Path: Xl.tags.giganews.com!local-2.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Sat, 20 Apr 2024 17:59:02 +0000 From: John Larkin Newsgroups: sci.electronics.design Subject: Re: spread-spectrum model Date: Sat, 20 Apr 2024 10:57:17 -0700 Organization: Highland Tech Reply-To: xx@yy.com Message-ID: References: <7qk22jlrrc9949ccrkdk058b4dinnnt75f@4ax.com> X-Newsreader: Forte Agent 3.1/32.783 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 55 X-Trace: sv3-Op6n3Iax7Rra7+QWLm7yXJYaV9995DCaLuwjWWMvVe7JKCRvZ6ESswKlhVfhQJ7kCUEoLST5rbyj9jk!aq2ut6CJLTudST5ALx5q2M8PDBnIJGu/ykrmJ2XGYebQL+oZtNDN5AwUHSPsThdt+frOxFsrts29!E85oPQ== X-Complaints-To: www.supernews.com/docs/abuse.html X-DMCA-Complaints-To: www.supernews.com/docs/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: 3453 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.