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let's see a demonstration of automatic
jitter analysis used using the jitter
wizard that provides software clock
recovery
so we have here a serial data signal you
can see if i just press stop
that you have edges in various positions
various width pulses
are these each of these edges in the
right timing position we really don't
know
we need to compare them to a reference
clock but there's no
real reference clock it's embedded in
the serial bus signal so we can use the
jitter analysis
to recover a clock in software
then do a comparison
of every edge find out how much timing
error there is
in order to do jitter analysis so i'm
going to go ahead and press run
and i'm going to get
thousands of edges on screen now right
now i'm at 500 microseconds per division
how many edges are there i'm not really
sure
let's make a measurement
i'm going to select
a pulse count measurement
how many positive pulses are there
well there's about
six and a half thousand that means
there's about 13 000 edges that we're
going to be
be performing timing measurements on
so
let's go into the jitter menu there's a
dedicated key on the front panel it says
jitter
and then this is called the jitter
wizard it assists you in setting up a
jitter measurement
the first thing we can select is
measurement thresholds
and we'll just use the defaults we'll
perform algeria
measurements relative to 50
then we can add a measurement
and there's various different types of
jitter there's data tie tie stands for
time interval error
clock jitter
in period jitter period period jitter
the most common is data tie so it's
going to measure
the timing error on all the data edges
next it automatically went to the clock
recovery menu
we can do this relative to a constant
frequency clock
or a first order pll phase lock loop or
a second order pll let's just do the
constant frequency clock say okay
okay
now how do we want to view the jitter we
can select measurement histogram
measurement trend
and jitter spectrum those are the three
different views that we have of
jitter so
let's press stop
and now
let's take a closer look at these three
different views of jitter
let me spread this out here you can see
the data edges again this purple
waveform here
is the jitter trend waveform it's
plotting timing error on the vertical
axis
versus time same time as the serial
signal on the horizontal axis and right
away i can see from this that we have
some sinusoidal
modulation going on on our signal
and also the sort of bluish plot down
here this is our histogram it plots it
statistically
and i can see that it's bimodal which
sinusoidal jitter or modulation would
give you this
bimodal distribution of jitter
if we had just random jitter you'd see
what would look like is a
gaussian distribution
the third trace is the jitter spectrum
let me change the scaling on that one so
we can see it more clearly
and i've made some measurements ahead of
time and i know this sinusoidal
modulation
is right around 20 kilohertz
and let me turn
this signal off
this
is the spectrum waveform and if we do a
search on it
frequency peaks
it finds a peak at exactly 21 kilohertz
with an amplitude
of 10.4 nanoseconds now
we could correlate that 10.4 nanoseconds
of jitter
back
to this
measurement trend waveform
and there probably is some
correlation as well as we can go to the
back to the histogram menu
and we have all sorts of statistical
information we have about 53 nanoseconds
of peak to peak jitter we have a
standard deviation
of about 10.9
nanoseconds
we have a max
reading a min reading and so you can get
various
specific
analytical information about the jitter
and you get various views now let's take
one more view of the jitter
let me press run again
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let me turn my waveform on
and let's look at this as an eye diagram
and with the eye diagram again we could
change the clock recovery if we like
let's just leave it where it was
and let's select auto setup
and there is our eye diagram
you can see the jitter
right away in the eye diagram relative
to the recovered clock
and it also also automatically measures
eye height
and eye width
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