Re: [SI-LIST] : LVDS signal observation

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From: Bradley S Henson (bhenson@notes.west.raytheon.com)
Date: Wed Feb 09 2000 - 07:13:27 PST


As I said, everyone has opinions and many are worthy of consideration. I have
used passive high BW resistive probes for almost 20 years; long before
contemporary authors started advocating them to SI engineers. In fact, I have a
rather demanding reputation for my usual insistance on good (usually resistive
custom made) probe technique. So. I don't disagree with your observation about
resistive passive probes ( I've used them). I find good uses for both my passive
resistive probes and good differental probes. I wonder why everyone that reads
probe specs assumes users will use them to their actual advertised BW? Things
like rambus and some FC (<=1Gbit = 500MHz upper frq fundemental) can be viewed
by a variety of techniques: the best depends on many things. I find that most of
my investigations are in very high common-mode environment with signals in the
50 to 100MHz range. Harmonic content beyond 300MHz is usually of little interest
in many of my cases. People should demo equipment (I always do) on test setups
that reflect their intended usage. It might be useful to consult a local
microwave engineer and have a reference or standard established by which you
compare the loaner equipment you evaluate. If you tell the vendor what you are
up to before hand, their apps folks can be amazingly helpful at getting the most
of your demo.

I've demo'd and find great uses for both types of probes described below. Users
should not get too wrapped up by analysis paralysis; they just need to use the
equipment within it's useful, and hopefully demonstrated, range. All kinds of
probes can yield poor results; it's up to the user to properly apply them and
know their idiosyncracies.

This can be a fun discussion, but I've noticed this list is more demanding of
contributors than I presently have time. Others can carry on...there are plenty
of opinions and lots of good experience ;)

Enjoy the process,
Brad Henson, Raytheon

Doug Smith <dsmith@corp.auspex.com> on 02/08/2000 10:25:24 AM

Please respond to si-list@silab.eng.sun.com
                                                                                
                                                                                
                                                                                

                                                              
                                                              
                                                              
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 Subject: Re: [SI-LIST] : LVDS signal observation
                                                              

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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