[SI-LIST] Re: via teardropping effect on signal integrity?
- From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- To: Pat_Diao@xxxxxxxx
- Date: Tue, 01 Jul 2003 20:10:22 -0700
Pat,
Risetime and bit rate are often not related at all. If the package
electrical length is greater than 1/10 of the signal risetime, then you
will see a signal disturbance with a mismatched package. Whether it is
an issue with your customer's signalling standard is their decision.
I've done designs even at extremely low frequencies (10 MHz) where
impedance matching was important for extremely long traces (>30 inches)
when low insertion and return loss were needed for analog applications.
Besides, 100 ohm impedance control is a "gimmie" and not hard to do.
regards,
scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
Pat Diao wrote:
>Mike,
>
>This raises an interesting question on impedance control.
>
>For differential pairs in a package, we are putting a lot of efforts to make
>sure the Zdiff is matched to 100 ohm, for us it's at the request of our
>customers.
>
>Looking at the speed, for a data rate of several hundred Mbps, the signal
>rise time is significantly longer than the package dimension. Any
>discontinuity or impedance mismatch is not seen by the signal.
>
>My point here is there are wasted efforts in the industry in doing imp
>control at low to mid frequency range.
>
>Of course it becomes necessary and much needed when the frequency is
>compariable to the structure. Much had been said on this.
>
>Any comments?
>
>
>Pat Diao
>ASAT Inc
>Fremont, CA
>
>-----Original Message-----
>From: mkhusid [mailto:mkhusid@xxxxxxxxxx]
>Sent: Tuesday, July 01, 2003 2:09 PM
>To: ivor@xxxxxxxxxxx
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: via teardropping effect on signal integrity?
>
>
>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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