ultra-wideband FM
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From: Vladimir Vassilevsky on 14 Jul 2010 16:51 Joerg wrote: > Paul Keinanen wrote: > >>On Wed, 14 Jul 2010 11:48:12 -0500, Vladimir Vassilevsky >><nospam(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. It depends. >>>>>Some part of the lower sideband >>>>>inevitably falls into the bandwidth of the useful signal. >> >>Are you referring to aliasing around zero frequency ? No. Think a little bit how PWM differs from FM. >>Looking at the problem purely in time domain, OK, looking at the problem purely in time domain: Think of PWM as if it is PCM which goes through the boxcar filter with the variable length (0...100% duty). The length of the filter varies with the instant amplitude of the signal. This creates nonlinear distortions. >> but I still do not >> understand why oversampling would be required. > > Don't worry, I don't either, so that makes two of us. I hope Vladimir > won't have our degrees pulled :-) Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: langwadt on 14 Jul 2010 17:15 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 if I remember correctly a theoretical 3rd order deltasigma moulator at 16x oversampling can make 60dB SNR thats "only" 16*300MHz = 4.8GHz -Lasse
From: Joerg on 14 Jul 2010 18:44 Vladimir Vassilevsky wrote: > > > Joerg wrote: >> Paul Keinanen wrote: >> >>> On Wed, 14 Jul 2010 11:48:12 -0500, Vladimir Vassilevsky >>> <nospam(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. > > It depends. > >>>>>> Some part of the lower sideband >>>>>> inevitably falls into the bandwidth of the useful signal. >>> >>> Are you referring to aliasing around zero frequency ? > > No. > Think a little bit how PWM differs from FM. > >>> Looking at the problem purely in time domain, > > OK, looking at the problem purely in time domain: > > Think of PWM as if it is PCM which goes through the boxcar filter with > the variable length (0...100% duty). The length of the filter varies > with the instant amplitude of the signal. This creates nonlinear > distortions. > It would not be very smart to let this run 0%...100%. If you reduce the gain and place an offset onto it then you can, for example, go from 25%...75%. Much less BW-demanding and it preserves the chance to correct errors based on amplitude differences. Which is one of the key parameters here because you LD and PD and the electronics around them can't be of infinite BW and cost. The real cat's meouw would be if the source signal allow the introduction of the occasional sync preamble, where you could have a 0V ref level and a max volt ref level. [...] -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 14 Jul 2010 18:55 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? > Another one. Used it before (on cables) and totally forgot. Probably because I didn't have a margarita yesterday. Send it plain old AM but in sampled blocks. A higher amplitude (higher than the signal will ever be) is sent inbetween samples, or once every so many samples. This is also sampled but then used on the receive side just like the black porch signal is on an analog TV receiver. It is used to adjust the gain to a possibly varying attenuation over the fiber. The falling edge of these high amplitude reference pulses are also used to sync-lock the sampler on the receive side. In order to get amplitude variations out of the laser diode you could use it's back facet photodiode as a feedback element. This is the diode that is normally used for a "governor" to avoid distroying the laser diode, and it still should be. Photodiodes are very linear devices and whatever happens after the fiberoptics connector is calibrated out at the receive side. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Paul Keinanen on 15 Jul 2010 01:18
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. With 3 GHz bandwidth, the slope would be 100 ps and to reach 3 ps accuracy, the amplitude accuracy requirement would be 1/30 of the pulse amplitude. With 30 GHz bandwidth and 10 ps slopes, the amplitude error and noise could be as high as 1/3 of the pulse amplitude. Thus, a practical system would require something between 3-30 GHz but definitely not 300 GHz BW. >if I remember correctly a theoretical 3rd order deltasigma moulator at >16x oversampling can make 60dB SNR >thats "only" 16*300MHz = 4.8GHz That would fall into the same ballpark. |