RE: [SI-LIST] : What speed scope should I consider?

tomda ([email protected])
Thu, 20 May 1999 14:15:19 -0700

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Looking at clock rate will give you an incorrect answer. You need to focus
on edge speed. If you have transitions of nsec I would recommend a scope
of
200 psec risetime (1.75 GHz BW). 200 psec edges should have a 40 psec
risetime (8 GHz BW). Shoot for 5X faster scope risetime than logic family
risetime. Remember the displayed risetime will be the root of the sum of
the squares of the logic and scope's risetime (neglecting probing effects).
If you have a logic family with a 1 nsec rise and a scope with 1 nsec rise
then what is displayed on screen will be 1.41 nsec. (all this assumes
risetimes are measured 10 to 90%, the way scopes are spec'ed).

Possibly, more important than the risetime is the probe that you use. I
would recommend throwing away the 10X passive 10Meg 10 pF probes that come
with most scopes and use either a low C FET or Zo probe. The reflections
caused by the 10 pF standard probe will kill you. You need to keep the C
loading below 1 pF for today's fast edges.

Tom Dagostino

-----Original Message-----
From: Chris Bobek [SMTP:[email protected]]
Sent: Thursday, May 20, 1999 11:33 AM
To: [email protected]
Subject: [SI-LIST] : What speed scope should I consider?

We are designing an SBC with a processor speed of 266MHz and bus speed of
66MHz. There are different logic families used on the board, some CMOS,
some TTL -- just standard stuff (no ECL or GTL, etc.).

Based upon that general scenerio, what bandwidth scope should be adequate
for doing SI analysis on our board? We are currently looking at a scope
with a 1.5GHz bandwidth (8GHz sampling rate). This seems sufficient, but I
would like other opinions...

Thanks,

Chris

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