[time-nuts] Switching regulator replacement for 7805

Attila Kinali attila at kinali.ch
Sun Dec 4 17:58:22 EST 2016

On Sun, 4 Dec 2016 16:22:02 -0500
Scott Stobbe <scott.j.stobbe at gmail.com> wrote:

> If you wanted to be nutty you wouldn't go PWM at all, just like
> fractional-N sythns don't just mash 2 divider values. You would sigma-delta
> modulate your power stage. I don't know if you can buy one COTS, but there
> are plenty of papers on rolling your own.

I guess you are refering to spread-spectrum techniques.
Such DC/DC converters exist, but are usually those with high power
ratings. IMHO it is also not worth the effort, as its main use is
to meet EMI emission requirements. The only application that comes
to my mind where spread-spectrum actually helps are high sensitiv
radio receivers where every spur is a nuisance. For most other
use, and time-nuts use in particular, it is much less useful.
The noise energy is not gone. It is still there, just spread over
a large bandwidth. In time measurement applications, noise is
integrated over time _and_ frequency. Thus even if the noise is
spread over a large bandwidth, the energy will still contribute
to the uncertainty and degrade the ADEV. It will be just harder
to identify as the peak is now much smaller and wanders in frequency.

It is much better to the design such, that as little as possible
of the switching energy leaks out of the DC/DC converter and filter
out the rest.

Depending on the application, another possible application is to
sync up the DC/DC converter to the "main" clock source. This makes
the switching noise then coherent to the system, which either makes
it average out completely, or possible to filter it out in the digital
domain using a deep notch-filter in receiver applications.

			Attila Kinali
Malek's Law:
        Any simple idea will be worded in the most complicated way.

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