Re: [SI-LIST] : LVDS signal observation

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From: David Instone (david_instone@uk.xyratex.com)
Date: Mon Feb 21 2000 - 02:30:29 PST


Actually I had a chance to do a direct comparison between the Tek
differential probe and a pair of Tek 500 ohm P6150.
The two p6150 probes were connected to measure a differential Fibre
Channel signal (150 ohm differential) across load resistors that with
the 2*500 ohm probes resulted in the corect load of 150 ohms.
FC (at 1 Gbd) has a max fundamental freq of 531MHz.

Rise times of the order of 250-300 ps 20%-80% were measured.
I then put the diff probe across while leaving the other probes in
place. The difference had to be seen to be believed. I didn't measure
the risetime but that 'square'wave went almost triangular!

Re the input capacitance of the diff probe,that 1 pf is the differential
capacitance, each side of the probe has 2pf to 'ground'
Dave Instone
_________

Doug Smith wrote:
>
> 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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-- 
Regards

Dave Instone. Compliance Engineer Test Systems, MP24/22 Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK. Tel: +44 (0)23-92-496862 (direct line) Fax: +44 (0)23-92-496014 http://www.xyratex.com Tel: +44 (0)23-92-486363

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