ultra-wideband FM
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From: Vladimir Vassilevsky on 13 Jul 2010 23:00 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. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: John Larkin on 13 Jul 2010 23:03 On Tue, 13 Jul 2010 17:03:27 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >On a sunny day (Tue, 13 Jul 2010 09:28:01 -0700) it happened John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in ><oo4p36hlsn8v9jrnm6jic5jk5uemrgjv8m(a)4ax.com>: > >>>Would not they use QAM maybe up to 64 constallation and a nice error correction scheme? >>>The data throughput would be much higher then with simpe FM / PM whatever. >> >>That would require digitizing the baseband signal on one end and >>DACing it on the other. That remains a possibility, but the numbers >>are intimidating. >> >>But as I mentioned, affordable fiber-coupled VCSEL or similar lasers >>are horribly nonlinear. Constellation-type modulations need a lot of >>linearity in the channel, and generally rely on error correction to >>push the channel limits. Lasers are usually used on/off, simple NRZ >>data. >> >>John > >OK, google gives lots of info, for example this on FM and its detection: > http://essay.utwente.nl/59384/ Interesting paper, thanks. John
From: Jim Thompson on 13 Jul 2010 23:12 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. > > [snip] Maybe. Having built more than a few in my audiophile lifetime, there's nothing like _real_ headroom. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Obama isn't going to raise your taxes...it's Bush' fault: Not re- newing the Bush tax cuts will increase the bottom tier rate by 50%
From: John Larkin on 13 Jul 2010 23:15 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. > That's what was done in part of the paper that Jan referenced. Looks messy to me. I understand pure digital data fiber links, and I understand A/D and D/A conversion, and I sort of understand FM. Combining a fiber data link with a digital or FM modulation scheme compartmentalizes the problem into managable chunks. But all that optical stuff is out of my league. >>> >>> 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? Preferebly without digitizing. At our bare-minimum 100 MHz signal bandwidth, and 10 bits, sampling at 250 MHz, just past Nyquist, we'd be shipping 2.5 gbits/second. That's feasible, but I thought we'd consider FM too, which would be simpler to do. > >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? We have a request from a customer to ship some wideband data over fiber. So we're thinking about it. It might turn into a standard product, too, especially if we make it accurate down to DC. Most fiber signal links are sort of ac-fuzzy... stick a signal in one end, get something sort of similar out the other. John
From: Paul Keinanen on 14 Jul 2010 01:13
On Tue, 13 Jul 2010 15:44:19 -0500, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > >For ~10 bit accuracy, the PWM rate must be ~20 times higher then the >highest frequency of the signal. > >Generating 10-bit linear ramp at 2 GHz is nontrivial. Why that high ? The input signal needs to be band limited to 150 MHz, then sampled (but not quantized) at something above 300 MHz (Nyquist), then feed it to a comparator with a 300+ MHz linear ramp and the output drives the LD. At receiver end, a sample&hold circuit is also required and a LPF. |