[SI-LIST] Re: A question for the instrument guys on TDR and TDT

Chris, it depends on what we want to know.  Certainly, for a lot of 
problems where we are "just" signaling with fixed thresholds, a lot 
of resolution isn't going to make go/no go much better.  For 
determining power supply response, we can get away with a reduced 
dynamic range if we can window it in the right range.
OTOH if I am verifying the design of some structure for a SerDes, 
I'll generally take as much resolution as I can get my hands 
on.  Ditto if I want to see a low milliohm impedance with a 50Ohm 
instrument, it is going to take 80-100dB dynamic range to get there.

The problems that you raise only get worse with increasingly complex 
and difficult to access packages.  We have in many places already 
reached the realm where direct external measurement is no longer 
feasible.  We either can't arrange access, or can't do so at an 
acceptable cost in time or money, or we can't do so without unduly 
affecting the thing we wish to measure.  This drives more and more 
designs with built in measurement capability of some form, and/or 
increased reliance on simulation.  In order to keep the simulations 
honest, Teraspeed constantly builds test vehicles that we then 
measure with as good equipment as we can lay our hands on.

Best Regards,


Steve

At 11:17 PM 11/6/2006, Chris Cheng wrote:
>Steve,
>
>May be it is one of these philosophical question like "if the scope 
>is not even accurate enough to measure it, does my active signal or 
>receiver care about it ?"
>
>You know, if the tree falls in the forest and no one is around to 
>hear it......
>
>I think I understand what TDA is doing but I think it will be even 
>better if they have access to impulse response rather than step response.
>
>Consider the probe/fixturing (you) and calibration (Istvan) problems 
>mentioned below.
>Other than build in SMA connection, show me a satisfactory probing 
>solution for VNA that can :
>a) be rigid but flexible enough for everyday end to end channel 
>measurements from bga pins, connectors to vias of different size.
>b) a non-coax calibration standard that can provide those repeatable 
>100db performance Istvan mentioned below
>c) doesn't need and additional $50K probe station
>
>Like I said below, the launch discontinuity can be calibrated out 
>through a FIR filter. Same way we equalize the real channel, instead 
>of the nightmarish SOLT or TRL process. Afterall, if FIR is good 
>enough for me to send a clear signal down that channel, it should be 
>good enough for me to use it to measure it. Sort of reverse "tree 
>falling in forest" analogy.
>
>
>
>----------
>From: steve weir [mailto:weirsi@xxxxxxxxxx]
>Sent: Mon 11/6/2006 10:02 PM
>To: Chris Cheng
>Cc: si-list@xxxxxxxxxxxxx
>Subject: Re: [SI-LIST] Re: A question for the instrument guys on TDR and TDT
>
>Chris, the VNA is up to the task.  TDA Systems, now Tektronix put out
>some nice papers in I believe 2003 / 2004 on using VNAs for TDR, and
>TDR as a poor man's VNA.  For signals there are a number of cases
>where either works quite well.  Designing the probe access and
>launches is the tricky part.  The same applies for the TDR.  It is
>pretty easy and all too common to see suboptimal launches mask away
>what is really going on whether using a VNA, or a TDR/TDT.  Each has
>its place.  I really prefer VNAs for power systems because of the
>broad frequency range, and wide dynamic range.  I have the luxury of
>access to both tools.
>
>Best Regards,
>
>
>Steve.
>
>
>At 08:08 PM 11/6/2006, Chris Cheng wrote:
> >I sure would like to see the VNA that can do real system channel =
> >measurements at 100db after fixture and cabling.
> >On the other hand, even at 40/60db range, a sampling scope may be good =
> >enough for a channel measurement up to a few GHz. Not a bad alternative.
> >
> >-----Original Message-----
> >From: si-list-bounce@xxxxxxxxxxxxx
> >[<mailto:si-list-bounce@xxxxxxxxxxxxx>mailto:si-list-bounce@freelis 
> ts.org]On Behalf Of Istvan Novak
> >Sent: Monday, November 06, 2006 7:50 PM
> >To: Chris Cheng
> >Cc: istvan.novak@xxxxxxx; si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: A question for the instrument guys on TDR and TDT
> >
> >
> >The 'why not' is an interesting question, and you may get several=20
> >different answers,
> >dependent on priorities.  My answer is that a scope is OK for low=20
> >dynamic range
> >or low-frequency measurements, in those applications it is cheaper than=20
> >VNA and
> >works fine.  On the other hand, VNAs can achieve 100+ dB dynamic range=20
> >easily;
> >to get that in a scope, the effective number of bits would need to be=20
> >16+, not
> >available today on fast scopes.=20
> >
> >If milliohms need to be measured, VNA is hard to beat.  For
> >tens and hundreds of milliohms or higher, scope would be acceptable.
> >
> >Regards,
> >
> >Istvan
> >
> >
> >Chris Cheng wrote:
> >
> > >I would agree. However, the question is not whether the scope is =
> >capable =3D
> > >of doing it but why not ?  The controls are almost there (for the =3D
> > >TDR/TDT) and de-embedding is a simple step of shifting the reference =
> >=3D
> > >plane or simple post processing FIR filter (something advance sampling =
> >=3D
> > >scope can already do). A $40K+ VNA vs. a free firmware upgrade on the =
> >=3D
> > >sample scope. Which one will you choose !?
> > >
> > >-----Original Message-----
> > >From: Istvan Novak 
> [<mailto:istvan.novak@xxxxxxxxxxx>mailto:istvan.novak@xxxxxxxxxxx]
> > >Sent: Monday, November 06, 2006 5:31 PM
> > >To: Chris Cheng
> > >Cc: si-list@xxxxxxxxxxxxx
> > >Subject: Re: [SI-LIST] A question for the instrument guys on TDR and =
> >TDT
> > >
> > >
> > >Chris,
> > >
> > >The answer is yes (as long as the network is 'sufficiently' linear). =
> >=3D20
> > >The finite rise time
> > >shows up as filtering, what happens in all simulations or =
> >measurements=3D20
> > >anyway,
> > >even if we neglect the effect of risetime.  If you need the impedance =
> >of =3D
> > >
> > >a power
> > >distribution network, the quantization of the receiving instrument =
> >will=3D20
> > >set the noise floor.
> > >
> > >Regards,
> > >
> > >Istvan Novak
> > >SUN Microsystems
> > >
> > >Chris Cheng wrote:
> > >
> > > =20
> > >
> > >>I can understand the merit of a step response and direct read out of =
> >=3D
> > >>   =20
> > >>
> > >=3D3D
> > > =20
> > >
> > >>impedance profile.
> > >>Will it be easy to also include a mode with a sharp pulse instead of =
> >=3D
> > >>   =20
> > >>
> > >=3D3D
> > > =20
> > >
> > >>step for an impulse response ? If you have a circuit to generate a =
> >35ps =3D
> > >>   =20
> > >>
> > >=3D3D
> > > =20
> > >
> > >>step, can you use it to generate a 35ps pulse also ? I think there are =
> >=3D
> > >>   =20
> > >>
> > >=3D3D
> > > =20
> > >
> > >>steps you can do to convert one to the other but a direct impulse =
> >=3D3D
> > >>response is just too good to pass up.
> > >>Network analyzer on the cheap.
> > >>Thanks in advanced,
> > >>Chris
> > >>
> > >>
> > >>=3D20
> > >>
> > >>   =20
> > >>
> > >
> > > =20
> > >
> >
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