Jitter method for serial PMDs
I think there are some issues with the jitter methodology that need some
discussion.
1. 1310nm test channel. I have asked two major fiber companies about the
availability of worst-case fiber in the 1310 region. Basically you need
fibers with a zero-dipsersion wavelength close to 1300nm and 1324nm. The
answers I received do not indicate that this is something that you could
buy in the market place. Now, one could argue that the method works as
specified, and that this is an implementor's problem. However, this
could make it quite difficult to test modules for compliance which I
think we want to avoid if possible.
2. RJ frequency spectrum. As currently written, the RJ "shall have a
uniform spectral content over the measurement frequency range of 40 kHz
to 5 GHz." The low frequency jitter is tested with sinusoidal jitter of
much larger amplitude than the specified RJ corresponds to, so what do
we gain by requiring the RJ to go down to 40 kHz. Is not the
mid-frequency range the most relevant for RJ testing. Very high
frequency RJ would not infuence the PLL significantly.
3. In a number of a the jitter sections it is stated that a PLL is not
strictly necessary to do the test, suggesting that you could use the
same clock source to synchronize the transmitter and the measurement
set-up. This could actually cause problems, because you could have
exactly the same jitter on the transmitted signal and the clock used to
trigger your measurement set-up. If there for some reason have is an
oscillation at e.g. 10 MHz in your "master" clock, that jitter would be
cancelled out in a measurement without a PLL. The way out of this is to
characterize the clock separately and then add the jitter of the clock
to the measured jitter. I think the cleanest way to write the standard
is to specify the use of a PLL for _conformance_ testing (which is the
main purpose of the standard). Then people can use the other method in
their lab and production environments as long as they know they measure
the right thing and characterize their clock and compensate for any
clock jitter.
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