[SI-LIST] Re: Probe impedance effect at high frequency
- From: David Instone <dave.instone@xxxxxxxxxx>
- To: TerenceHsieh@xxxxxxxxxxxx
- Date: Fri, 17 Mar 2006 10:03:37 +0000
Terence,
I've been using the next model down (P7350) for over a year now on
SATA to 1.5GHz (also 100ohm differential) and it's effect on the sata
signal was just visible, but for practical purposes insignificant.
However, I would recomend that, where you can, you use a direct input to
the 'scopes 50 ohm inputs rather than a probe. In other words use the
scope inputs as the load to the transmitter. Further you can satisfy
yourself on the effect of the probe by then probing on the junction of
the coax to the UUT and comparing what you see.
You might also care to take a look at the following TeK application note
about your probe, where they discuss the effects of probe loading etc.
http://www.tek.com/Measurement/App_Notes/51_17892/eng/51W_17892_1.pdf
Dave
TerenceHsieh@xxxxxxxxxxxx wrote:
>I have a problem about differential probe. When measure a DUT with low
>output impedance, it is appropriate to use high impedance probe. If the
>probe impedance is ten times than DUT, the probe impedance could ignore
>and the measurement result will more accurate. I checked the technical
>note of differential probe P7380. The DC impedance of P7380 is 100K ohm
>and this is larger enough for most DUT. However, the impedance is just
>about 300 ohm at about 2GHz. If I want to measure a DUT that operate at
>1.25GHz (PCIE Gen.1) and the differential impedance of DUT is 100 ohm.
>Could P7380 measure accurate result? How the low AC Loading of probe
>affect the result? Thanks
>
>Terence
>
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