ultra-wideband FM
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From: Joerg on 13 Jul 2010 18:47 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. >>> Generating 10-bit linear ramp at 2 GHz is nontrivial. >>> >> He needs only 150MHz signal bandwidth. > > 150 x 20 = 3000 > Again, how do you arrive at 20 times? >> >>> (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. >> The loop filter in that servo won't exactly >> be trivial but looking at their product portfolio just about everything >> they build pushes the envelope a bit farther. > > 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 :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 13 Jul 2010 18:52 Jan Panteltje wrote: > On a sunny day (Tue, 13 Jul 2010 14:10:46 -0700) it happened Joerg > <invalid(a)invalid.invalid> wrote in <8a432sFuidU1(a)mid.individual.net>: > >>>> 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 :-) >>> >> As long as it doesn't spit out the cleaning tape like ours kept doing >> yesterday. Looks like another weekend honey-do project :-( > > I digitised my VHS tapes about ten years ago. > If you care about any of your videos you should do the same, > and put the VHS with the trash. > Some people claim tapes last longer then DVDs .. I have seen CDs and DVDs that have delaminated. The last (expensive) DVD we lost began to grow a blue cloud in the middle. It played to about half the movie, then cut out. Most probably last a long time but if they do delaminate it can happen within months. Tapes all lasted >10 years. Some are 20 years and still fine. > But I have seen too many damaged tapes and dropouts. > If yoy digitise with the PC, then you can also play with the PC, > PC as media center. A PC in the living room? No way. SWMBO won't be happy about that. We have structured wiring so we could put it in the basement but then I'd have to design an RF remote that works in our RF-unfriendly house. > My 10 year old CDs with DivX and all my DVDs I burned still play fine. > next is burn it to flash, an also to a huge harddisk. > That is why I have the 1TB Seagate external, most movies just a few keys away. We don't watch many anyway. Maybe an old Humphrey Bogart, or a Western. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Bill Sloman on 13 Jul 2010 19:35 On Jul 14, 1:29 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> 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. http://www.onsemi.com/PowerSolutions/parametrics.do?id=586 The ECLips range of digitally programmable delays might be what yoou had in mind. Most of them have an analog bandwidth of up to 1GHz, up to 1.5GHz for the faster ones. The maximum programmable delays vary from something like 2nsec to to 10nsec, and you probably don't want to reprogram the delay while there is an edge propagating through the device. Putting a couple in parallel and combining the outputs isn't going to be an option - production tolerances are large, and the delays you get for a given digital input can vary quite a lot with temperature. You were probably thinking of the MC10198 http://www.onsemi.com/pub_link/Collateral/MC10198-D.PDF which still seems to be being made, if not recommended for new production. Minimum output pulse width is 10nsec, which isn't entirely compatible with your 1GHz carrier frequency. It is a nice part, and it is reasonably easy to manipulate the monostable period - see figues 4 and 5 on the data sheet. <snip> -- Bill Sloman, Nijmegen
From: John Larkin on 13 Jul 2010 20:52 On Tue, 13 Jul 2010 16:35:27 -0700 (PDT), Bill Sloman <bill.sloman(a)ieee.org> wrote: >On Jul 14, 1:29�am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> 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. > >http://www.onsemi.com/PowerSolutions/parametrics.do?id=586 > >The ECLips range of digitally programmable delays might be what yoou >had in mind. Most of them have an analog bandwidth of up to 1GHz, up >to 1.5GHz for the faster ones. Actually, no. They would require me to digitize the signal first, which is what an FM scheme would avoid. Once it was digitized, I may as well just ship the data. John
From: Phil Hobbs on 13 Jul 2010 22:57
Joerg 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 build current-tuned diode laser gizmos from time to time, and they work great for many things. The main difficulty is that the output power as well as the optical frequency changes with the bias current, which constrains the available detection mechanisms. (Laser noise cancellers do amazing things at moderate speeds, but not in the gigahertz.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net |