Hi Ken and all others, Thanks again for your comments and inputs. I checked the data sheets of the two antennas that I'm using in the 30M-6G range and considering CISPR 16-1-4 sec.4.5.3 "c) The return loss of the antenna with the antenna feeder connected shall not be less than 10 dB. A matching attenuator may be part of the antenna feeder cable for antennas if needed to meet this requirement."
I'm leaning towards the following change: 30 MHz - 1 GHz range : the bilog antenna I'm using has a typical VSWR way above 1.9 below 200 MHz, corresponding to RL way below 10 dB -> 6dB pad moved to antenna end of cable 1 - 6 GHz range: the horn antenna I'm using has a typical VSWR of 1.5, corresponding to RL of 14 dB -> 6dB pad to be removed from the chain, relying on the nominal 50 ohm input impedance and on the auto attenuation of the EMI receiver, and hoping that the minimum 10dB does not increase the noise floor too much. I also need to get notch filters for the 2.4G and 5G BL and Wi-Fi signals that I get more and more frequently in our tested EUTs. Does that sound ok? Thanks again Paolo On Sun, Jul 16, 2023 at 12:26 AM Ken Javor <ken.ja...@emccompliance.com> wrote: > The typical spectrum analyzer / EMI receiver input only looks like 50 Ω > with some input attenuation. Typically, they specify vswr in a 50 Ω system > with 10 dB internal attenuation selected. > > > > -- > > Ken Javor > > (256) 650-5261 > > > > *From: *Richard Nute <ri...@ieee.org> > *Reply-To: *<ri...@ieee.org> > *Date: *Saturday, July 15, 2023 at 4:28 PM > *To: *<EMC-PSTC@LISTSERV.IEEE.ORG> > *Subject: *Re: [PSES] 6dB pad > > > > > > 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> > *Reply-To: *Paolo Roncone <paoloc...@gmail.com> > *Date: *Saturday, July 15, 2023 at 10:29 AM > *To: *<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> > 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> > *Reply-To: *Paolo Roncone <paoloc...@gmail.com> > *Date: *Friday, July 14, 2023 at 3:53 PM > *To: *<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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