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From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: spread-spectrum model
Date: Sun, 21 Apr 2024 11:23:10 -0400
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On Sun, 21 Apr 2024 08:50:23 -0400, legg <legg@nospam.magma.ca> wrote:

>On Sat, 20 Apr 2024 10:57:17 -0700, John Larkin
><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>
>>On Sat, 20 Apr 2024 10:34:46 -0400, legg <legg@nospam.magma.ca> wrote:
>>
>>>On Thu, 18 Apr 2024 12:14:04 -0700, John Larkin
>>><jjSNIPlarkin@highNONOlandtechnology.com> wrote:
>>>
>>>>On Thu, 18 Apr 2024 13:16:04 -0400, Joe Gwinn <joegwinn@comcast.net>
>>>>wrote:
>>>>
>>>>>On Thu, 18 Apr 2024 08:26:56 -0700, John Larkin
>>>>><jjSNIPlarkin@highNONOlandtechnology.com> 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