[SI-LIST] Re: TDR impedance measurement and rise time
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: Nagy István <buenos@xxxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 7 Apr 2009 07:39:20 -0700
Concerning measuring Z0:
The TDR will report the same characteristic impedance of your trace regardless
of risetime, assuming your trace is long enough and there aren't significant
variations in impedance along its length.
Typically, we have very similar 6" coupons for all our controlled impedances.
The board manufacturer will typically measure them with an HVM-compatible TDR,
probably about 200 ps risetime. We verify the impedances with our ~17ps TDR.
For simulations, on the other hand, you'll probably want a risetime faster than
the projected risetime of your device (I'd guess about 2x; I don't remember
seeing it quantified). I typically see folks just go with the risetime of the
equipment, ~17ps, and ensure simulation match those measurements. They may be
a little conservative, but probably less work in the long run than trying to
exactly justify any particular risetime.
The advantages/disadvantages I can think of off-hand for fast risetimes are:
1) fast R.T. = resolution of finer features (discontinuities). Unfortunately,
this can also erroneously lead you to believe you need to fix things that are
"invisible" at your risetime of interest. Filtering to your risetime of
interest can help you decide whether a discontinuity is significant or not.
2) fast R.T. = smaller probing geometries. It doesn't make sense to try to
maintain a 15 ps risetime through a launch structure with 30 mil vias spaced
100 mils apart (such as might be used for manufacturing testing). Living with
slower risetimes can allow you to adopt much more HVM-friendly launch
structures, including pogo-pinned probe connections.
3) fast R.T. = less ESD protection. It's very easy to damage a TDR head from
static discharge - HVM-compatible TDR machines with slower risetimes have ESD
protection.
If the scope or post-processing software doesn't have the ability to slow your
risetimes, you can buy filters from Picosecond Pulse labs (buy a filter at
0.35/RT). They also sell hardware to put out very fast risetimes.
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Nagy István
Sent: Tuesday, April 07, 2009 4:59 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] TDR impedance measurement and rise time
hi
If we measure PCB test coupons with a TDR to determine characteristic
impedance, should we set the rise time to be the same as the signal's rise
time? is it possible to set it at all?
what i found on the internet, the TDR manufacturers try to make rise time to be
as low as possible, like 15ps..., and thats it.
If the rise time is always 15ps, then i think it will always measure the
impedance on a very high frequency, 2/t_rise or something, so several
gigahertz. Usually on a board we have different signals, some are running
100MHz analog, some other are 800MT/s digital, or 2.5Gb/s digital.
shouldn't we do different setups for these, to get impedances on the signal's
operating frequency?
Someone from a Fab told me, that the "TDR is not frequency dependent". so they
dont take the signal's frequency into account.
what is the correct handling of signaling frequency for impedance measurements,
and simulations?
regards,
Istvan Nagy
CCT
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