Ben Miller Hi, everyone.
I'm Ben Miller and this is Life in the Fast Lanes.
In this episode, I'm talking to Mike Beyers, product planner for Keysight's High Performance
oscilloscopes about the migration from non-return-to-zero, or NRZ, signaling to pulse amplitude modulation,
PAM, or multi-level signaling.
Ben Miller With the increasing demand for higher data
rates in wired communications, NRZ systems are starting to show their bandwidth limitations.
So pulse amplitude modulation has started to arise as a really popular alternative to
that because of its higher data rates and spectral efficiency.
However, there are some challenges involved with migrating from an NRZ signaling standard
to a PAM four signaling standard.
I'll be talking to Mike about those challenges and how to overcome them, as well as all the
different benefits of migrating from NRZ to PAM systems.
So without further ado, I'd like to throw it over to Mike.
Mike Beyers Hey, Ben.
Yeah, sure.
This is Mike Byers.
I am the Infinium product planner for Keysight Technologies.
In particular, that makes me responsible for future product definition and application
definition for Keysight's UXR-Series of oscilloscopes.
Ben Miller So let's just jump right into it.
What do you think is the real challenge for using NRZ today?
And why are standards moving away from it?
Mike Beyers Sure.
So if we go back to the very beginning of high speed serial with PCI Express gen 1,
it was 2.5 gigabit per second NRZ, and that required a 13 GHz oscilloscope.
So we had lots of bandwidth relative to the baud rate that we were trying to push through
a channel.
As data rates have gone from 2.5 gigabits per second per lane on PC Express that I'm
using as an example, all the way to the newest generation, PC Express Gen 6 is 64 gigabits
per second.
So scaling that bandwidth, right, fundamentally, the channel is not that different.
So we're just not going to get the hundreds of gigahertz of bandwidth it would require
if we kept that same ratio of bitrate to bandwidth.
So we have to look for how to put a different way to put more bits through that, through
fundamentally the same set of connectors and technology.
Ben Miller And what's the difference between NRZ and
PAM encoding?
Mike Beyers Sure.
Well, technically, NRZ is PAM, right?
It's pulse amplitude modulation, but it's PAM2.
It's two levels.
PAM takes that further and uses PAM four, uses four different voltage levels.
PAM8 uses eight different voltage levels.
And so we use those extra levels to get more data using the same symbol rate.
But there's a limit, right, that as we go to double the number of levels from NRZ or
PAM2 to PAM4, we go from one bit per symbol to two bits per symbol, which is great, but
if you go to PAM Eight, now you have eight levels, but you only get one more bit, right?
You quickly run out of signal to noise ratio as you try to use more and more levels of
PAM in order to get more bits.
Mike Beyers Yeah, so really it's about channel cost, right?
If we had optical or something, we'd have many terahertz of bandwidth, but really it's
about that using more levels to shove more bits into the same baud rate.
We don't want to go to more expensive board materials, more expensive connectors.
We want to live with the same inexpensive channel.
Ben Miller So what challenges are engineers going to
face as they move from NRZ to PAM in terms of signal processing requirements?
Mike Beyers So, increasingly, the baud rates are pushing
closer and closer to the channel bandwidth i.e., to that fundamental limit of how much
we can get through a channel that's described by the Nyquist criteria, right?
As we push closer to that, our eyes get closed, more closed, and so we end up applying more
equalization, more processing to be able to reliably extract the data bits out.
Beyond that, even after all the processing we do, it's still not perfect, right?
So in the old days, we expected the transmission to be almost error free.
In today's world, often or almost always when we're using PAM encoding, we're pushing so
close to the limit that not only do we have to use a lot of equalization to open the eyes
and extract the data, but we have to use error correction as well.
So the fundamental data error rate we're shooting for is no longer 1e-12 or 1e-15 but maybe
1e-4, or in some cases, 1e-6, in some cases as low as 1e-2.
But error free transmission is no longer the goal that we require.
At least without the forward error correction, the FEC coding.
Ben Miller So are some of those challenges because of
the increased sensitivity to noise that's inherent with PAM modulation?
Mike Beyers Yes, right.
It's really both those things together.
So smaller signal height, more voltage levels, equals a lot less spacings between our levels.
But the other part is the eyes closing due to the channel bandwidth.
And so there's a variety of equalization techniques used to try to recover that eye.
To open that eye back up.
There's FFE (feed forward equalization) typically used in the transmitter But then also in the
receiver, there's CTLE (continuous time linear equalization) as well as DFE discrete feedback
equalization, which is a nonlinear equalization technique.
The advantage of the nonlinear equalizer, the DFE, is that it can open the eye without
adding noise.
One of the disadvantages though, is that it's relying on, it's making the decision of what
symbols are which correctly.
So as you use long DFE equalizers, if you get one answer wrong, it can make the next
answers wrong as well.
And that poses a challenge for the forward error correction.
Forward error correction works great for randomly distributed errors.
When you get bursts of errors, it can fail.
So that's one of the things that we're looking for is not only errors and how often the errors
are in the traditional bit error rate, but also anytime there's bursts of errors that
can lead to FEC failure or what we call frame loss, it's no longer bit error rate, but it's
frame loss or frame loss rate.
Ben Miller So does that lead to some of the timing and
synchronization issues with PAM?
Mike Beyers Yeah.
So if we think about jitter, we've been talking about the closure of the eye vertically, but
jitter is the closure of the eye horizontally, which is still a challenge.
As we are, we're talking about 32 gigabaud signals and above.
Jitter fundamentally has two contributions.
There's intrinsic jitter of a signal that is due to say, the clock, right?
Then there's the jitter due to noise.
And the faster the Slew rate, the less the noise matter.
Right?
That second contribution is really the noise divided by the slew rate.
So if we had infinite slew rate, it wouldn't be a problem.
But again, because we have limited bandwidth, we have a limited slew rate.
And so therefore this noise over slew rate contribution, especially in PAM, is becoming
an increasingly important part in comparison to the intrinsic jitter that we found mostly
in NRZ.
Mike Beyers So again, normally when we talked about jitter,
we thought about jitter separate from noise.
But now we have this new component of jitter in our PAM signals that we really need to
think about much more about the noise and the signal noise ratio in our systems.
Ben Miller So how do the test and measurement challenges
differ between NRZ system and the PAM system?
Mike Beyers Well, the old way we used to make jitter measurements
was apply a defined equalizer, a reference equalizer inside the oscilloscope, and then
make the jitter measurement on the open eye.
But in today's world, the specification doesn't want to talk as much about exactly what the
equalizer is.
And so they require the jitter measurements to be made on the signal without equalization
applied i.e the closed eye.
So how do you make jitter measurements on a closed eye?
Or noise measurements on a closed eye?
And these are some of the new measurements you find for PAM, including twelve edge jitter
we call it twelve edge jitter because we're measuring at twelve specific places in that
waveform.
If you think about it, we're going to start from a 0123 and we're going to go to, well,
one of the other three symbols.
So there's twelve possible transitions, right, et cetera.
You go through all the possible combinations, there's twelve possible combinations, and
we've called out in the specification twelve specific places in that pattern that we're
going to make this jitter.
So that's how we make that jitter measurement on that closed eye.
And then also from vertical noise perspective, we also make SNDR measurements.
So SNDR has been around a long time for analog signals where you put in a sine wave and then
you measure what you get the best fit sine wave versus the noise and distortion.
But this is SNDR relative to a digital pattern, very much the same techniques, but really
it's a new way of being able to measure the noise and distortion.
Both matter, right, because we're talking about a linear output on a closed eye.
Ben Miller So let's talk a little bit about some of the
standards that are using PAM today.
Could you tell me what those are?
And do you think it's going to be a trend that more and more standards are going to
be adopting PAM in the future?
Mike Beyers Sure.
Where it started was the very fastest signals, the ethernet signals, the IEEE 802.3ck on
the electrical side and the OIF equivalent on the optical side.
That's a 53 gigabaud PAM four signal for 100 gigabit per lane, ie. 400 gigabit Ethernet.
We've also already mentioned PCI Express Gen 6 is moving to PAM4, USB is moving to PAM3
with USB4 Version 2.0.
Beyond that, various DDR and MIPI standards are also talking and starting investigating,
moving to either PAM3 or PAM4 type symbols or signals.
And then last but not least, there's a variety of research going on where people are looking
at even PAM6 and PAM8.
So we don't have any specifications calling those out quite yet, but those are supported
in our Uxrb software if you're in that arena.
Last but not least is the coherent optical communication space.
It's completely different in the sense that it uses quadrature amplitude population (QAM),
which is really PAM in both the vertical and the horizontal axis.
So if you have a PAM4 I and a PAM4 Q signal, that creates 16 different possible combinations
in quadrature and that's QAM16.
Ben Miller So when we talk about transmitter testing
for like PAM signal transmitting, what are some of the key specifications or requirements
that you're going to need for an Oscilloscope when you're doing that testing?
Mike Beyers Sure, good question.
So I think first and foremost is bandwidth.
We've talked about trying to push an increasing amount of data through the same channel, so
you need enough bandwidth to see the output of the transmitter.
Take for example, the next generation Ethernet standards at 112 gigabaud PAM4.
That'll require an 80 GHz Oscilloscope to view.
So the UXR-B, best-in-class bandwidth up to 110 GHz, can cover the most demanding standards.
But beyond that, the jitter, the noise, the things that close our eyes horizontally and
vertically are increasingly important.
The intrinsic jitter of an oscilloscope.
An oscilloscope has both a noise-induced jitter and an intrinsic jitter component, just like
the transmitters.
And the intrinsic jitter component of the UXR-B is 15 femtoseconds RMS.
So incredibly low.
As well as best-in-class noise, right, because especially as we're dealing with closed eyes
and whether or not we're applying equalization, we're looking at the end of a very lossy channel,
so there's just not much signal left.
So not only having enough bandwidth but having a low enough noise to be able to make those
measurements.
Last but not least, even with the best-in-class oscilloscopes, these measurements are a challenge.
So increasingly we look to compensate the noise of the oscilloscope.
It's always best to start with the lowest noise, widest bandwidth scope you can get,
but also then when it comes to the twelve-edge jitter measurement, the SNDR measurements,
we can try to compensate for the oscilloscope contribution to that noise, to back out a
bit of that contribution and get the customer some more test margin.
It's really an emerging measurement science area, and it's an area where you're going
to want to definitely get the most recent revision of the software for your Infinium
oscilloscope, because we're constantly making improvements on that.
The UXR-B has the bandwidth necessary for the highest, most demanding standards, the
lowest noise, the lowest jitter, to give you the most test margin on both your vertical
measurements like SNDR , and your horizontal twelve inch jitter measurements, as well as
the most recent, most advanced measurement science to compensate for the noise of the
scope, as well as have the processing speed to make the measurements these increasingly
complex, increasingly demanding measurements in, well, a reasonable amount of time.
Ben Miller Thank you again to Mike for his time today.
As you can tell from our discussion, the industry trend toward PAM signaling is not going to
end anytime soon.
So it's extremely important for developers and designers to learn how to design, measure,
and test using multilevel signaling and have the right tools to do so.
If you have any questions about how to do that, or about signal integrity and multilevel
systems, check out the resources below.
Until next time, this has been life in the fast lanes.