Oh boy, have I seen this, in the distant past. Today, there is likely an IEC standard which defines how this measurement should be performed.
When I was a development engineer at a small d.c. power supply company in the 1990s we grappled with this same issue. We eventually designed a custom voltage probe which measured differential ripple & noise into 50 ohms with a 20 MHz bandwidth. It provided a repeatable measurement of output noise into a stabilized impedance while rejecting common-mode contribution. Its implementation settled most arguments on how this measurement was done since some customers at the time were challenging our results when we were merely using an unbalanced 10X scope probe with any convenient oscilloscope on hand. Any, well considered, implementation for a noise probe is probably just as good so long as it is used consistently, and the method disclosed to those who need to know. Ralph From: doug emcesd.com <[email protected]> Sent: Monday, July 1, 2024 4:18 PM To: [email protected] Subject: [PSES] power lsupply musings #1 Hi Everyone, I thought I would post a bit about power supplies. Something as simple as trying to measure ripple on the output can be very inaccurate, overstating ripple amplitude by a lot, 100% over stated is not all that unusual. One problem arises from common mode noise on the output that gets into the structure of the probe used for the measurement. Most probes have modes resulting in display of voltages that are not actually present. If you doubt this, just connect both terminals of a scope probe to the low end, say ground, of a power supply output and you will often see a significant signal that is not actually there. Whatever one measures with a shorted probe on the ground side of the supply output is the error in the measurement and can easily exceed the actual ripple voltage present on the output. Have you seen this? I cover this in detail in my presentations. Doug <https://lh4.googleusercontent.com/_HuR3Ky2TF_XhFHyxnYRmiq7nHQldnMsPNYFaLG6k b5T4y8MeCe-BDC_BscJtSFgszSSjssihHS-pjM3-jwNP8S0CwE-gN8fsRsPkojiAlmpBwb20vIVi zS-siCUywW_jqrefbVr> _____ This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to [email protected] <mailto:[email protected]> All emc-pstc postings are archived and searchable on the web at: https://www.mail-archive.com/[email protected]/ <https://www.mail-archive.com/[email protected]/%20> Website: https://ewh.ieee.org/soc/pses/ <https://ewh.ieee.org/soc/pses/> Instructions: https://ewh.ieee.org/soc/pses/list.html (including how to unsubscribe) <https://ewh.ieee.org/soc/pses/list.html> List rules: https://ewh.ieee.org/soc/pses/listrules.html For help, send mail to the list administrators: Mike Sherman at: [email protected] <mailto:[email protected]> Rick Linford at: [email protected] <mailto:[email protected]> For policy questions, send mail to: Jim Bacher at: [email protected] <mailto:[email protected]> _____ To unsubscribe from the EMC-PSTC list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC <https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC&A=1> &A=1 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to [email protected] All emc-pstc postings are archived and searchable on the web at: https://www.mail-archive.com/[email protected]/ Website: https://ewh.ieee.org/soc/pses/ Instructions: https://ewh.ieee.org/soc/pses/list.html (including how to unsubscribe) List rules: https://ewh.ieee.org/soc/pses/listrules.html For help, send mail to the list administrators: Mike Sherman at: [email protected] Rick Linford at: [email protected] For policy questions, send mail to: Jim Bacher: <[email protected]> _________________________________________________ To unsubscribe from the EMC-PSTC list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC&A=1
