[time-nuts] Low noise power supplies?

[Bob Camp]

Hi

I think the comparison of PSD on a power supply to phase noise and phase
noise plots is a pretty good one in this case. 

For most applications nV/sqrt(Hz) is a pretty good way to check things out
on a supply or regulator. It's not quite the same thing as dbc / sqrt(Hz)
but it conveys the same sort of information. Unless you have a *very*
sensitive part, anything below 10 nV/ sqrt(Hz) is likely to be "a very quiet
supply". The main limit you hit is in the sub 100 Hz region where you likely
see things like popcorn noise. 

AC ripple is no different than spurs in phase noise testing. You sometimes
see people who ignore them when plotting phase noise. I'd suggest that they
are an important part of characterizing a power supply. 

As with phase noise, frequency ranges are going to be application dependant.
I may not care about 0.1 to 10 Hz phase noise for project A. It may be the
only thing I care about for project B. Same thing with power supplies. 

With a good enough voltmeter you could carry the analogy one step further
and compute an ADEV like number on the output voltage. I suspect that's
carrying things a bit far.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Tom Van Baak
Sent: Thursday, January 31, 2013 9:43 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Low noise power supplies?

Hi Rick,

I'll answer the question with anther question -- how does one properly
measure power supply noise? Does it boil down to a single number, a couple
of key numbers, or is it a plot, or several plots?

I ask because without some sort of standard test and reporting method it
just becomes a word game. Like, "use batteries because they are better";
"use my design because it is quiet"; "this has lower noise than that". RMS
AC ripple or nV/rtHz sounds like a good start, but I'm wondering if there's
something more complete.

I was never quite satisfied with the outcome of comparing a half dozen power
supplies this way:
http://www.leapsecond.com/pages/tbolt/noise.htm

We have rigorous ways to compare and report oscillator performance; both as
numbers and as plots. Is there something equivalent for power supplies?

Thanks,
/tvb

> I know this topic has been discussed in the past on the list, but
> a colleague is asking if there are any off the shelf low
> noise power supplies for testing oscillators.  Something
> a cut above an HP "brick" lab power supply etc.  They are hoping
> to avoid having to homebrew a power conditioning circuit.
> Did we ever arrive at a concensus as to the state of the art
> in homebrew power conditioning circuits?
> 
> Any help would be appreciated.
> 
> Rick Karlquist N6RK


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