Measuring power supply ripple with an oscilloscope - Page 1
Forum section: electronics / beginners

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: June 03, 2020, 05:50:56 am
content: Hi,I have an old reel to reel recorder where I'm trying to measure the ripple voltage of the power board. I got hold of an oscilloscope (Rigol MSO1104Z) and proceeded with utmost caution and fear trying to figure this thing out and not blow anything (last time I used a scope I ruined the piece I was working on -- guess I put the probe ground lead in the wrong place or something).After some consideration I placed the probe across C506 (marked in the attached schematic), which worked fine. I put the scope coupling to AC and measured between 15-25mV of Vpp. I assumed that basically was the ripple voltage.However, I then proceeded to turn of the device I was measuring --- and the "ripple" stayed the same. Tried again this morning, the "ripple" was a bit higher (up to 30mV), I then proceeded to remove the probe from the scope and the Vpp still displayed around 15-20mV.Could there be interference from somewhere else? Or am I just doing it wrong?? The devices are next to each other on a table, far away from any other electronics.Any help would be most welcome!

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 03, 2020, 07:40:25 am
content: If the regulator is working well then it's 'normal' ripple is about an order lower (about 1 mV, +-).The voltage you see is some parasitic common-mode voltage, which became when you connect an oscillograph.I can't say how to avoid it (in short), it can be a ground (an earthing)-connected issue.What frequency is this triple? 100 Hz or much higher?I usually have such an issue only with high-frequency (>20 kHz) devices when every wire is an inductor and an antenna.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: June 03, 2020, 08:24:36 am
content: Quote from: Vovk_Z on June 03, 2020, 07:40:25 amWhat you mean by frequency triple? The only oscillator in the device is ~85Khz for recording bias.

author: rdl
author_rank: Super Contributor
timestamp: June 03, 2020, 11:15:06 am
content: There is always some low level noise on my Rigol scope. Best guess is that it's from the local college radio station.

author: David Hess
author_rank: Super Contributor
timestamp: June 03, 2020, 03:24:00 pm
content: The power supply output ripple may have been lower than the broadband noise from various sources.Ripple is distinct from noise and usually related to the input regulation or AC rejection. One way to make a very sensitive measurement of ripple is to set the oscilloscope to "line" triggering which will remove the correlation of broadband noise. In addition to line triggering, DSOs can use averaging to further reduce broadband noise and show very low levels of ripple.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 03, 2020, 06:50:52 pm
content: Quote from: mvno_subscriber on June 03, 2020, 08:24:36 amI mean "ripple", not "triple"I can guess that you see some parasitics from that 85 kHz ripple.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: June 03, 2020, 07:29:52 pm
content: Thanks! I'll try to see what filtering options I have. In the meantime, I managed to get something out of it by using 1X probe and unplugging nearby wall warts.Images are with device on and off, respectively. You can see the 50Hz ripple, swamped with the noise from my living room 😊

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 04, 2020, 04:06:44 pm
content: I can't remember right now but there is several (at least two or three) books about noise black magic. If I remember - I'll write here.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 04, 2020, 04:10:47 pm
content: Look here, for example:Recommended Books on EMC. One of the books is like a bible: "Noise Reduction Techniques in Electronic Systems" by Henry W. Ott.

author: JimS
author_rank: Contributor
timestamp: June 06, 2020, 09:38:58 am
content: Try setting the bandwidth to it's lowest setting.Should help minimize the high frequency noise being picked up.The supply ripple should be at 2x the line freq.

author: calzap
author_rank: Frequent Contributor
timestamp: June 06, 2020, 06:19:56 pm
content: The scope is mains powered and so is the DUT, so something needs to be isolated for safety. Can use an isolation transformer for the DUT or a differential probe or a probe isolator. "Being careful" isn't good enough. The DUT may not conform to the schematic, especially if it has problems.Mike in California

author: bob91343
author_rank: Super Contributor
timestamp: June 06, 2020, 06:29:52 pm
content: Many years ago my company was trying to build power supplies with very low ripple, 1 mV or less. We learned a lot about ESR and common mode and ground loops.You must first define what voltage you are measuring. Ground is not ground, so don't use it as a reference. You need differential probes and a very good 'scope front end.With only 1 mV allowable, there is no room for noise or other extraneous signals. If you want to measure the voltage across a particular capacitor, you need to connect your measuring device across that capacitor. Both leads, including signal and return. You need to connect the 'scope input with a short (while leaving the return lead on the DUT 'cold' side) to see the residual noise. If it's more than 100 uV or so, you can't make a solid measurement.You also need to define the frequency range of interest. If you want just power line ripple, use a low pass filter. If you include device noise, you need wide band sensitivity.

author: mvno_subscriber
author_rank: Regular Contributor
timestamp: June 08, 2020, 09:53:21 am
content: Thanks for all the input! I've tried figuring out how to use two probes to remove noise ("add/invert") - but I didn't have the same stuff on my scope as in whatever tutorials I found, so had to give up on that one. Would love to find out how, though.In the end, it all worked out with using the BW limit, 1X probe and lowering the display intensity.