Charles,
are you using the same probe for both measurements? I have found that probing
a Vcc plane accurately requires a probe with decent common mode rejection.
About the best is to solder a coaxial cable in place.
Dave Cuthbert
Micron Technology
From: Charles Grasso
Mr. Novak makes some excellent points. I was under the assumption that the
phrase voltage ripple implied conducted emission measurements at a LISN
port. Hence my comments on mode separation. Across a spectrum of even 30
MHz, any normal scope probe I know of (1 or 10 M Ohm in parallel with
Zero span can show rep rate or modulation but it cannot correlate the MHz
bandwidth waveform amplitude the scope sees with the amplitude of any
particular spectral component to which the spectrum analyzer is tuned.
Further, the spectrum analyzer is a much more sensitive device than the
scope, and
As I mentioned earlier, the waveform from a switching power supply has two
distinct components, due to the fast rise-time driving current into ground
(common mode) and the pulse itself which is differential mode. Separating
modes allows you to time window properly to really resolve the waveform
Charles,
Doing this kind of correlation is very difficult for the following reasons:
- unless you measure a very simple and dummy system, hardware today is
so complex that you cant predict for sure its activity; it is a strong
function
of time.
-tThe spectrum analyzer and scope will look at the
I read in !emc-pstc that Charles Grasso cgrassospri...@earthlink.net
wrote (in ekeeipjkkmpklafoobmcaelkcfaa.cgrassospri...@earthlink.net)
about 'Q on Correlation of Votage ripple with a Spectrum Analyser' on
Sat, 26 Jul 2003:
I first calibrated myslef using a known source - a sine wave.
Really?
I read in !emc-pstc that Charles Grasso cgrassospri...@earthlink.net
wrote (in ekeeipjkkmpklafoobmcoelhcfaa.cgrassospri...@earthlink.net)
about 'Q on Correlation of Votage ripple with a Spectrum Analyser' on
Sat, 26 Jul 2003:
Has anyone tried correlating the voltage ripple as seen on a scope
Charles,
Instead of using the 1GHz single-ended scope probe, have you tried to
connect the same coax cable that you made with the seriers 50 ohms for the
SA to connect to the scope? With a 50-ohm input impedance setting on the
scope, the loading of the planes would be exactly the same.
As I
Then the answer is the difference between the sum of all spectral components
measured with the scope vs. the individual components themselves. If you
want correlation you have to go the FFT route. From a strictly EMC
point-of-view only the spectral components matter, the only point of
This is getting pretty intense for a Sunday!!
Both Mr. Javor and Mr Novak make excellent
observations. Both center on the method of measurment
as a point of concern.
To measure the voltage ripple I used a high
badwidth (1GHz) sigle ended probe with very
short leads. In order to establish the
Hi all,
Actually I was using a good ole Spectrim Analyser
so I sidestepped the windowing issue/software issues
altogether.
What I was(am)trying to do was match the max voltage
as measured on a scope with the value as measured
on a SA.
I first calibrated myslef using a known source - a sine
Charles Grasso wrote:
What I was(am)trying to do was match the max voltage
as measured on a scope with the value as measured
on a SA.
Try zero span on the SA. Compare THAT with the scope.
Cortland
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc
I presented a paper on that very subject about a decade ago at one of the
EMC TD magazine EMC symposia. I used a Fluke Scopemeter and some FFT
software that came with it. The Fluke interfaced to the PC through an
optically isolated RS-232 protocol. It worked quite well from a
pre-compliance or
Hi All,
Has anyone tried correlating the voltage ripple
as seen on a scope with the amplitudes measured
on a Spectrum Analyser?
I tried doing that the other day with ..umm. minimal
success. I think that due to the comples convoltions
that would have to occur when FFT'ing an irregular
voltage
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