ultra-wideband FM
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From: John Larkin on 13 Jul 2010 11:29 Hi, One of the nasty things about cheap fiber-coupled lasers is that they have terrible amplitude stability and linearity, full of mode jumps and such. Given that, sending a signal over a fiberoptic link using amplitude modulation is usually done with a stable CW laser feeding a lithium-niobate modulator. The modulator itself is nonlinear and expensive and a nuisance to drive and bias. Digitizing and sending samples is OK, up to a point. It gets messy at some 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 frequency that's reasonably easy to work with, say 1 GHz, it could maybe be done. The laser driver and receiver aren't too difficult. The issues are the modulator, the demodulator, and the pure signal theory necessary to turn the time-domain behavior of the link into classic measures 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 sort of fractional modulation bandwidth. And if they did, a varicap modulating an LC oscillator would probably distort like mad. (Faint echoes of the capacitor charge debate?) The modulator may have to be some EclipsLite version of a 555 on steroids. Or a multi-GHz VCO heterodyned down. Yuk: sounds like RF. On the theory side, does anyone know of (or have?) one of the high-end math tools that could do a quantitative signal-quality analysis of such a link, given, say, approximate experimental data on the time-domain behavior of the laser link? Hiring a consultant to do this would be a desirable alternate to getting and learning this stuff ourselves. Any thoughts? John
From: Tim Wescott on 13 Jul 2010 11:43 On 07/13/2010 08:29 AM, John Larkin wrote: > > > Hi, > > One of the nasty things about cheap fiber-coupled lasers is that they > have terrible amplitude stability and linearity, full of mode jumps > and such. Given that, sending a signal over a fiberoptic link using > amplitude modulation is usually done with a stable CW laser feeding a > lithium-niobate modulator. The modulator itself is nonlinear and > expensive and a nuisance to drive and bias. > > Digitizing and sending samples is OK, up to a point. It gets messy at > some 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 frequency > that's reasonably easy to work with, say 1 GHz, it could maybe be > done. The laser driver and receiver aren't too difficult. The issues > are the modulator, the demodulator, and the pure signal theory > necessary to turn the time-domain behavior of the link into classic > measures 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 sort > of fractional modulation bandwidth. And if they did, a varicap > modulating an LC oscillator would probably distort like mad. (Faint > echoes of the capacitor charge debate?) The modulator may have to be > some EclipsLite version of a 555 on steroids. Or a multi-GHz VCO > heterodyned down. Yuk: sounds like RF. > > On the theory side, does anyone know of (or have?) one of the high-end > math tools that could do a quantitative signal-quality analysis of > such a link, given, say, approximate experimental data on the > time-domain behavior of the laser link? Hiring a consultant to do this > would be a desirable alternate to getting and learning this stuff > ourselves. > > Any thoughts? Frequency modulate what? I assume you're going to frequency modulate the 1GHz carrier, then amplitude modulate the laser with that -- correct? I think the most important "high-end" math tool in this case is the consultant's brain -- Scilab you can get off the web for free and go buy a nice car with the money you would have spent to buy Matlab. With either Scilab or Matlab you still need a nice squishy pile of neurons that knows how to feed in the questions the right way and interpret the results. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
From: John Larkin on 13 Jul 2010 11:59 On Tue, 13 Jul 2010 08:43:13 -0700, Tim Wescott <tim(a)seemywebsite.com> wrote: >On 07/13/2010 08:29 AM, John Larkin wrote: >> >> >> Hi, >> >> One of the nasty things about cheap fiber-coupled lasers is that they >> have terrible amplitude stability and linearity, full of mode jumps >> and such. Given that, sending a signal over a fiberoptic link using >> amplitude modulation is usually done with a stable CW laser feeding a >> lithium-niobate modulator. The modulator itself is nonlinear and >> expensive and a nuisance to drive and bias. >> >> Digitizing and sending samples is OK, up to a point. It gets messy at >> some 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 frequency >> that's reasonably easy to work with, say 1 GHz, it could maybe be >> done. The laser driver and receiver aren't too difficult. The issues >> are the modulator, the demodulator, and the pure signal theory >> necessary to turn the time-domain behavior of the link into classic >> measures 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 sort >> of fractional modulation bandwidth. And if they did, a varicap >> modulating an LC oscillator would probably distort like mad. (Faint >> echoes of the capacitor charge debate?) The modulator may have to be >> some EclipsLite version of a 555 on steroids. Or a multi-GHz VCO >> heterodyned down. Yuk: sounds like RF. >> >> On the theory side, does anyone know of (or have?) one of the high-end >> math tools that could do a quantitative signal-quality analysis of >> such a link, given, say, approximate experimental data on the >> time-domain behavior of the laser link? Hiring a consultant to do this >> would be a desirable alternate to getting and learning this stuff >> ourselves. >> >> Any thoughts? > >Frequency modulate what? A carrier. With the baseband signal. That's how FM is usually done. > >I assume you're going to frequency modulate the 1GHz carrier, then >amplitude modulate the laser with that -- correct? Yup. The laser would actually run on/off at the (modulated) carrier frequency. > >I think the most important "high-end" math tool in this case is the >consultant's brain -- Scilab you can get off the web for free and go buy >a nice car with the money you would have spent to buy Matlab. With >either Scilab or Matlab you still need a nice squishy pile of neurons >that knows how to feed in the questions the right way and interpret the >results. Hence the option to have a consultant, a real RF signals guy familiar with the tools, furnish the neurons. John
From: Vladimir Vassilevsky on 13 Jul 2010 12:08 John Larkin wrote: > > Hi, > > One of the nasty things about cheap fiber-coupled lasers is that they > have terrible amplitude stability and linearity, > 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 frequency > that's reasonably easy to work with, say 1 GHz, it could maybe be > done. > On the theory side, does anyone know of (or have?) one of the high-end > math tools that could do a quantitative signal-quality analysis of > such a link, given, say, approximate experimental data on the > time-domain behavior of the laser link? Hiring a consultant to do this > would be a desirable alternate to getting and learning this stuff > ourselves. > > Any thoughts? 1. Bandwidth and throughput can be traded in many different ways, but there is no free lunch. 2. The bandlimiting, amplitude and phase distortion in the FM channel results in the nonlinear distortion of the modulating signal. This represents intractable problem except for the simple cases like a pure sine wave input. However the worst case estimates can be drawn. 3. Math tools: all that needed is a C compiler (or PowerBasic, if you like). Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Paul Keinanen on 13 Jul 2010 12:08
On Tue, 13 Jul 2010 08:29:45 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >Digitizing and sending samples is OK, up to a point. It gets messy at >some 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 frequency >that's reasonably easy to work with, say 1 GHz, it could maybe be >done. The laser driver and receiver aren't too difficult. Have you considered PWM (or power position modulation) ? Should be easier to modulate than high modulation index FM. |