[time-nuts] Aside about Triangle Waveforms
robert8rpi at yahoo.co.uk
Tue Feb 2 19:48:52 UTC 2010
Modern radar altimeters also use triangular wave FM modulation but at around 4.2GHz. Mix the return signal with a sample of the transmitter and you get an audio tone directly proportional to the round trip delay and thus height. works down to a few feet, pretty good for a real time time interval measurement. Some old techniques are hard to beat :-)
--- On Tue, 2/2/10, Brucekareen at aol.com <Brucekareen at aol.com> wrote:
From: Brucekareen at aol.com <Brucekareen at aol.com>
Subject: [time-nuts] Aside about Triangle Waveforms
To: time-nuts at febo.com
Date: Tuesday, 2 February, 2010, 15:14
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.
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