[SI-LIST] Re: FET Probe
- From: Doug Smith <doug@xxxxxxxxxx>
- To: James_R_Jones@xxxxxxxx
- Date: Thu, 31 Oct 2002 13:04:46 -0800
Hi James,
Your experience is not all that uncommon with active probes. They
often and work well to about 1/4 to 1/2 of rated frequency, but many
specs do not explicitly show input impedance, which is your problem.
The problem is the measurement that rates frequeny response often (not
sure of this case) has no relation to real use. That is not to say the
measurement method is wrong, it is just a standard method just like
some of the EMC/ESD standards that do not necessarily reflect real use.
Take a look at the article:
http://emcesd.com/tt2002/tt080102.htm
having to do with probe input impedance. For active probes, input
impedance can easily be a factor of 10 to 100 lower than you think in
the upper octave of its rated frequency range, as low as 20 Ohms even
for very short attachment leads.
This information is usually somewhere in a probe instruction manual in
the form of an equivalent circuit, but the informaton is not explicit.
I like the way the manual is written for the 1158a active probe, as
well as its low Q design which I have been advocating for many years
(and is finally here in the 1158a). The URL is:
http://cp.literature.agilent.com/litweb/pdf/01158-97001.pdf
An alternative 1.8 GHz differential probe by Fischer Custom
Communications that may work is described at:
http://emcesd.com/products/bcp-2.pdf
Doug
James_R_Jones@xxxxxxxx wrote:
>
> Gurus,
>
> Does anyone have a basic schematic for Tektronix FET probe? I don't need
> proprietary details about the probes, but just a basic schematic of what we
> see looking into the probe, and relative to scope ground.
>
> The reason that I am asking is that I have a board that will not come up
> properly when the positive terminal of the probe is hung off of a DDR strobe
> line. No ground attachment required. With the probe disconnected from the
> scope at the scope, the problem does not exist. Anyone seen this before?
>
> Thanks in advance,
>
> James R. Jones
> ESG Signal Integrity
> Dell Computer Corporation
> (512) 725-1809
>
>
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