Re: [volt-nuts] Practical power supply noise testing

[Scott Stobbe]

If you are looking for sub 10 Hz noise you could just null bridge two of
them with an ordinary multimeter.

On Fri, Jul 1, 2016 at 8:47 AM, John Ackermann N8UR  wrote:

> I have several supposedly low noise bench power supplies that I want to
> check out.  They seem to work well but are quite old, so of course one
> wonders about the caps, etc.
>
> Can someone suggest a practical test regime to verify power supply DC
> noise performance?  Preferably one that doesn't require building up custom
> low noise amps?
>
> Thanks!
>
> John
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Re: [volt-nuts] Practical power supply noise testing

[Bruce Griffiths]

Its certainly a good way to quickly check if anythings drastically wrong and 
one could even compare them with a known source such as an LM317 based 
regulator with no ADJ pin bypass. If one can hear the noise then its possible 
that a sound card could make some meaningful measurements of the headphone amp 
output.
John, do you have any specs for the supplies?It would be nice to know the 
likely best case output noise level.
To measure the ripple of most supplies I usually just connect my USB scope (dc 
coupled) to the supply output.This certainly works (with my 14 bit scope at 
least) with E3610A/E3611A series supplies. For lower ripple supplies I use AC 
coupling.
The 100Hz (in my case) ripple shows up nicely in the FFT. 

Bruce
 

  From: Poul-Henning Kamp <p...@phk.freebsd.dk>
 To: Discussion of precise voltage measurement <volt-nuts@febo.com>; Brooke 
Clarke <bro...@pacific.net> 
 Sent: Saturday, 2 July 2016 11:28 AM
 Subject: Re: [volt-nuts] Practical power supply noise testing
   

In message <5776e31d.7090...@pacific.net>, Brooke Clarke writes:

>Instead of headphones something like the HP 4395A [...]

Well, there is a slight difference in price there, isn't there ? :-)

-- 
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
p...@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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Re: [volt-nuts] Practical power supply noise testing

[Poul-Henning Kamp]

In message <5776e31d.7090...@pacific.net>, Brooke Clarke writes:

>Instead of headphones something like the HP 4395A [...]

Well, there is a slight difference in price there, isn't there ? :-)

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@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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Re: [volt-nuts] Practical power supply noise testing

[Poul-Henning Kamp]

In message <2656f010-8e12-4bad-b9ae-713cefee1...@febo.com>, John Ackermann N8UR 
writes:
>I have several supposedly low noise bench power supplies that I want to check 
>out.  They seem to work well but are quite old, so of course one wonders about 
>the caps, etc.
>
>Can someone suggest a practical test regime to verify power supply
>DC noise performance?  Preferably one that doesn't require building
>up custom low noise amps?

Listen to them ?

All the "hard" noise is in the audio-range, so a series capacitor
a low-noise amp and a pair of headphones...

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@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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Re: [volt-nuts] Practical power supply noise testing

[David]

In the past when I did this, I used a high resolution digital
voltmeter like a Fluke 8505A in sample mode with a simple 10 Hz low
pass filter.  I took samples for 10 seconds to get down to 0.1 Hz and
calculated the standard deviation to get RMS noise.  I made a recent
post about this in connection with very low noise DC amplifiers.

To measure output ripple, I use a DSO set to line triggering.
Averaging removes all noise which is not synchronous with the AC line
frequency.

A low noise preamplifier with offset capability or a very low cutoff
frequency is still needed for lower level noise measurements.  I have
a Tektronix 7A22 now but its AC coupled low pass cutoff is only
specified as better than 2 Hz and its offset capability is somewhat
limited.  It works great to help with higher frequency bandpass
measurements though.  A 7A13 has slightly better offset capability but
is also higher noise.

You said you want to avoid building a custom low noise amplifier
however for more sensitive measurements, even a simple operational
amplifier based design will help considerably.  I always hack together
an AC coupled amplifier using a low noise JFET operational amplifier
as needed; the quality of the AC coupling capacitor is what limits low
frequency performance.

Check out Linear Technology application notes 83, 124, and 159 for
some ideas and a discussion of the AC coupling capacitor problem:

http://cds.linear.com/docs/en/application-note/an83f.pdf
http://cds.linear.com/docs/en/application-note/an124f.pdf
http://cds.linear.com/docs/en/application-note/an159fa.pdf

On Fri, 1 Jul 2016 08:47:18 -0400, you wrote:

>I have several supposedly low noise bench power supplies that I want to check 
>out.  They seem to work well but are quite old, so of course one wonders about 
>the caps, etc.
>
>Can someone suggest a practical test regime to verify power supply DC noise 
>performance?  Preferably one that doesn't require building up custom low noise 
>amps?
>
>Thanks!
>
>John
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Re: [volt-nuts] Practical power supply noise testing

[Bruce Griffiths]

I've used a pair of minicircuits phase modulators, a low noise OXCO, and a 
Timepod to do this.The phase modulators work best with about 9V input.The noise 
of my E3610A's is clearly evident as is the noise of an LM317 based regulator 
with unbypassed ADJ terminal.To achieve lower system noise a pair of preamps 
(one per phase modulator) is useful, however the noise need not exceptionally 
low as cross correlation lowers the effective preamp noise.
Bruce


  From: John Ackermann N8UR 
 To: volt-nuts@febo.com 
 Sent: Saturday, 2 July 2016 12:47 AM
 Subject: [volt-nuts] Practical power supply noise testing
   
I have several supposedly low noise bench power supplies that I want to check 
out.  They seem to work well but are quite old, so of course one wonders about 
the caps, etc.

Can someone suggest a practical test regime to verify power supply DC noise 
performance?  Preferably one that doesn't require building up custom low noise 
amps?

Thanks!

John
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