Hi I'm Erin East and welcome to Scopes University. In this episode we're going
to learn how a diagram can be used to identify noise and jitter in your signal
that can ultimately cause errors in the data that you transmit.
I'm actually going to discuss this topic in two parts, so in this first episode we're going to
learn the basics of what makes up an eye diagram, why you want to use one to
analyze your clock signal and your serial data, and then how to quickly set
one up on the scope. In the next Infiniivision episode we'll analyze that eye
diagram even further by performing jitter measurements on it to figure out
how we can eliminate any errors or anomalies that we're seeing. Eye diagrams
are helpful in testing the physical layer fidelity of clock signals in
serial data. So what it is, is a layered view of each of the different bit
transition combinations and there are eight of those in total. So this provides
a composite picture of the overall quality of the physical characteristics,
like amplitude variations, timing uncertainties, and infrequent glitches.
The first thing we need to do before we set up the eye diagram is take a bit
rate measurement. So the bit rate is basically just a pulse width of each of
the bits. So you can see the signal I'm working with on screen here is serial
data with a lot of jitter riding on it. To figure out what's a bit rate of
the signal is I want to stop the acquisition, then I want to zoom out and
find one of the smallest pulses I can see in this signal and center that on
screen. Now I can go into my measurements menu and add a measurement. We're going
to add a bit rate measurement. Now I can see a rough estimate of what my ideal
bit rate should be: around 6.2 megabits per second. Now that we have this
measurement we're ready to get started with our eye diagram, but my first
question when I was learning about eye diagrams was: "how do you get it to look
like this with all the bit transitions layered on top of each other like that?
A normal trigger couldn't do that." The answer was clock recovery. So some
signals use an explicit clock which just can be driven right into one of the
channels of the scope, but other signals use an embedded clock. With an embedded
clock you have to have some way to de-embed or recover it, so that's when
you use clock recovery. So what the clock recovery actually does is calculate the
exact ideal bit rate of the signal that you're working with.
So let's take a look at some of the different clock recovery options that we
have to work with. So the first thing I want to do is turn off my measurements
and press Auto scale and then we can go into the analyze menu and in here we're
going to go to features and select real time eye, but we don't want to turn it on
yet. So we only want to select it once. Then we'll go into the clock recovery
menu and in here we can see the different data rate options that we have.
So there's fully automatic which you would want to use in the case that you
have no idea what the bit rate of the signal that you're working with is and
you want the scope to completely calculate that exact ideal bit rate for
you. Semi-automatic should be used when you
have an estimate of what that bit rate is and you want the scope to use that
estimate to figure out what the ideal bit rate should be. Manual is used when
you know that exact ideal bit rate that you want to work with. So since we have a
rough estimate of what the bit rate of the signal is we want to use the
semi-automatic option, so the scope will use that bit rate that we found in order
to calculate the exact ideal bit rate of the signal. So in the nominal data rate
box we want to plug in that 6.2 mega bit bit rate that we just measured and we
can also see there are a few different methods of clock recovery as well so
there's constant frequency, first order and second order PLL, and explicit. So
again an explicit clock is when you have that clock signal that you're actually
able to plug into one of the channels. Constant frequency is what you use when
we're working with an embedded clock and we'll talk about the first and second
order PLL's in the next video. So let's leave it at constant frequency
and select okay and now all we have to do to set up the eye diagram is select
Auto setup and that's going to automatically set up the eye diagram in
the center of the screen. So the scope will set up that eye diagram centered
around that ideal bit rate that it calculated and it uses color grading to
display this information. So if we go into the color grade menu and select
show color key we're able to see just how frequent or infrequent the signal is
coming through certain areas of the screen. So this will allow you to see if
your signal flipping bits correctly or if there are any delays or jitter. Having
jitter or glitches in your signal can be bad because that means when the data is
transmitted it can be interpreted wrong on the receiving
end because the bits aren't coming in correctly. So you're actually going to be
getting the wrong data on the receiving end.
Notice when we opened up the eye diagram we got two custom measurements that
popped up in the sidebar: the eye height and the eye width. So this shows you the
largest amplitude and the widest opening of the eye and you can verify and
compare that with your specifications for the signal and debug if you aren't
seeing what you expected to. We can take a look at the statistics of those
in the measurements menu by turning on the statistics display and we can see
the mean of those measurements. So this is going to give us a better
understanding of what's happening on average in this signal and we can learn
more information from this eye by really quickly bringing up a histogram. To do
that we just draw a box on the screen and select histogram. So what is this
histogram actually mean? Well you'll have to find out in the next episode. So
we just learned some of the basics of what makes up an eye diagram, why you
want to use one to test your signal, and how to set it up on the scope. In the
next episode Infiniivision episode we'll be doing an even deeper analysis of this
eye diagram using the jitter measurements to figure out what might be causing the
anomalies that we're seeing in this signal. Thank you for joining me! If you
want to learn a little bit more about eye diagrams and how to do mask testing
with them make sure to check out the application note Can Eye Diagram Mask Testing.
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