This discussion not only applies to EMI testing, it also applies to high-frequency and fast risetime pulse voltage measurements. A 6 dB (2x voltage attenuation) or 10 dB (~3x voltage attenuation) in a 50-ohm system which would otherwise be subject to reflections due to impedance discontinuities (i.e., which create the VSWR) will give a more accurate voltage measurement (which would otherwise be dependent on the length of the cable due to VSWR). The attenuator makes the input impedance look like nearly 50 ohms regardless of actual load impedance. Same for a source impedance that is not 50 ohms.
I’m not sure that you need a pad at both ends. If the input to the receiver is 50 ohms throughout the frequency band, then there are no reflections from the receiver end and no VSWR that would distort the measurement. Arguments? Best regards, Rich From: Ken Javor <ken.ja...@emccompliance.com> Sent: Saturday, July 15, 2023 10:15 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] 6dB pad Receiver vswr is specified with 10 dB attenuation because the mixer itself is not 50 Ω. Antenna vswr is usually best in the middle of the usable frequency range. Worst case at the low end for EMI antennas used down to 30 MHz, because except for half-wave dipoles, they are electrically short at and near 30 MHz. Antenna vswr is typically stated at the antenna port, without any added attenuation. Putting attenuation at the EMI receiver input takes care of mixer mismatch, and padding the antenna takes care at the opposite end. You need a pad at both ends to completely control vswr-related uncertainty. Unfortunately, you often cannot stand the desensitization of all the extra attenuation. -- Ken Javor (256) 650-5261 From: Paolo Roncone <paoloc...@gmail.com <mailto:paoloc...@gmail.com> > Reply-To: Paolo Roncone <paoloc...@gmail.com <mailto:paoloc...@gmail.com> > Date: Saturday, July 15, 2023 at 10:29 AM To: <EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG> > Subject: Re: [PSES] 6dB pad Hi Ken Thanks for your feedback. Why should the VSWR be specified with 10 dB attenuation? That would alter (for the better) the specified VSWR itself. I checked th e data sheets of 3 antennas that I'm using (a hybrid bilog, a log-periodic and a horn) and in all of them a "typical" VSWR is specified, without any mention to a 10dB attenuation. Paolo On Fri, Jul 14, 2023 at 11:33 PM Ken Javor <ken.ja...@emccompliance.com <mailto:ken.ja...@emccompliance.com> > wrote: Not expressing an opinion, just listing some facts. Absent any input attenuation, vswr will be higher than manufacturer specifies, because the specification is with 10 dB input attenuation (typically). Therefore, the use of a 6 dB pad, absent any internal attenuation, will not meet the manufacturer’s spec for vswr, and thus your uncertainty budget increases. If your ambient source is brush noise, preselection should help with that, once you are out of band to it. Band stop filters will help with known transmitters. -- Ken Javor (256) 650-5261 From: Paolo Roncone <paoloc...@gmail.com <mailto:paoloc...@gmail.com> > Reply-To: Paolo Roncone <paoloc...@gmail.com <mailto:paoloc...@gmail.com> > Date: Friday, July 14, 2023 at 3:53 PM To: <EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG> > Subject: [PSES] 6dB pad Hi all, I'm having a discussion with my colleague about the use of a fixed 6dB attenuation pad at the input of the EMI receiver for radiated EMI in the range 30MHz - 6GHz. The pad I'm using is a Weinschel 6dB N-type 50ohm. My colleague says the pad is an unnecessary element in the measurement chain, because the receiver built-in attenuator (typically with 10dB steps) and proper adjustments in the amplitude settings are enough. The reason I'm using the 6dB pad is that - based on my EMI testing experience, the 6dB pad is a good trade-off between the need to avoid receiver overload and maintaining a good enough noise floor. Without the pad the noise floor is of course lower and everything is fine as long as the input levels are low enough. But in the majority of the test scenarios I'm working on the input levels are not so low and the 6dB pad is just enough to avoid triggering the auto built-in 10dB attenuation, that kicks in when the receiver attenuation is in auto mode, and that is oftentimes an overkill, raising the noise floor too much, especially in the 1-6 GHz range. I have the 6dB pad calibrated for cable loss once a year together with the N-cable connecting to the antenna. And also the cable calibrated without the pad. The most typical sources of overload are transient noise generated by motors (especially brush DC motors) and signals from radio modules like 2.4G wi-fi, Bluetooth and 5G wi-fi. In addition to overload protection, the 6dB pad improves the VSWR at the cable-receiver interface, as explained in C.J.Paul's EMC Introduction to EMC (John Wiley 2nd Ed.) Ch.7. Based on the above I think that, being a resistive network stabilizing the 50ohm termination impedance at the receiver end, the 6dB 50ohm pad is a good practice that can prevent overload or excessive auto-attenuation, and worst-case doesn't do any harm. Or, am I missing something here? I'd like to know if there is someone else in this community who has experience using fixed attenuation pads at the receiver end of EMI receivers/spectrum analyzers, or any opinion about it. Thanks a lot in advance for any feedback! Paolo _____ This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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To post a message to the list, send your e-mail to All emc-pstc postings are archived and searchable on the web at: https://www.mail-archive.com/emc-pstc@listserv.ieee.org/ 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: msherma...@comcast.net <mailto:msherma...@comcast.net> Rick Linford at: linf...@ieee.org <mailto:linf...@ieee.org> For policy questions, send mail to: Jim Bacher at: j.bac...@ieee.org <mailto:j.bac...@ieee.org> _____ 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. 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