[SI-LIST] Re: How to Measure Ground Noise
- From: "ospyng" <ospyng@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 07 Apr 2005 17:57:16 -0000
thanks alot to all the rest of you have have reply to me in private.
not really the radiation, I was trying to measure the ground noise.
here what happen.
we are at customer site recently, which complain that our internal
PLL(part of the chip design) is generating clock with huge jitter.
output clock changing from 80-100 Mhz.
so of course I measure the VDDPLL, and sure enoungh there are a 1v p-p
noise on it, and the vddpll is suppose to be 2.8 v. this is alot of
noise and it will mess up the vco for the pll.
and then I try to measure the ground noise. As to the vco potential
differnet is important. using the techquie that I mention, shorting
the ground of probe, tip of probe and ground tp on board. and conclude
that there is a 600mVp-p noise on the ground
It was later point out to me that, such a techquie does not seem to be
correct.
the customer has since replace their test setup and the guy who taught
me hoe to measure ground noise is from a previous company. :(
I just keep thinking about it. what am I measuring ? and how could I
measure noise on the ground? yes there is noise on the vddpll but if
the gnd pll is actual the same as vddpll. then there is really no
potential differert. ?
is there a way to measure ground noise on a PCB board?
thanks alots
pyng
--- In si-list@xxxxxxxxxxxxxxx, Zhangkun <zhang_kun@xxxx> wrote:
> Dear Istvan
>
> Do you mean connecting the probe to the ground pin? And measure the
radiation in the environment.
>
> In our measurement, there is often such kind of situation. When the
pigtail of the probe is long, the radiation will couple into the
measured signal. Is there any technique about shielding probe?
>
> Best Regards
>
> Zhangkun
> 2005.4.7
> ----- Original Message -----
> From: "Istvan Novak" <istvan.novak@xxxx>
> To: <ospyng@xxxx>
> Cc: <si-list@xxxx>
> Sent: Thursday, April 07, 2005 8:03 PM
> Subject: [SI-LIST] Re: How to Measure Ground Noise
>
>
> > Hi pyng,
> >
> > Beyond the comments you already got from the other list member,
> > here is one additional item you may want to consider: taking a
> > baseline reading of the noise with an oscilloscope in the way you
> > stated is a good practice to establish the noise floor of the setup.
> > If you connect the shorted signal and ground pins of the probe to
> > the ground test point near a signal test point, the recorded signal
> > level will tell you what is the noise picked up by the various ground
> > loops in the setup, and it establishes a level below which you cannot
> > trust subsequent measurement data taken at the signal test point.
> >
> > So the 'ground noise' expression may not be the best choice to
> > describe of what you were told to do, but nevertheless it is a good
> > practice. Make sure that when you take this baseline measurement
> > you have everything else in the system as close as possible to the
> > settings of your actual later measurements. This refers to geometries
> > and activities alike. Two simple examples: many active scope probe
> > heads have very little shielding, so their relative position with
respect
> > to active devices in your system is important. Likewise activity also
> > matters: turning on and off subsystems, bus activities, loads on the
> > power mains (lights) all may create large common-mode noise in the
> > ground loops.
> >
> > Best regards,
> >
> > Istvan Novak
> > SUN Microsystems
> > .
> > ospyng wrote:
> >
> > >hi,
> > >I was taught to measure ground noise, by having the ground of the
> > >probe connect to ground TP of the board and the tip of the probe
> > >measuring the same TP.
> > >
> > >is this the correct where? and Why?
> > >is it because of the ground cable (3 inch) cause a loop,
therefore the
> > >reference and the tip of the probe has a different voltage?
> > >
> > >thanks
> > >pyng
> > >
> > >
> > >
> > >
> >
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