Hi my name is Greg LeCheminant and I work for Keysight Technologies, specifically in
the group that produces sampling scopes, commonly known in the industry as the 86100 DCA-X. Today
I'd like to talk about how we can configure a DCA to do PAM-4 measurements.
When we think about configuring a DCA to do PAM-4 work, there's really two things we're concerned
about. How do we configure the hardware in the DCA to be able to accept a PAM-4 signal,
and the second thing that we worry about is, how is it that we actually make the measurements
on the PAM-4 eye diagram.
So first let's talk about hardware. When we're configuring a scope to make a measurement,
we often ask, what bandwidth do we need to make a measurement? The DCA is capable of
several different bandwidths depending on the plug-in module that you put into the mainframe.
But, essentially we've got plug-ins that do 20 GHz, 30 GHz, 50 GHz, even up to on the
order of 80 GHz bandwidths. What is going to be appropriate for your PAM-4 measurement?
When we think about a PAM-4 signal, for example take a PAM-4 50 Gbps signal, the data rate,
50 Gbps, but the key thing in terms of bandwidth for the DCA is to think in terms of how fast
that signal is switching, what the baud rate is. So a 50 Gbps PAM-4, would be a 25 Gbaud
signal. So 25 Gbaud has a fundamental frequency of 12.5 GHz. If we were want to try to look
at the 3rd harmonic of that, we'd be on the order of 37 GHz, even up to the 5th harmonic,
you're a little bit above 60 GHz. So, in terms of configuring a bandwidth in the DCA to make
that measurement, we have a lot of flexibility and plenty of range to make those measurements,
even up to 50 Gbps.
In the optical space, they're also going to be doing PAM-4 modulation at 50 Gbps, but
the way bandwidth is defined for optical measurements has historically been a little bit different.
For example, if you had a 10 Gbps NRZ signal, the bandwidth of that was set by what's called
a reference receiver where the bandwidth is 75% of the data rate (7.5 GHz). For 25 Gbps
NRZ signal, we had 75% of that, 19 GHz bandwidth. What's going to happen when we go to PAM-4
in terms of optical reference receivers? Well, the standards haven't defined it yet, but
what we anticipate is going to happen is, just like we described with electrical signals,
that it's not the bit rate that counts, but rather the baud rate. So a 50 Gbps NRZ optical
signal would have a baud rate of 25 Gbaud. The reference receiver for that we'd expect
would be roughly 75% of that baud rate, which would be 19 or so GHz. Effectively what happens
is the very same reference receiver that we used for 25 Gbps is likely to be the reference
receiver that would be used for a 50 Gbps PAM-4 signal.
So, in terms of hardware channels, really then, the hardware that we have today for
NRZ signals is going to be applicable to the PAM-4 signal.
There are certain measurement scenarios where clock recovery is required. And, in a sampling
scope that is done in hardware, not software. So, we've got different scenarios. We've got
external accessories that do clock recovery, we have modules that have clock recovery built
in - what would we have to do to change that clock recovery hardware to work with a PAM-4
signal? Turns out, not much. The hardware responds to edges, the edges come through
in a PAM-4 signal similar to the way that they did in an NRZ signal. Really the only
difference is that we have to tell the DCA not what the bit rate, but in this case the
baud rate. So, the range of our clock recovery system will go to 32 Gbps for NRZ, and effectively
64 Gbps for PAM-4, but we have to tell the DCA in that case what the baud rate is, which
is, for PAM-4 half of the data rate.
Finally, in terms of making the analysis on the PAM-4 waveform, that's also quite simple.
Essentially what we have to do is give the DCA sort of a slightly modified personality,
a new measurement suite that is PAM-4 aware. And, with PAM-4, there's essentially a license
that we install as well as a firmware upgrade, gives it a new set of PAM-4 measurements.
Many of the PAM-4 measurements that are made on that eye diagram are similar to what was
done on the NRZ eye. Things like eye width, eye height...except that if you notice here,
instead of an eye width, there are 3 eye widths, 3 eye heights. So, when you press the eye
width measurement, it reports 3 values.
Also, as the standards have developed, there are some new measurements that are specific
to the PAM-4 signal. We've been trying to be involved in all those standards committees
and evolve and adapt as the new measurements are required and those have been built in
and are up-to-date with our PAM-4 analysis capability.
So, the bottom line is, if you're going to be encountering PAM-4 measurement needs, the
DCA is ready to go today. The hardware is applicable as it stands, just add the new
measurement capability, you'll be good to go, ready to make PAM-4 measurements.
Thanks for your time. Good luck with your PAM-4 measurements.