Re: [time-nuts] Low noise power supplies?

Thu, 31 Jan 2013 14:01:50 -0800 

There are at least two methods.
First, the broadband or time domain method that uses a low noise amplifier and oscilloscope, and specifies noise voltage within a certain bandwidth. For details check application notes from LTC and Analog Devices. 


The narrowband or frequency domain method specifies noise voltage within 
a normalized bandwidth (nV/sqrt(Hz)) at a specific frequency.
A typical test setup is included in the HP 3048A phase noise measurement 
system as so-called base-band noise measurement. For higher noise 
voltages, the 3561A dynamic signal analyzer alone is used with a DC 
blocking capacitor. For lower noise voltages, the signal is first 
amplified by the LNA that is part of the 11848A phase noise test set. 
For details please see the HP 3048A manual that can be downloaded from 
http://www.hparchive.com/hp_equipment.htm
Note the Opt K23 DC blocking filter. There is an insertion loss of 6 dB 
to be entered in the 3048A software when using the DC block in 
conjunction with the 11848A.
Obviously, any FFT analyzer plus LNA and coupling cap can be used for 
that method. The major advantage of the 3048A system is the automatic 
generation of plots.


Adrian

John Ackermann N8UR schrieb:

And this (very interesting) thread brings up the question of 
measurement methods.  Some time ago I searched around and didn't find 
much on a standard way to measure noise on low voltage DC supplies.


John
----

On 1/31/2013 11:36 AM, Bob Camp wrote:

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: [email protected] [mailto:[email protected]] 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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