[SI-LIST] Re: Jitter measurement floor on different high bandwidth oscilloscopes
- From: Kirby Goulet <kgoulet@xxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 10 Jun 2010 03:20:40 -0700 (PDT)
Many years ago I saw an application note which suggested the input gain be
maximized when doing jitter measurements.
One problem with that is that some samples around the threshold crossing may
get clipped. The loss of signal information can then affect the interpolation
results, depending on the interpolation method.
As part of an open source signal analysis project, I did some basic analysis
for linear interpolation on the affect of slew rate, sampling frequency and
quantization. The notes are available
athttp://wavu.sourceforge.net/linear-interpolation-error2.html
Of course, the scope software is much more complex, but it does show how
complex a simple case can be.
I wonder if scope manufacturers could (or maybe already do) tune their sample
clocks up and down a few 100 ppm in order to help remove errors caused by the
relation between sample and signal frequency.
--- On Thu, 6/10/10, Robert Szumowicz <robert.szumowicz@xxxxxxxxxx> wrote:
From: Robert Szumowicz <robert.szumowicz@xxxxxxxxxx>
Subject: [SI-LIST] Re: Jitter measurement floor on different high bandwidth
oscilloscopes
To: si-list@xxxxxxxxxxxxx
Date: Thursday, June 10, 2010, 9:14 AM
I have been doing jitter measurements for a few years using several models
of scopes and a dedicated jitter analysis software as well by post
processing exported data captured by a scope. I made enough measurements to
reserve a distance before any conclusions from measurements are drawn. It
happened to me that measuring the same signal I got better results (lower
jitter) using a slower oscilloscope with declared worse parameters.
That is true that a signal slew rate and a scope noise floor affect
measurements, that's a reason why a signal (which has a different slew rates
at different points) virtually loses some amount of jitter as it travels
through components which should add some extra jitter.
Typically (as recommended in many jitter application notes) the vertical
scale of the scope should be scaled to maximize a dynamic range of the
sampler, the signal should reach both vertical limits of the scope. I do not
use this approach, I often scale the signal much more so it is clipped by
the scope, the signal is the vertically offset to show a middle portion of
edges (or any other point of interest, but typically we measure jitter at
50% of the amplitude). Doing this I reduce the effect of the noise floor of
the sampler and increase a slew rate of the signal seen by the sampler. I
wonder if anyone else tried this, probably not all scopes are suitable for
this technique. I have performed a number of measurements of the same signal
using different vertical scales and different sampling speeds, results are
really surprising.
Other quite interesting thing which I learnt myself is that not used
channels of the scope may significantly affect the measurement even the
channels are switched off. For that reason for accurate jitter measurements
I mechanically disconnect scope probes from all channels but the one I use.
I crime would be to measure a jitter on one channel leaving a second probe
connected to a different signal. This issue can be also scope dependant, but
in my case it was usually important.
Attaching probes to the DUT has also some effect to results of measurements.
I prefer to use a cheap, homemade divider probe which I solder to a DUT if a
measured signal can afford to be loaded by it. Usually I compare the same
measurements using different probes, attached differently (as probe loop can
also pick up some noise which is not a real jitter) and most often I decide
to continue using divider probes, the measurements are (in my case) as good
or even better as using active probes although I do not have experience with
12GHz BW oscilloscopes.
I do not know if selecting a scope for jitter measurements can be only based
on scope parameters. For me a parameter of the greatest importance would be
the own jitter of a sampler clock. There are companies other than scope
manufactures which provide jitter analysis software. Several years ago I saw
a comparison for jitter limits which can be measured using different
oscilloscopes.
Good luck
Robert
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of prasad
Sent: 10 czerwca 2010 05:58
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Jitter measurement floor on different high bandwidth
oscilloscopes
Hi every one....
i am evaluating high bandwidth oscilloscopes (12GHz) from different
vendors. I was looking the data sheets of them. One of the
them(DSO91204A) has very good noise floor compared to others. Though
its a good thing for me but when it comes to the jitter measurements ,
the lowest jitter that can be measured on that is dependatnt on the
slew rate of the signal ,which is actually true(since the voltage
noise will have a second order effect on the timing of the signal).
But when i looked at one more vendor (SDA13Zi) the noise floor is poor
compared to other. In which case the lowest jitter that can be
measured (jitter measurement floor)should be higher than earlier. But
if you look at the datasheet, they have specified a fixed value for
this which is very less .
My question is , if the noise floor is high in the second box how
would the jitter measurement floor be less?
second one is , since the timing noise(jitter) is dependatnt on slew
rate, how a fixed value is given in datasheet?
please help me understand. Am i missing some other factor here?
Welcome all your suggestions and ideas...
thanks in advance...
prasad
h
On 09/06/2010, colin_warwick@xxxxxxxxxxx <colin_warwick@xxxxxxxxxxx> wrote:
> (Note: I sent this info to Hermann off-list but he suggested it might be
of
> general interest. Send flames to me, not Hermann, if it isn't.)
>
> In ADS the implementation is:
>
>
> "Fast" corner
> (a) the max values are selected for all the I-V data (Pullup, Pulldown,
> Power Clamp a Ground Clamp) and for the waveform data (Ramp, Rising
Waveform
> and Falling Waveform), and
> (b) the min values are selected for all R, L, C, delay and TT data.
>
>
> "Slow" corner is the reverse obviously
> (a) the min values are selected for all the I-V data (Pullup, Pulldown,
> Power Clamp a Ground Clamp) and for the waveform data (Ramp, Rising
Waveform
> and Falling Waveform), and
> (b) the max values are selected for all R, L, C, delay and TT data.
>
>
>
> -- Colin
>
> -----Original Message-----
> .
> .
> .
>
> Any feedback from the tool vendors how they implemented this selection ?
>
> Thanks and Regards
>
> Hermann
>
> EKH - EyeKnowHow
> Hermann Ruckerbauer
> www.EyeKnowHow.de
> Hermann.Ruckerbauer@xxxxxxxxxxxxx
> Veilchenstrasse 1
> 94554 Moos
> Tel.: +49 (0)9938 / 902 083
> Mobile: +49 (0)176 / 787 787 77
> Fax: +49 (0)3212 / 121 9008
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--
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