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