Re: [SI-LIST] : measuring radiation

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From: S. Weir (weirsp@a.crl.com)
Date: Sat Feb 19 2000 - 13:40:56 PST


Yasser,

When your scope see's things that your circuit does not, there is a good
likelihood that your scope is operating as a near-field antenna. For board
level signals, this is almost always the result of the loop created by the
ground lead on the scope. You need to minimize the variation between your
scope connection and a perfect coaxial connection. How you do this depends
on the type of probes that you have. Most probes have available a spring
ground clip that attaches to a ground ring just behind the tip. You can
sometimes fashion a crude one with solid wire. You then need a ground
connection very close to the signal that you are measuring.

If this is still not good enough, then an SMA connector can be used, but
this must be part of the board, and is quite inconvenient, if not totally
impractical after the fact. SMA connectors are very common on evaluation
boards for high-speed A/D's as well as switch mode power supplies.

As far as your rise times go, you need to understand the specifications of
both your scope and your probe as they relate to the signals you are
looking at. This gets a little bit more complicated if you have a digital
scope, but basically:

1. What is the impedance of your 'scope probe to your signal? The 'scope
probe's capacitance is usually the big problem. If you research the
archives of the Si-list in the past month, you will find considerable
discussion of resistive probes, as well as favorite probes of individuals
for different purposes. You can also refer to Dr. Howard Johnson's book
"High Speed Digital Design", www.signalintegrity.com, or Doug Smith's web
site www.dsmith.org.

2. What is the bandwidth of your oscilloscope's amplifiers?

3. What is the bandwidth of your oscilloscope's display mechanism? If it
is an analog scope, this is essentially the same as 2. If it is a digital
scope, it depends on whether the signal is repetitive, and what the capture
scheme of the 'scope is.

Good luck,

Steve.

At 05:29 PM 2/19/2000 +0200, you wrote:
>Dear Sir :
>
>I have two problems :
>
>1- I need to know a simple method to measure the radiation from my
>switched mode power supply using the scope. If you can help me by telling
>me for a technical document for this issue.
>
>2- When i make a measurements by the scope on my circuit some times i
>discover that the signal rise times or fall times is not as i expect and i
>may see a noise on the supply that makes supply drop for 1 v and i have in
>my circuit a power faile detection if the supply falls for 0.5 v and no
>reset ocuurs. Can any one help me to know the better measurment using
>scope and any technical documents available on the web.
>
>With my best regards
>Yasser El-haddad

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