain
some pretty complex components.
Bruce Hunter
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