Steve. Caps are ok but they are bi-directional and narrow band. They work great when you have all the frequencies figured out or in a narrow band app. I have seen them work great for say 10mhz applications but then there is a 300-500 mhz tone causing problems what do you do? This happens all too often on digital boards. We live in a wide band world now and so that is why I partially dissagree. Richard > Date: Mon, 15 Sep 2008 14:09:05 -0700 > From: weirsi@xxxxxxxxxx > To: r_jungert@xxxxxxxxxxx > CC: mrose@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] Re: plane-to-plane decoupling > > Richard, in my experience, linear regulators can be effective for > cleaning up PLL power on the PCB but they are usually completely > unnecessary. The bandwidth of even a good linear regulator by itself is > still in the audio range. If your PLL is insensitive to audio frequency > disturbances by virtue of a good loop, then what you are really buying > with the linear regulator is a very expensive series L/R that works with > the bypass caps on the PLL side to filter noise from the dirty digital > realm. With a little care you can usually save a lot of: board > real-estate, money, and complication by designing a passive filter to do > the same or better job. > > Steve. > > Richard Jungert wrote: > > Mike. > > > > I would recommend putting the PLL circuit near the edge of the board and > > also cut a power and ground plane split 270 degrees around or under the PLL > > circuit. Cut them both the same shape. If you put it near the center of > > the board there is a very high probablility for much more jitter. > > > > Also, in my experience its not a great idea to power the PLL with a common > > voltage. Put a small low power voltage regulator in to just power the PLL > > circuit and this will isolate him from the rest of the noisy digital logic. > > Power and ground plane noise will sneak right into the PLL and modulate > > your clock. > > > > I have found on three other designs that this approach is very effective in > > cutting down phase noise on the clock circuits. > > > > Another trick that cuts noise on ground/power planes is decoupling with > > caps out to the chassis ground. Decouple out to the chassis ground and > > watch the noise decrease. This is also a trick used to supress EMC > > problems. > > > > Richard Jungert > > > > > > > > > >> Subject: [SI-LIST] Re: plane-to-plane decoupling > >> Date: Mon, 15 Sep 2008 12:14:31 -0500 > >> From: zabinski.patrick@xxxxxxxx > >> To: mrose@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx > >> > >> Mike, > >> > >> In answer to your last question, I believe the lowest-inductance option > >> involves capacitors placed directly between the two power planes (and > >> not through the ground plane). > >> > >> That said, the scenario scares me. > >> > >> For many components that require multiple planes (e.g., FPGAs w/ > >> SerDes), some of their supplies are very sensitive to noise. For > >> example, nearly all SerDes cores have an analog supply for phase-locked > >> loops (PLLs), and noise injected into them can severely degrade jitter > >> performance. Similarly, noise on the I/O supply of a parallel bus can > >> degrade SI of the output waveform. In many instances, the chip supplier > >> provides guidelines for isolating such voltages, often recommending > >> specific isolation circuits. Adding decoupling between the various > >> voltages will/can defeat the isolation circuits and inject noise from > >> one plane onto the next. > >> > >> Although adding decoupling between the power planes will help with > >> discontinuities of the signals traversing the split, you could be > >> inadvertently creating other problems. As such, I suggest you look > >> closely at the power planes that you're about to inject noise into and > >> ensure they are immune to outside influence. > >> > >> Good luck, > >> Pat Zabinski > >> Mayo Clinic > >> > >> > >> > >>> Some diff pairs on L4 will cross power plane splits (actual different > >>> power sources and loads) and I wanted to provide an effective AC path > >>> for any common-mode return currents. I was thinking about placing some > >>> nearby decoupling caps from plane-to-plane across the split. Do you > >>> think it would be better to decouple from plane-to-ground on > >>> both sides > >>> to steer the current through the L6 ground layer? L5 and L6 > >>> are already > >>> coupled through the inter-plane capacitance (they're about > >>> 4mils apart). > >>> Which will provide a lower inductance path? > >>> > >> > >> ------------------------------------------------------------------ > >> 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 technical documents are available at: > >> http://www.si-list.net > >> > >> 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 > >> > >> > >> > > > > _________________________________________________________________ > > Get more out of the Web. Learn 10 hidden secrets of Windows Live. > > http://windowslive.com/connect/post/jamiethomson.spaces.live.com-Blog-cns!550F681DAD532637!5295.entry?ocid=TXT_TAGLM_WL_domore_092008 > > ------------------------------------------------------------------ > > 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 technical documents are available at: > > http://www.si-list.net > > > > 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 > > > > > > > > > > > -- > Steve Weir > Teraspeed Consulting Group LLC > 121 North River Drive > Narragansett, RI 02882 > > California office > (866) 675-4630 Business > (707) 780-1951 Fax > > Main office > (401) 284-1827 Business > (401) 284-1840 Fax > > Oregon office > (503) 430-1065 Business > (503) 430-1285 Fax > > http://www.teraspeed.com > This e-mail contains proprietary and confidential intellectual property of > Teraspeed Consulting Group LLC > ------------------------------------------------------------------------------------------------------ > Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC > > ------------------------------------------------------------------ > 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 technical documents are available at: > http://www.si-list.net > > 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 > > _________________________________________________________________ Stay up to date on your PC, the Web, and your mobile phone with Windows Live. http://clk.atdmt.com/MRT/go/msnnkwxp1020093185mrt/direct/01/ ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net 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