[SI-LIST] Re: via teardropping effect on signal integrity?
- From: "mkhusid" <mkhusid@xxxxxxxxxx>
- To: <ivor@xxxxxxxxxxx>
- Date: Tue, 1 Jul 2003 17:09:06 -0400
Ivor,
If one uses a TDR for the measurements, a short discontinuity typically
shows up as a blip. As the signal risetime increases (that is, the
signal is getting slower), the blip becomes flatter and wider until it
literally disappears. Typically, a discontinuity is considered
insignificant if it is less than half of the signal rise time.
A 66MHz PCI signal has a rise time around 2ns. Given the approximate
180ps/in delay in striplines and 150ps/in delay in microstrip layers,
that's several inches of a trace. So, a signal will only be able to
detect a discontinuity which is that long and has significantly
different impedance from the rest of the trace.
The teardrop effect is likely to be comparable to the via size, that's
40mils, and is much much smaller than anything your signal will notice.
Regarding the analog LVDS signals: you can use the risetime of those
signals and the above reasoning to determine whether the 40mil teardrop
is long enough to affect the signals.
Mike
----------------------------------------------------------------
Michael Khusid
Ansoft Corporation
SI/HF Application Engineer
25 Burlington Mall Road, 6th floor
Burlington, MA 01803-4100
Tel 781-229-8900 Ext. 134 <--- New extension
Fax 781-229-8624
---------------------http://www.ansoft.com ---------------------
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Ivor Bowden
Sent: Tuesday, July 01, 2003 4:56 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] via teardropping effect on signal integrity?
Hi, if I have a fairly standard stackup (sig,plane,sig,sig,plane,sig;
signal traces are 80 ohms) with .006"/.006" trace/space and .040" via
pads with .008" finished hole size, signal trace lengths of 2.5" and
7.5", would there be any significant issues with teardropping the vias?
The signals are either PCI (66MHz clock) or analog/LVDS.
tia,
Ivor Bowden
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