Re: [SI-LIST] : Probing PCI slew rate test

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From: Istvan NOVAK (istvan.novak@worldnet.att.net)
Date: Fri Nov 11 1994 - 05:41:23 PST


Andy,

if you are interested in the frequency response of your scope probe, and
have a vector network analyzer on hand, you can measure it easily. You also
need a scope with sufficient bandwidth which has output connection at one of
its analog front end amplifiers (some digital scopes still have it). First
you calibrate out the response of the scope by connecting VNA port1 to the
scope input directly and port2 to the scope analog output. Second, you
connect your probe in series to this link, by connecting the robe inputs
across a 50-ohm termination of the vna port1 output, and the probe output
connected to the scope. You may see that simple homemade 1kohm probes
easily can get a 3dB bandwidth up to 1-5GHz.

thanks

Istvan Novak
SUN Microsystems

----- Original Message -----
From: Ingraham, Andrew <Andrew.Ingraham@compaq.com>
To: 'si-list@silab.eng.sun.com' <si-list@silab.Eng.Sun.COM>
Sent: Monday, November 22, 1999 11:13 AM
Subject: RE: [SI-LIST] : Probing PCI slew rate test

> >I have been working on trying to probe PCI buffers against the PCI
> >spec slew rate load (1k to vss, 1k to vdd, 10pf to vss)
> >
> >These are pretty fast edges (spec up to 4v/ns) and what I have
> >been looking at so far is to use a 8pf scope probe as a replacement
> >for the 10pf load in the spec.
>
> >I know that at fast edge rates a passive resistive load is better,
> >but with a 1k scope input being on the same order of magnitude as
> >the spec load I am not sure it is a good idea.
>
> As far as loading is concerned, you could use either the 8pF scope probe
> (properly compensated) and ~2pF of stray capacitance and 1K
> pull-up/pull-down resistors, or the 1K passive scope probe (which serves
as
> the 1K pull-down) in parallel with just a 1K pull-up and 10pF.
>
> But find out how good the 8pF scope probe is. Does it have the bandwidth?
> Including calibration (compensation) error? What about its resonance?
How
> short/small can you make the probe+return wire loop? What is its
impedance
> vs. frequency? Some "hi-Z" scope probes may have a big loss factor at
> higher frequencies, which cannot be represented by a 10M resistor || XpF
at
> several MHz.
>
> Regards,
> Andy
>
>

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