ultra-wideband FM
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From: Joerg on 15 Jul 2010 10:33 Paul Keinanen wrote: > On Wed, 14 Jul 2010 14:15:22 -0700 (PDT), "langwadt(a)fonz.dk" > <langwadt(a)fonz.dk> wrote: > >> On 14 Jul., 22:16, Paul Keinanen <keina...(a)sci.fi> wrote: >>> On Wed, 14 Jul 2010 11:48:12 -0500, Vladimir Vassilevsky >>> >>> <nos...(a)nowhere.com> wrote: >>> >>>> Joerg wrote: >>>>> Vladimir Vassilevsky wrote: >>>>>> Joerg wrote: >>>>>>> Vladimir Vassilevsky wrote: >>>>>> PWM is a kind of angular modulation. As such, it creates infinite >>>>>> sidebands on both sides of the carrier. >>> So does FM, however, the Bessel function drops of quite rapidly even >>> with a high modulation index. >>> >>>>>> Some part of the lower sideband >>>>>> inevitably falls into the bandwidth of the useful signal. >>> Are you referring to aliasing around zero frequency ? >>> >>>>>> How much of >>>>>> trash gets into the signal? It depends. Ballpark: for 60dB of rejection, >>>>>> the PWM carrier should be ~ x20 times of the highest signal frequency. >>>>> I've done a _lot_ better than 60dB. >>>> How about doing some math before breaking physical laws? >>> Looking at the problem purely in time domain, for 60 dB headroom, you >>> should be able to measure the pulse width with an accuracy of about >>> 1/1000 of the pulse period. >>> >> doing so would be like 10 bits per pulse, so for 150MHz signal >> bandwidth you'd >> only need 300MHz pulse rate. >> >> making a 1/1000 cycle at 300MHz is 300GHz > > Some back of the envelope calculations: > > Assuming 333 MHz sampling rate (3 ns pulse repetition time), thus for > 60 dB (10 bit) accuracy, the pulse width would have to be measured > with 3 ps accuracy. > > A square wave low pass limited to about 300 MHz would have a rise and > fall time in the order of 1 ns. To reach the 60 dB goal, the threshold > stability and amplitude noise would have to be less than 1/300 of the > pulse amplitude. > That's only 50dB SNR. I guess John's fiber isn't going across the Atlantic :-) [...] -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: quiettechblue on 16 Jul 2010 21:53 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? > >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. By the way Tim; you should put up a site with [pointers to] current versions of SciLab and matching versions of your exercizes from your book.
From: quiettechblue on 16 Jul 2010 22:01 On Tue, 13 Jul 2010 08:59:09 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >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 recommend against on/off of the laser. Use bright/dim instead and servo versus temperature etc., to stay above critical current for dim and below max output for bright. And use "square wave" drive to the LD modulator. Hello fast edges. >> >>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: quiettechblue on 16 Jul 2010 22:09 On Tue, 13 Jul 2010 13:31:43 -0700, Tim Wescott <tim(a)seemywebsite.com> wrote: >On 07/13/2010 08:59 AM, John Larkin wrote: >> 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. > >Yes it is. I just wanted you to confirm that you weren't thinking of >modulating the color of the laser and calling it "FM". It's >theoretically possible, although it would probably be insanity to >attempt with today's lasers. > >>> >>> 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. > >It all sounds interesting. I assume (well, gather from other posts in >the thread) the ultimate goal is to be able to send nice clean analog >signals while dodging any need to digitize? > >IFAIK this is done: there are (or used to be) off-the-shelf fiber-optic >links that you can stick analog video into and get analog video out. I >vaguely remember a hand-waving explanation that included mention of a >high-frequency FM signal that then amplitude modulated the light. > >Are you trying to replicate one of these, only cheaper? Ditto, only >better? Reinventing the wheel because it feels good? Who knows? JL has yet to discuss the bandwidth of the modulating signal. Sorry, but i just reading the thread.
From: quiettechblue on 16 Jul 2010 22:14
On Tue, 13 Jul 2010 13:45:53 -0700, Joerg <invalid(a)invalid.invalid> wrote: >Tim Wescott wrote: >> On 07/13/2010 08:59 AM, John Larkin wrote: >>> 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. >> >> Yes it is. I just wanted you to confirm that you weren't thinking of >> modulating the color of the laser and calling it "FM". It's >> theoretically possible, although it would probably be insanity to >> attempt with today's lasers. >> > >Guilty. Done it, modulated the wavelength. But not at anywhere close to >the speed John would need although I wouldn't see what would have >prevented that if using a fat RF BJT or an LDMOS. > >>>> >>>> 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. >> >> It all sounds interesting. I assume (well, gather from other posts in >> the thread) the ultimate goal is to be able to send nice clean analog >> signals while dodging any need to digitize? >> > >I think Paul's suggestion to do PWM was right on. John, IMHO math tools >are less important here. What I feel is more important is to find the >parts that can do the job with enough margins. The only tool I used on >my last laser project was an HP11C. And that wasn't even mine, >technically it belongs to my wife, I just confiscated it around 15 years >ago :-) > > >> IFAIK this is done: there are (or used to be) off-the-shelf fiber-optic >> links that you can stick analog video into and get analog video out. I >> vaguely remember a hand-waving explanation that included mention of a >> high-frequency FM signal that then amplitude modulated the light. >> > >I've seen FM ones but those are 10MHz BW, or less, usually. > >[...] Well the old C-band satellite television system used 70 MHz FM IF per channel for a modulating signal of NTSC or similar. Some of the later models used 140 MHz FM IF. Used to be able to get parts. |