Set your scope for AC coupling. Set your scope probe for 1x rather than 10x. 
Use the absolutely shortest scope grounding you can. That’s what those spring 
looking things that came with it are for. I typically use the spring gizmo and 
probe on an SMD cap. The ground wire with an alligator clip will just pick up 
far more noise than you’ll be measuring. This is how I was able to measure the 
noise and ripple of the SC189Z switcher feeding the OCXO in my GPSDO. I got 
measurements of ~4 mV P-P that way. Be careful you don’t get the probe and 
ground reversed - your scope won’t likely have an isolated ground from your DUT 
and that would therefore be bad.

You’re going to want to check the supply’s performance under load. For that, 
you’ll may want to get yourself a dummy load. I got one from Tindie for testing 
my Pi Power design: https://www.tindie.com/products/arachnidlabs/reload-2/



> On Oct 14, 2016, at 11:00 AM, Cube Central <cubecent...@gmail.com> wrote:
> 
> How would one go about testing power supplies and seeing how noisy they are?  
> I have the standard suite of tools, an oscilloscope and a little (dangerous) 
> know-how.  I am just not sure what to look for or how to safely hook it up to 
> test.
> 
> Thanks in advance for any tips!  
> 
>       -Randal R.
>               (at CubeCentral)
> 
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Chris 
> Albertson
> Sent: Friday, 14 October, 2016 02:29
> To: Discussion of precise time and frequency measurement <time-nuts@febo.com>
> Subject: Re: [time-nuts] For those that insist on using switching power 
> supplies
> 
> On Thu, Oct 13, 2016 at 6:05 AM, Van Horn, David 
> <david.vanh...@backcountryaccess.com> wrote:
>> To be fair here, phone chargers have almost no requirement to be quiet other 
>> than conducted and radiated emissions limits.
>> It's charging a battery.
> 
> Not only that but,  the 5 volts comping out of the larger is almost certainly 
> the input to another DC/DC power supply and NOT used directly.
> You can't charge a Lithium battery with the 5 volts the charger outputs.
> 
> If you don't know about LiPo batteries, they need a constant current power 
> source and then as they get close to charged the charger switches to constant 
> voltage (VERY roughly) at about 4V per cell.
> 
> I have a project right here on my desk as I type.  I'm using the output of a 
> generic USB hub.  The circuit is  a cap from 5V to GND and
> then a low dropout regulator to get 3.3 volts.    I don't care to much
> if there is huge ripple on the 5.0 volts coming in as long as it stays above 
> the LDO limit.
> 
> Also it looks like they tested the USB chargers with no load.  A typical load 
> might have a say, 0.01uf cap to short the noise to ground.  So in use the 
> power might be better?
> 
> It was no surprise the counterfeit chargers were horrible.  The manufacturers 
> are by definition of "counterfeit" being dishonest slim balls. Why would he 
> care about anything other then that he can fool
> some people into buying his product.   There are third party chargers
> that are not trying to copy a well known brand, these are usually much better
> 
> 
> -- 
> 
> Chris Albertson
> Redondo Beach, California
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