ultra-wideband FM
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From: Joerg on 13 Jul 2010 17:03 Vladimir Vassilevsky wrote: > > > Joerg wrote: > >> John Larkin wrote: >> >>>>> One of the nasty things about cheap fiber-coupled lasers is that they >>>>> have terrible amplitude stability and linearity, >>>>> So the idea of using FM pops up. >>> >>> Rob thinks I should just digitize the signal and ship it 8b/10b. > > Rob is right. > Doing a 5Gbps transmission is doable these days but a bit of a challenge. He'd have to digitize somewhere around 400MSPS, times 10 bits plus some sync overhead. >> >> If 10 bits is enough you could also just PWM, as someone had already >> suggested. > > 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 :-) > Generating 10-bit linear ramp at 2 GHz is nontrivial. > He needs only 150MHz signal bandwidth. > (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. 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. -- 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 17:10 Vladimir Vassilevsky 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. > This is where experiments come in. Lots of stuff such as LD behavior just cannot be modeled, on my first laser project I learned rather quickly to hang up those efforts and flipped the switch on the Weller. Same goes for the photodiode. Ok, we needed less than half of John's BW and used phase modulation but at a heck of a lot more than 10bits worth of dynamic range. And we got it. >> >> 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 :-( -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Vladimir Vassilevsky on 13 Jul 2010 17:12 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. >>Generating 10-bit linear ramp at 2 GHz is nontrivial. >> > He needs only 150MHz signal bandwidth. 150 x 20 = 3000 > >>(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. > 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 :-) Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Jan Panteltje on 13 Jul 2010 18:10 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 .. 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. 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.
From: langwadt on 13 Jul 2010 18:25
On 13 Jul., 18:08, Paul Keinanen <keina...(a)sci.fi> wrote: > On Tue, 13 Jul 2010 08:29:45 -0700, John Larkin > > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >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. > > Have you considered PWM (or power position modulation) ? > > Should be easier to modulate than high modulation index FM. or maybe delta-sigma? -Lasse |