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 -----Original Message----- From: ANAND KURIAKOSE [mailto:Anand.Kuriakose@xxxxxxxxxx] Sent: Thursday, August 08, 2002 11:08 AM To: Larry Smith Cc: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Noise on BGA core voltage rail Hi Larry, The board has 2 power-ground pairs. The spacing b/n the power and the ground plane is 4 mils. The power planes carry 2.5 V rail (the noisy rail) along with other volttages on the board. I have one question. In addition to having power-ground pairs yeilding low impedances at high frequencies above 100Mhz (which seems to be the only solution according to you), what other precautions should be taken during the design or layout to have low target impedances from DC to around 200Mhz-600Mhz? Anand. From: Larry Smith <Larry.Smith@xxxxxxx> on 08/09/2002 12:03 AM Please respond to Larry Smith <Larry.Smith@xxxxxxx>@SMTP@Exchange To: si-list@xxxxxxxxxxxxx@SMTP@Exchange, ANAND KURIAKOSE/Apex Data Inc/01@Apex Data Inc cc: Subject: Re: [SI-LIST] Noise on BGA core voltage rail Anand - It is difficult to do anything about power distribution noise at 200 MHz and above by using discreet decoupling capacitors. Your best ally at high frequency is the PCB power planes. They are effective at high frequency but may have cavity resonances which depend upon the dimensions of the board. The best way to make the PCB power planes effective is by defining a stackup that has power/Gnd plane pairs next to each other. The dielectric thickness between the planes determines the capacitance and spreading inductance of the planes. Thinner is better. 4 mils between planes is good, 2 mils is better. Several years ago, it was common to see power and ground planes separated by 14 mils or so in order to accommodate 2 signal layers between the planes. With that spacing, the performance of the decoupling capacitors is limited by the spreading inductance of the power planes. If you have more than a few dozen ceramic capacitors, you must use adjacent power planes in the stackup in order to make them effective. Otherwise, the impedance of the planes dominates over the impedance of the capacitors at high frequency. regards, Larry Smith Sun Microsystems > Delivered-To: si-list@xxxxxxxxxxxxx > X-Lotus-FromDomain: APEX DATA INC > From: Anand.Kuriakose@xxxxxxxxxx > To: si-list@xxxxxxxxxxxxx > Date: Thu, 8 Aug 2002 22:06:59 +0630 > Subject: [SI-LIST] Noise on BGA core voltage rail > Mime-Version: 1.0 > Content-Disposition: inline > Content-Transfer-Encoding: 8bit > X-archive-position: 3678 > X-ecartis-version: Ecartis v1.0.0 > X-original-sender: Anand.Kuriakose@xxxxxxxxxx > X-list: si-list > > > > > Hi, > > Heres a situation where i have a BGA (chipset with interface to processor, DDR > memory and other high speed proprietry buses) with sufficient decoupling > sprinkled around the BGA. The decoupling on the core voltage rail (2.5V, which > is also the I/O voltage for DDR interface) basically consists of 2 high value > bulk capacitors, six 1uf caps, ten each of 0.1uF and 0.01uF caps. I am noticing > noise around 150mv of noise during activity on the DDR (using software > utilities) and roughly 80-100 mV during almost no activity across the chip. > This amplitude is significantly more than the noise noticed at other high speed > chips on the board. Using the FFT function on the DSO, i figured out that the > frequencies where it peaks are 200Mhz and integral multiples of 200Mhz. > > I tried a couple of things: > > 1> Since i am seeing peaks at 200Mhz and its integral multiples, i thought > that there could be insufficient high frequency decoupling and hence i replace > the 0.1uF caps with 1000pF caps. > 2> Secondly, fearing that there could be some resonance happening due the > different values of caps used, i replaced all the 0.1 uF caps with 0.01uF caps > (in addition to existing 0.01uF caps). > > Both the above strategies failed to reduce the noise. There was no change in the > amplitude of the noise and also the frequencies where peaks were noticed. > > Can somebody out there throw some light on what is lacking in the strategies > mentioned above and how to reduce this noise satisfactorily. > > Thanx in advance. > Anand. > > > ------------------------------------------------------------------ > 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: > http://www.qsl.net/wb6tpu > > ------------------------------------------------------------------ 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: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ 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: http://www.qsl.net/wb6tpu