Re: [SI-LIST] : LVDS signal observation

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From: Doug Smith (dsmith@corp.auspex.com)
Date: Tue Feb 08 2000 - 10:25:24 PST


Hi All,

Specs on test equipment can be misleading unless one reads between the
lines...

According to the specs below, the probe's input impedance is on the
order of 100
Ohms at the high end of its advertised bandwidth (typical for probes
with a
capacitive input impedance). Personally, I don't use probes where
capacitance
dominates the input impedance (as in this case) at frequencies higher
than 1/4
or at most 1/2 of their stated bandwidth because of input impedance and
other
concerns and the way the specs are written.

A 1500 Ohm resistive, passive differential probe (I call it a Balanced
Coaxial Probe) has
a much higher input impedance at these frequencies. Barth Electronics
has a 180
degree combiner that will give 40 dB CMRR to 1 GHz to use with the
Balanced Coaxial
Probe structure.

Below: the input Z = 1/(2Pi*1.5GHz*1pF) or about 100 Ohms (which swamps
the 200K parallel
resistive component of the input impedance).

I wonder if a 100 Ohm load will change the signals? Hmmmmm............

Doug

Bradley S Henson wrote:
>
> Opinions on this kind of question cover the entire spectrum. On top of my wish
> list for high speed differential signal characterization equipment is the TEK
> TDS694C real time sampler with the P6248 1.5GHz differential probe: 1pf and
> 200K_ohm in differential mode. Nice stuff IMHO. We'll see if I can get it :)
> Good for LVDS up to gigabit fibre channel etc.
>
> Brad Henson, Raytheon
>
> Christopher wilson <christopher.wilson@sd.conexant.com> on 02/02/2000 10:05:33
> AM
>
> Please respond to si-list@silab.eng.sun.com
>
>
>
>
>
>
>
> To: si-list@silab.eng.sun.com
>
> cc: (bcc: Bradley S Henson/RWS/Raytheon/US)
>
>
>
> Subject: [SI-LIST] : LVDS signal observation
>
>
> I need to buy a new oscilloscope and probes in part to
> observe LVDS signals between devices (as opposed to
> terminating at the scope). A FET probe seems to be
> the least intrusive. Any suggestions for probe type
> or specific scope/amp/probe recommendations? Anyone
> looked at the new Tek CSA8000 for SI applications?
>
> Thanks in advance,
> Chris Wilson, Conexant Systems
>
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-- 
----------------------------------------------------------
    ___          _          Doug Smith
     \          / )         Manager EMC Development & Test
      =========             Auspex Systems
   _ / \     / \ _          2300 Central Expressway
 /  /\  \ ] /  /\  \        Santa Clara, CA 95050-2516
|  q-----( )  |  o  |       Phone/FAX: 408-566-2157/2020
 \ _ /    ]    \ _ /        Email: dsmith@auspex.com
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