Hi!The best measurement technique I have found for doing noise analysis on power rails is: 1)Use 50 ohm coaxial cables RG178 with 2.6mm dia. 2)Remove the cap, and solder the cable on the pads. 3)terminate the scope to 50ohm. Bob --- Larry Smith <Larry.Smith@xxxxxxx> wrote: > > Anand - 1 inch of ground lead on the probe is a lot > these days. That > is probably the source of a lot of your noise. You > want the total loop > area including your ground lead and hot side of the > probe to be 1/4 > square inch or less. > > I prefer to use 50 Ohm transmission line probes. > They are much higher > impedance than any power distribution system that > you are likely to > measure. The loop area is measured in square mils. > Just be sure to > flip the measurement instrument into the 50 Ohm > termination state, > otherwise you will have excessive ringing at the 1/4 > wavelength of your > transmission line. > > One more thing, be sure and remove the decoupling > capacitor from it's > pads before you use the pads for measurement. If > you don't, the > capacitor mounted on the pads will have a self > resonant frequency and > greatly color your noise measurements. > > regards, > Larry Smith > Sun Microsystems > > > From: ANAND KURIAKOSE <Anand.Kuriakose@xxxxxxxxxx> > > To: pwelling@xxxxxxxxxxxxxx > > Cc: Larry.Smith@xxxxxxx, si-list@xxxxxxxxxxxxx > > Subject: RE: [SI-LIST] Re: Noise on BGA core > voltage rail > > Date: Thu, 8 Aug 2002 20:19:22 -0700 > > MIME-Version: 1.0 > > > > Hi philips, > > > > Basically i used an FET probe with roughly 1 inch > ground length b/n > > probe and test board ground. The measurements were > taken either directly > > across the high frequency caps which (quite a few) > are placed directly under > > the BGA or on the BGA vias. However i will > validate the test setup in a > > manner u have mentioned below. > > > > Thanx, > > Anand. > > > > > > > > From: pwelling@xxxxxxxxxxxxxx on 08/09/2002 04:16 > AM > > To: ANAND KURIAKOSE/Apex Data Inc/01@Apex Data > Inc, > > Larry.Smith@xxxxxxx@SMTP@Exchange > > cc: si-list@xxxxxxxxxxxxx@SMTP@Exchange > > > > Subject: RE: [SI-LIST] Re: Noise on BGA core > voltage rail > > > > Anand, > > > > Have you checked the local probe environment to > see if you are > > inductively > > picking up EMI noise from the components around > the measurements > > area? I am > > already assuming that you have a very short > ground (less than 1/2 > > inch) from > > the scope probe ground to a low impedance ground > on the board under > > test. > > Often, we will put ground pads on our boards to > minimize the loop. I > > am also > > assuming that there are not alot of traces run > between the ground > > connection > > and the test measurement point. This is only one > of the reasons why > > we limit > > routing on outside layers of the board. > > > > > > There is an easy test to see if you are creating > measurement error > > (the > > small difference between the 2 measured values - > may suggest this) > > by the > > local environment. > > > > That test is to short the oscilloscope probe > ground to the > > oscilloscope > > probe tip. Then move the probe near the area you > were measuring. You > > will > > then "see" the measurement environment as a > single turn loop. If the > > noise > > is there at nearly the same levels, the loop area > of the ground is > > suspect - > > or - look at the rest of the measuring > environment. The rest of the > > measuring environment is made up of the probe > body (in some less > > expensive > > probes), probe cable, oscilloscope environment, > etc... It sounds > > funny, but > > if you think of a debug station is , you have > power supplies with > > long > > cables, power cords, Personal Computers, Display > Monitors, keyboard > > and > > mouse cables, etc... I actually walked into a > situation once with a > > high > > resolution monitor at a debug station that > coupled noise to the > > oscilloscope > > probe cable causing a false reading (CRT card > emissions, and Yoke > > emissions). > > > > Aside from this possibility, you have received > good advice from the > > LIST > > contributors. One thing I wanted to mention is > that when we think of > > capacitance from the power plane structure, we > often consider it > > high > > quality capacitance dominated by capaitance not > inductance. As vias > > penetrate that plane capacitance structure, the > planes become more > > inductive > > (capillary inductive action) the quality of that > capacitance > > degrades. With > > high pin count BGAs, like the 1152 pin packages, > really perforate > > the planes > > and the buried capacitance suffers. > Unfortunately, that is where we > > need the > > plane capacitance for the higher frequency output > drivers and core > > voltages. > > Creative device fanout helps, and using > micro-vias help that > > situation, but > > are expensive. > > > > Has anyone performed research on how the plane > capacitance is > > degraded by > > large BGAs perforations? Both donut style and > full package > > perforations? > > > > Good Luck. > > > > Philip Ross Wellington > > Mgr. Signal Integrity & EMI > > L-3 Communications CSW > > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in > the Subject field > > or to administer your membership from a web page, go > to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the > Subject field > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are > viewable at: > === message truncated === __________________________________________________ Do You Yahoo!? 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