Path: ...!Xl.tags.giganews.com!local-2.nntp.ord.giganews.com!news.giganews.com.POSTED!not-for-mail NNTP-Posting-Date: Sun, 21 Apr 2024 15:23:10 +0000 From: Joe Gwinn Newsgroups: sci.electronics.design Subject: Re: spread-spectrum model Date: Sun, 21 Apr 2024 11:23:10 -0400 Message-ID: References: <7qk22jlrrc9949ccrkdk058b4dinnnt75f@4ax.com> User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 70 X-Usenet-Provider: http://www.giganews.com X-Trace: sv3-OkGR/917wuxpBtBnY9hw4zjoQkkb9OMZnHDyXO9MWaCEGtOGRJtzUe/JFClhcKirgmUYWxUSwVTVljg!4NnLtw+lP1OjQrIt487bGmjzsYKM724qmQ7fHbiFJCLeY2/iJIgFYes34Nd7YvValxaW390= X-Complaints-To: abuse@giganews.com X-DMCA-Notifications: http://www.giganews.com/info/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 4082 On Sun, 21 Apr 2024 08:50:23 -0400, legg wrote: >On Sat, 20 Apr 2024 10:57:17 -0700, John Larkin > wrote: > >>On Sat, 20 Apr 2024 10:34:46 -0400, legg wrote: >> >>>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin >>> wrote: >>> >>>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn >>>>wrote: >>>> >>>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin >>>>> wrote: >>>>> >>>>>>I'm designing a switching power supply module and could reduce EMI by >>>>>>going spread-spectrum on the switching frequency. The simple one below >>>>>>reduces things by 20 dB. Probe the SS node and FFT. >>>>>> >>>>>>The ss inside switching reg chips is no doubt more sophisticated. In >>>>>>an FPGA, we could do some sort of pseudo-random thing. >>>>>> >>>>>>On a multi-channel power supply, there may be some small advantage to >>>>>>have a separate spread per channel. That would be easy. >>>>> >>>>>I'd check for cross-correlation as well, so no ganging up in systems >>>>>using multiple channels in some signal path. >>>> >>>>When my engineers get too fussy about stuff like that, I remind them >>>>"it's just a power supply." >>> >>>Noise at the local level is best correlated, as it is more >>>predictable - you avoid low-frequency beat frequencies in the >>>local regulators - which can and will show up in a detector's >>>BW and in the regulators' outputs. >> >>But...but... it's just a power supply! >> >>Presumably uncorrelated spread-spectrum will make wideband noise at an >>output, not a beat. >> >>> >>>A master clock, phase shifted for various local users, can be dithered >>>for the system (box), which is the actual, final radiator. >> >>Our box has a 50 MHz clock that is bussed to all the plugin modules, >>and it can be locked to other boxes or to a 10 MHz reference, so we >>can't usefully dither that. I guess each module could have its own >>VCO, but that would mess up synchronizing modules, and complicate >>things. Spread-spectrum sounds easier. >> >>> >>>Your engineers can get REAL fussy, if the system's non-compliant >>>way past the development's due date. >> >>Eventually, some giant customer may want CE stickers, so we'll do the >>easier things now, to improve our chances of passing an EMI test. A >>bit of VHDL in the FPGAs would be easy. > >Unsynchronized power supplies on the same board can >influence each other, unpredictably with load, to produce >audible harmonics. > >Ignore the effects at your peril. Yes. Fix the shielding and grounding story until these effects no longer matter. This is done in radar, as discussed upthread. Joe Gwinn