Re: EMI, Pigtails, and Isolators
Jay,
Great analysis! Very informative! We all need to focus more on this class of
discussion of PAM vs. Serial and Multichannel issues.
Best Regards,
Rich
--
jay.hoge@xxxxxxxxxxxxxx wrote:
>
> Good Morning All,
>
> I've been giving some thought to the 10G Serial TDM solution. If you look
> at OC-192 Xcvr's, the DFB's are typically biased to 1.5 - 2.0 mW min. facet
> power at room temp before adding modulation current. With lower bias
> currents, edges go to hell. Add modulation current, double for connector
> repeatability issues (assuming the use of a fiber stub) and you're looking
> at a drive at low temps of <60 mA....maybe as much as 100 mA, with a rise
> time of >50 pS. And this ~an inch from the bulkhead which will be full of
> mounting holes. The EMI implications, irrespective of data patterns, will,
> I believe, require pigtailing.
>
> Moreover, unless people are willing to convert to APC connectors throughout
> the system, the back reflection from PC polished connections means we'll
> have to deal with an ORL of -14 dB. The out of phase return signal will act
> as a seed oscilation in the lasers cavity, killing your optical SNR (bear
> in mind that the grating's wavelength is constantly changing as a function
> of junction temperature: single mode is sort of reletive). The answer to
> this is to incorporate a Faraday Rotator and polarizer in the optical path.
> While not particularly expensive in terms of component cost (~$20),
> isolators are notoriously fiddly to set up and don't lend themselves to
> automation. We once built a connectorized isolated laser and found that the
> ORL varied by 12 dB at a fixed temp, so we abandoned the idea. Again....a
> pigtail.
>
> EMI should be sustantially reduced using PAM-5; first because of the
> halving of the frequency and second because of the reduction in dI/dT owing
> to the reduced current swing per baud. The reduction in frequency means
> that biasing can be reduced as the rise time requirements are halved. ORL
> may still be a problem and an isolator may (probably will) still be
> required.
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