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From: legg <legg@nospam.magma.ca>
Newsgroups: sci.electronics.design
Subject: Re: spread-spectrum model
Date: Sun, 21 Apr 2024 08:50:23 -0400
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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.

RL