[SI-LIST] Re: FET Probe

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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