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

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From: Douglas C. Smith (doug@dsmith.org)
Date: Mon Nov 22 1999 - 22:39:04 PST


Hi All,

There is nothing magic about making the input impedance of a resistive
probe 1K. You can make it 10K if you compensate for the capacitance of
the resistor and account for the extra loss. I would recommend putting
10 1K small surface mount resistors in series to minimize the
capacitance and compensate for what capacitance there is using the
method posted on my website under the link to plans for the 1 GHz
probe (www.dsmith.org). You can probably get by without the 50 Ohm
termination at the end of the coax so the 10K probe will have a probe
factor of about 200X. At 1 mV per division you would get 200 mV per
division, adequate for looking at most signals.

Doug

"Ingraham, Andrew" wrote:
>
> >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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