[SI-LIST] Re: high speed test measurement equipment?
- From: "Sanchez, Louis" <louis.sanchez@xxxxxxxxx>
- To: "'doug@xxxxxxxxxx'" <doug@xxxxxxxxxx>,"Sanchez, Louis" <louis.sanchez@xxxxxxxxx>
- Date: Wed, 6 Feb 2002 13:50:47 -0800
Hello Douglas and other SIers..........
It's difficult to respond to the original query regarding the proper
selection of Scopes and Probes for SI work because, as Tom Dagostino pointed
out, "It is impossible for an outsider to tell you what you need. You need
to look at what signals you are exploring." Tektronix has a 57 page tutorial
entitled "ABCs of Probes",located at http://www.tektronix.com/available .
Follow the links to 'Oscilloscopes', and then to 'probes and accessories'.
The following is an excerpt from the Tektronix tutorial I mentioned.
"Manufacturers of oscilloscopes specify bandwidth or rise time to the probe
tip when the scope is used with specific probe models. This is important
because the oscilloscope and probe together form a measurement system, and
it's the bandwidth and rise time of the system that determine its
measurement capabilities. If you use a probe that is not on the scope's
recommended list of probes, you run the risk of unpredictable measurement
results." This statement would seem to apply to the high frequency
applications that are the source of the original query.
Doug, your points are well taken. It may be that after everyone has stated
their opinions, they could add up to a very lengthy document about a complex
subject, which is seemingly very simple. I think it's difficult to convey in
a few short paragraphs what Tektronix did in a 57 page tutorial. I would
recommend the tutorial by Tektronix is an excellant source of information
for anyone who wants to know more about scope probes.
Lou A. Sanchez
-----Original Message-----
From: Douglas C. Smith [mailto:doug@xxxxxxxxxx]
Sent: Wednesday, February 06, 2002 12:20 PM
To: louis.sanchez@xxxxxxxxx
Cc: 'tom_dagostino@xxxxxxxxxxx'; 'jvalle@xxxxxxxxxxxxxxxxxxxxxx';
si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: high speed test measurement equipment?
Hi Louis,
It should be pointed out that probe risetime as seen in your circuit may
have nothing to do with its specification! Probes are usually specified
with a 50 Ohm source and then used in an environment that is not 50
Ohms. EMI filters and some immunity test equipment it treated the same
way leading to problems there as well. Another result of such a
specification is that you can not generally use a probe anywhere near
its stated bandwidth because the probe input impedance near the
bandwidth spec is usually comparable to 50 Ohms! The result is to slow
down the risetime charging up the probe capacitance. The resulting
circuit loading can have a significant effect on the waveform in the
circuit being probed.
Doug
"Sanchez, Louis" wrote:
>
> Scope bandwidth required, is also based on how much error you are willing
to
> accept when measuring the rise time on the scope. Using a factor of either
3
> or 5 is a good rule of thumb. The rise time observed on the scope is the
> root sum square of the scope rise time, probe rise time, and the rise time
> of the waveform under observation. Given a permissable % or error in
> measured risetime, and a particular probes risetime, then the root sum
> square relationship can be massaged to yield the minimum scope bandwidth.
I
> have used a Mathcad worksheet to graph percentage of error as a function
of
> scope bandwidth. This proved useful in helping us to evaluate our
> requirements prior to purchasing.
>
> Although the relationship between the bandwidth of a scope and its
> equivalent risetime is expressed by BW=.35/risetime for a 10% to 90%
> measurement, the rise time of many high frequency signals are measured
from
> the 20% to 80% points on the waveform, e.g., LVDS and PECL. Using a single
> pole response model, the relationship between bandwidth and risetime
becomes
> approximately: BW=.22/risetime. I mention this because I think its
important
> to consider whether the risetime of a pulse is measured using the 10%/90%
> method or the 20%/80% method. Particularly if you're performing any
analysis
> that reduces everything to an equivalent bandwidth.
>
> -----Original Message-----
> From: Dagostino, Tom [mailto:tom_dagostino@xxxxxxxxxxx]
> Sent: Wednesday, February 06, 2002 9:31 AM
> To: 'jvalle@xxxxxxxxxxxxxxxxxxxxxx'; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: high speed test measurement equipment?
>
> Scope bandwidth required is determine by the fastest risetime you need to
> observe, look at the IBIS models or SPICE models you have. Compute the
> bandwidth of that risetime (falltime). For a risetime expressed from 10
to
> 90% use the equation BW = .35/risetime. For example a 350 psec risetime
> yields a 1 GHz bandwidth. To properly display this risetime you will need
3
> to 5 times the bandwidth in the scope and probe system. So you need to
look
> at the bandwidth at the probe tip. Remember to keep your probe's ground
> lead as short as possible. The scope probe should have minimum loading on
> the circuit, less than 1 pF.
>
> It is impossible for an outsider to tell you what you need. You need to
> look at what signals you are exploring. Rise/fall times for modern
> component range from about 1 nsec to 15 psec which result in bandwidth
> requirements from over 1 GHz to way over 30.
>
> Tom Dagostino
> Modeling Manager
> Mentor Graphics Corp.
> SAS
> tom_dagostino@xxxxxxxxxx
> 503-685-1613
>
> -----Original Message-----
> From: Javier del Valle [mailto:jvalle@xxxxxxxxxxxxxxxxxxxxxx]
> Sent: Tuesday, February 05, 2002 11:27 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: high speed test measurement equipment?
>
> Thanks for the info but, could someone also tell me which test equipment
> models they use for signal integrity?. I mean the oscilloscope should have
> 500Mhz, 1GHz bandwith or more, and also is it needed a special probes for
> signal integrity measurements?.
>
> Thanks a lot again,
>
> Javi
>
>
> -----Mensaje original-----
> De: Bob Patel [mailto:whizplayer@xxxxxxxxx]
> Enviado el: martes 5 de febrero de 2002 19:22
> Para: jvalle@xxxxxxxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Asunto: Re: [SI-LIST] high speed test measurement equipment?
>
> Hi! JAvier,
> Books: 1) High speed digital design- A handbook of
> Black magic by Howard Johnson
> 2)High Speed Digital System Design: Hall Hall McCall
> Web sites: 1)www.sigcon.com
> 2)www.ultracad.com
> 3)www.dsmith.org
> Equipment: I prefer a VNA with built in TDR, this way
> you can do both frequency domain & time doamin
> measurements--Contact Agilent
> Bob
> --- Javier del Valle <jvalle@xxxxxxxxxxxxxxxxxxxxxx>
> wrote:
> >
> > Hi to all,
> >
> > I am quite junior at high speed/signal integrity
> > design and I would like to
> > learn more about it. I have seen some demos of
> > programmes like Specctraquest
> > and hyperlinx, that some of you use for your
> > designs. I would like to know
> > also which is the test and measurement equipment
> > needed, and the required
> > specifications of these equipment for the design of
> > boards similar to PC
> > motherboards that integrates microprocessors like
> > Pentium IV, PowerPC with
> > high speed buses. I also would appreciate if someone
> > could recommend me some
> > books for learning about how to perform high speed
> > measurements, what to
> > measure (timing measurements, ...) etc. (I just have
> > read a book about this
> > field but was not much practical).
> >
> > Thanks a lot and best regards,
> >
> > Javi
> >
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