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 <[email protected]> Reply-To: Paolo Roncone <[email protected]> Date: Friday, July 14, 2023 at 3:53 PM To: <[email protected]> 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. 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/[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 at: [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 - ---------------------------------------------------------------- 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
