It is also wise to measure the noise floor of the test setup as fully connected as possible. For one of those usb charger cubes, you can use a power bar with a power switch (which will only switch the hot line on/off, earth and neutral are permanently connected, one would hope). You will see a lot of other noise sources before you even power up your DUT.
On Fri, Oct 14, 2016 at 2:16 PM, Nick Sayer via time-nuts < time-nuts@febo.com> wrote: > 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.vanhorn@ > 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 > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.