ultra-wideband FM
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From: quiettechblue on 16 Jul 2010 22:31 On Tue, 13 Jul 2010 15:56:03 -0500, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > > >Joerg wrote: > >> I think Paul's suggestion to do PWM was right on. > >It depends. John didn't post any requirements except bandwidth. > >> John, IMHO math tools are less important here. > >Math is very important there, as it will clearly illustrate the >limitations of PWM or FM. > >> What I feel is more important is to find the >> parts that can do the job with enough margins. > >Besides the parts nonideality, there are fundamental problems. > >> >> I've seen FM ones but those are 10MHz BW, or less, usually. > >Well, analog VCRs record FM signal to tape (THD ~ 1%, SNR ~40dB). You >only have to do x50 frequency upscaled version :-) > >VLV It seems that only the audio part was FM and only on later models. http://en.wikipedia.org/wiki/VHS
From: quiettechblue on 16 Jul 2010 23:09 On Tue, 13 Jul 2010 22:00:27 -0500, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > > >Joerg wrote: > >> Vladimir Vassilevsky wrote: >>>Joerg wrote: >>>>Vladimir Vassilevsky wrote: >>>> >>>>>For ~10 bit accuracy, the PWM rate must be ~20 times higher then the >>>>>highest frequency of the signal. >>>>> >>>>Why that high? I bet John would spring for more than an RC lowpass >>>>here :-) >>>> >>> >>>Even if he puts ideal brickwall, that won't help. >> Of course not, but the rate doesn't have to be 20 times higher. >> Again, how do you arrive at 20 times? > >PWM is a kind of angular modulation. As such, it creates infinite >sidebands on both sides of the carrier. Some part of the lower sideband >inevitably falls into the bandwidth of the useful signal. 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. > > >>>>(This accounts for negative feedback in transmitter. Without NFB, the >>>>>results are going to be several times worse). >>>> >>>>Well, yeah, it would have to be some, as John called it, one-shot on >>>>steroids. This will not be very accurate and linear so it needs to be >>>>servoed at the transmitter. >>> >>>Loop cutoff ~ PWM rate/8. Do the math. >>> >> This part has to be a little more nifty than just a simple loop. Ideally >> there should be a characteriziation plus tempco-LUT so you can "steer" >> the PWM generator to the points where the signal wants it to, with >> little residual error. > >This could buy some accuracy. But, still loop cutoff ~ PWM rate/8. Or, >if you want to really push phase margin, PWM rate/6. Consequently, the >feedback is going to be quite shallow and it can't be the cure for all >sins of PWM. > >>> >>>Even John Larkin can't exceed physical limits :-) >>> >> >> Why not? The guys that designed the 1000W PMPO amp powered by a wall >> wart must have, at least on paper :-) > >Not that I support this sort of audiofoolery, but there is a bit of >sense there. For undistorted audio, there is generally no need for an >audio amp to sustain power more then ~1/8 of the peak power. > > 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.
From: Paul Keinanen on 17 Jul 2010 01:25 On Fri, 16 Jul 2010 19:31:08 -0700, quiettechblue(a)yahoo.com wrote: >On Tue, 13 Jul 2010 15:56:03 -0500, Vladimir Vassilevsky ><nospam(a)nowhere.com> wrote: > >> >> >>Joerg wrote: >>> I've seen FM ones but those are 10MHz BW, or less, usually. >> >>Well, analog VCRs record FM signal to tape (THD ~ 1%, SNR ~40dB). You >>only have to do x50 frequency upscaled version :-) >> >>VLV > >It seems that only the audio part was FM and only on later models. > >http://en.wikipedia.org/wiki/VHS 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.
From: Michael A. Terrell on 18 Jul 2010 16:01
quiettechblue(a)yahoo.com wrote: > > 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. 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. |