ultra-wideband FM

John Larkin · 2010-07-13T15:29:45+00:00

Hi,One of the nasty things about cheap fiber-coupled lasers is that theyhave terrible amplitude stability and linearity, full of mode jumpsand such. Given that, sending a signal over a fiberoptic link usingamplitude modulation is usually done with a stable CW laser feeding alithium-niobate modulator. The modulator itself is nonlinear andexpensive and a nuisance to drive and bias.Digitizing and sending samples is OK, up to a point. It gets messy atsome point from a sheer speed standpoint.So the idea of using FM pops up. If my baseband analog signal were,say, DC to 150 MHz, and I picked the highest carrier center frequencythat's reasonably easy to work with, say 1 GHz, it could maybe bedone. The laser driver and receiver aren't too difficult. The issuesare the modulator, the demodulator, and the pure signal theorynecessary to turn the time-domain behavior of the link into classicmeasures like s/n and distortion of the recovered baseband signal.Asymmetrically bandlimiting an FM signal is computationally messy.I'd expect that commercial VCOs wouldn't have anything like this sortof fractional modulation bandwidth. And if they did, a varicapmodulating an LC oscillator would probably distort like mad. (Faintechoes of the capacitor charge debate?) The modulator may have to besome EclipsLite version of a 555 on steroids. Or a multi-GHz VCOheterodyned down. Yuk: sounds like RF.On the theory side, does anyone know of (or have?) one of the high-endmath tools that could do a quantitative signal-quality analysis ofsuch a link, given, say, approximate experimental data on thetime-domain behavior of the laser link? Hiring a consultant to do thiswould be a desirable alternate to getting and learning this stuffourselves.Any thoughts?John

ultra-wideband FM

Jul 13, 2010 63 Replies

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quiettechblue 16 years ago

It seems that only the audio part was FM and only on later models.

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quiettechblue 16 years ago

But just a few percent (at best) of peak is a little ridiculous don't ya think? Especially with todays highly compressed dynamic range. I wonder just what the maximum voltage available is, just what the wall wart rated power is.

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Paul Keinanen 16 years ago

Both NTSC and PAL VHS recorders separated luminance from chrominance, since there was no way to main the critical phase relationship of the chroma signal as required by both NTSC and PAL.

The chroma signals are frequency modulated at low frequency (1 MHz) subcarriers, while the luminance is modulated into the 5-10 MHz (to gain some signal to noise ratio), before feeding into the helical scan heads.

Originally and for compatibility, the audio was recorded on longitudinal track(s) on the side of the track. The HiFi recorded the FM modulated audio using helical scan, as the video.

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Michael A. Terrell 16 years ago

Some of us still have parts. I also have a 70 MHz microwave IF link analyzer and a Microdyne C-band test set for testing and analyizing C-band equipment.

Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge.

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