[time-nuts] How to measure regulator noise?

Poul-Henning Kamp phk at phk.freebsd.dk
Mon Jan 7 04:37:01 EST 2008

In message <079101c8510f$af8c35a0$7900a8c0 at athlon1200>, "Dave Brown" writes:

>You won't need much of a cap for DC blocking as inputs are hi-Z....too 
>big and the chargeup may well trigger the overrange condition.

You need to put a shunt resistor after the capacitor if the input is hi-Z
but an even better way is to offset the gnd clip with a couple of fresh
batteries so that you can avoid the capacitor.

Batteries have very low, but not zero noise, so what you do is:

Put six fresh 1.5 volt batteries in series, so that three of them
is the "wrong way" and the resultant voltage is zero and then you
measure their noise.

Then you put three of them "the wrong way" on your 5v supply, so that
the output voltage is only .5V and then you measure the noise.

That worked fine for me using a HP6885B

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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