[time-nuts] LT3042, etc. Re: HP E1938A schematics.

[Poul-Henning Kamp]

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In message <20190709053037.2D244406063 at ip-64-139-1-69.sjc.megapath.net>, Hal Mu
rray writes:

>What's the advantage of a PSRR in the MHz range?  Is it as simple as reducing 
>the number and size of the caps needed?

The caps have only ever acted as a low-pass filter, to move the
noise down in the frequency range of the power-supply regulation.

The digital switching noise happens in (pico- and nano-)Coloumbs,
not in volts or amps, which means that as the supply voltage
decreases, it becomes percentwise larger voltage noise.

To deal with that passively you would need bigger caps with
better high frequency performance, and lower ESR and more,
expensive PCBs (more layers etc.)

The trend is therefore to move the corner frequency of the passive
low-pass filter higher, that allows you to use ceramic capacitors,
instead you must increase the bandwidth of the final power-supply,
typically a LDO, and move it physically closer to the load.

On high-end kit, it is not uncommon to see a big chip surrounded
by a ring of tiny LDO's spaced as little as a centimeter apart along
its periphery.

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