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Re: Equalization


Yes, this data would be very useful. The question is, if the DMD
response is truly time-variant, what is the time constant of this
variation relative to the baud rate.

Abhijit Phanse

ps: A reponse of a random stream of pulses, oversampled to the
extent permitted by the sampling oscilloscope, as a function of time
(on a bad DMD fiber) would be very useful.

Jonathan Thatcher wrote:

> Vipul,
> Not a punishment, just something I've been noodling over and wasn't sure if
> we were ready to ask.
> Specifically, I have been curious about the potential for the DMD response
> (assumes no launch control) to be non-deterministic. In short, what happens
> to that thing we used to call "modal noise" in a DMD-sensitive system?
> It seems likely that a simple experiment could be created if one had a BERT,
> Scope, LX transceiver, and a spool of the "bad DMD fiber" to see if the
> general shape of the pulse into the Rx would change under various types of
> "abuse." I'm not talking about dynamic changes like wrapping it around a
> mandrel or anything that would not be "typical" in an installed
> infrastructure. Just fiber blowing in the wind, the temperature of the Tx
> changing... stuff like that.
> Now, you could argue that these types of things would be "random" in nature.
> Even so, if the response of the system to these changes is significantly
> non-linear, then the effect could end up unmanageable. I hope otherwise.
> jonathan
> >-----Original Message-----
> >From: Vipul Bhatt [mailto:vipul.bhatt@xxxxxxxxxxx]
> >Sent: Thursday, July 20, 2000 5:25 PM
> >To: Jonathan Thatcher; HSSG
> >Subject: RE: Equalization
> >
> >
> >Jonathan,
> >
> >If I didn't know better, I would have thought you are punishing me
> >for starting this thread!
> >
> >Actually, this one is easy, almost elementary. (I suspect you read
> >"impulse response" as "impulse", and  the 7th cup of coffee made you
> >quickly pull the trigger...)
> >
> >Still, here goes. Simple models of communications links assume the
> >channel to be linear, time-invariant. "Channel" is copper cable,
> >fiber, air, whatever. In time domain, we describe its behavior with
> >an impulse response. If this impulse response, the h(t) curve, is
> >the same no matter when you look, we call it a time-invariant
> >response. The impulse response of a singlemode fiber, for example,
> >can be called time-invariant. If not, it's time-variant. (Purists,
> >please cut me some slack; I know, nothing is truly time-invariant.)
> >If it is time-variant, we have to ask if it varies randomly or
> >deterministically.
> >
> >My contention was that a multimode fiber with DMD can be viewed as a
> >channel whose impulse response is changing with time in a random
> >fashion. Hence the phrase "randomly time-variant impulse response".
> >
> >Regards,
> >Vipul
> >
> >=============================
> >
> >
> >> -----Original Message-----
> >> From: Jonathan Thatcher
> >> [mailto:Jonathan.Thatcher@xxxxxxxxxxxxxxxxxxxx]
> >> Sent: Thursday, July 20, 2000 3:07 PM
> >> To: 'vipul.bhatt@xxxxxxxxxxx'; HSSG
> >> Subject: RE: Equalization
> >>
> >>
> >> Vipul,
> >>
> >> You have really caught my attention on this one. What
> >> exactly is a "
> >> time-variant impulse?"
> >>
> >> jonathan
> >>
> >