How To Measure Power Supply Noise - Page 1
Forum section: electronics / beginners

author: mdijkens
author_rank: Regular Contributor
timestamp: June 17, 2017, 06:23:38 pm
content: I've build a small linear power supply which I want to make some simple noise measuring with.It is powered with a Lipo-battery to have minimal ripple and noise.The supply is under 900mA load @5VI have looked at EEVblog #594 about measuring ripple and noise and tried 3 different methods with the equipment I have:(Scope is on AC/peak detect/20MHz limit/1mV/1mS)1. Single x1 probe without ground lead connected to banana-bnc adapter on power supply. I measure 5mV noise.2. Two x1 probes each with inline 50 ohm terminator. I measure CH1-CH2 = 1.77mV * (50+315 ohm probe / 50 ohm terminator) = 13mV noise.3. One coax directly connected to banana-bnc adapter on one end and inline 50 ohm terminator on scope end. I measure 22mV noise.I did all measurements several times.Which of these measurements is most accurate?Why these differences? What am I doing wrong?

author: ocset
author_rank: Super Contributor
timestamp: June 17, 2017, 07:48:23 pm
content: All you need for your purpose is a good quality bit of coaxial cable with a BNC connector at one end to go into the scope (not the 50ohm input). Then the other end you kind of split it and solder the coax sheath to your gnd, and the inner to your power rail, right next to each other.Don’t worry about the capacitance of the cable, because on a regulator output you should have a biggish cap anyway.Maybe also some relevance here…http://www.edaboard.com/thread358562.htmlhttp://www.edaboard.com/thread254629.html

author: danadak
author_rank: Super Contributor
timestamp: June 17, 2017, 08:24:57 pm
content: Put your scope on infinite persistence and let it run fora minute or two. Then you will see pk-pk broadband noise.The longer you let it run the greater the accuracy of theresult.If you wanted to look at it in a specific BW some scopeshave the ability to filter a signal and perform infinite persistenceon the filtered signal.http://literature.cdn.keysight.com/litweb/pdf/5989-3020EN.pdf?id=677260Regards, Dana.

author: mdijkens
author_rank: Regular Contributor
timestamp: June 17, 2017, 08:50:40 pm
content: Thanks for the quick replies@treezIf I leave out the 50 ohm terminator, I measure 50mV - 70mV depending on how I move the coax cable/connectors: So it is very sensitive to my setup@danadakDid that also, but still the different results with the 3 different methods...I may have cheap equipment all the way, but still want to find the 'best' or 'least worst' way to measure. Now I'm still not sure if my power supply has noise of 5, 15. 25 or 50 mV ....

author: alm
author_rank: Super Contributor
timestamp: June 17, 2017, 09:17:55 pm
content: Check Dave's video on common mode noise (short your test leads near the power supply output terminals). Especially with different methods, common mode noise could easily be (partly) responsible for the differences.

author: mdijkens
author_rank: Regular Contributor
timestamp: June 17, 2017, 09:44:06 pm
content: Yes, that could be the reason. Saw the video.But it's kind of difficult in my home to detect the source(s)...The single probe measurement (with lowest noise) has almost no spikes and so could be the cleanest measuring method?Or is that just wishfull thinking ?It also has least adapters/leads/connections/etc and does not respond to touching or bending the probe lead.I use a small springclip on the probe to connect to positive and negative of the power supply...

author: alm
author_rank: Super Contributor
timestamp: June 18, 2017, 07:14:40 am
content: What you see in EEVblog #441 is that Dave disables the output (the classical way is to connect the probe tip to ground at the DUT). That does not require any extra gear or investment. Same with switching off the power supply. Do you still see noise with any of the methods?Is the power supply grounded? In that case ground loop currents could increase the apparent noise. In that case, using a differential set up would be preferable.#1 (assuming you use a grounding clip) should work best with a powers supply where both rails have a high impedance to ground (think battery). Common mode signals should not be an issue, and it keeps the loop area between probe tip and ground as small as possible.#2 As for termination, I don't agree with Dave's method of using a 50 Ohm terminator with 1x probe. It attenuates an already weak signal and screws up the frequency response (look at the probe compensation signal). I would rather ground the DUT using some other method (probe ground shield or even separate grounding wire). In that case this method should work well for DUTs where common mode is an issue (think grounded power supplies or power supplies with a capacitive coupling to ground). However, the loop area between the probes is much greater, increasing the potential for inductive pickup of external signals (e.g. lighthing). Also CH1-CH2 will have a limited common mode rejection.#3 Should work fine with termination, but only if the output voltage is low enough as not to exceed the rating of the terminator (which might only be 0.5 W) and if the 50 Ohm termination will not load the power supply appreciably. The noise of power supply tends to be higher under load. Try adding a 50 Ohm resistor across the power rails with the other methods to see if it makes a difference. The banana to BNC adapter should be pretty good for inductive pickup (small loop area).

author: Andreas
author_rank: Super Contributor
timestamp: June 18, 2017, 09:28:44 am
content: Quote from: mdijkens on June 17, 2017, 08:50:40 pmHello,some rules for measurement of noise:- place the DUT in a shielded cabinet (I use a cookies box, others a paint can) connected to the GND-point of your scope.see also AN124 Figure A1:http://cds.linear.com/docs/en/application-note/an124f.pdf- do not use pig-tails only short spring connections are allowed- for commercial power supplies a pomona connector might be an option (if the supply is in a metal case)https://www.digikey.de/products/de?keywords=501-1032-ND- determine the bandwidth for which you want to measure the noise (e.g. 10 Hz - 100kHz wideband noise or 0.1-10Hz 1/f noise)- set the filters on your scope accordingly (or use a filter amplifier)- for low noise power supplies you will need a amplifier 40-60 dB for wideband noise or even 80 dB for 1/f noise.- always determine the noise floor before beginning measurement.the noise floor needs to be at least a factor of 3 below the expected measurement to get correct (within 10%) readings.with best regardsAndreas

author: mdijkens
author_rank: Regular Contributor
timestamp: June 18, 2017, 11:18:10 am
content: Thanks again.Did all measurements also when turning off DUT and guess what... noise is only a few mV less with DUT off. So am I only measuring common mode noise with all three setups?DUT is battery powered, scope also tested without AC ground but no difference, Not using pig-tails only spring-clip (least noise) and direct BNC connections ( I do use the so called pomona connector)The measured noise is all in the range of 1 MHz and aboveAs a load I use several high power resistors (3x2 ohm) with 50cm AWG14 leads maybe also picking up noise...I don't have a metal 'case' big enough to hold the complete test-setupPfff... it is difficult to make even a gestimate of noise with the cheap equipment/cables/connectors I have

author: Andreas
author_rank: Super Contributor
timestamp: June 18, 2017, 12:22:04 pm
content: Hello,of course I missed one point:- switch off your cell phone- switch off all energy saving illumination: only incandescend bulbs (without electronic transformer) allowed.With best regardsAndreas

author: vindoline
author_rank: Unknown
timestamp: June 18, 2017, 01:40:55 pm
content: Quote from: mdijkens on June 18, 2017, 11:18:10 amYes, as Andreas points out, even a simple sounding measurement like "how noisy is my supply" has many subtle details that require attention if one desires an accurate and precise answer. It can be a great learning experience though! "The Art of Electronics, 3rd ed." has a very good chapter on noise and noise measurement.

author: mdijkens
author_rank: Regular Contributor
timestamp: June 18, 2017, 03:27:24 pm
content: Thanks for all advice and tips.I'll try to create a 'clean' environment.But what I still don't understand, even if I measure a lot of common noise, how could measuring with a single x1 probe connected with a spring-clip to the 'ponoma' bnc-banana adapter only result in 5 or 6 mV noise measure?I can't imagine my DUT having a maximum 6mV noise? On the scope this is the only measurement without any spikes.Could there be circumstances that I measure 'to little' noise?

author: Andreas
author_rank: Super Contributor
timestamp: June 18, 2017, 05:10:32 pm
content: Hello,I do not know your cirquit. A linear regulator should not have too much noise.On a 7815 standard voltage regulator I measured about 1mVpp or 124uVeff at 100 kHz measurement bandwith.(which is near the datasheet value).of course you measure more with 20 MHz bandwidth.With best regardsAndreasEdit: usually on voltage regulators the noise scales with output voltage. So at 5V a 7805 should give about factor 3 lower values.