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From: legg <legg@nospam.magma.ca>
Newsgroups: sci.electronics.design
Subject: Re: spread-spectrum model
Date: Sat, 20 Apr 2024 10:34:46 -0400
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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.

A master clock, phase shifted for various local users, can be dithered
for the system (box), which is the actual, final radiator.

Your engineers can get REAL fussy, if the system's non-compliant 
way past the development's due date.

RL