There is another issue which is different which may be what the FCC is after (I didn't read the referenced part 15 paragraph). FCC calls it pulse desensitization. It was what the mil and aerospace world used to call a broadband signal. If a signal is in the passband of a receiver for less time than it takes to charge the IF bandpass filter, then the filter output is the average of the input. For instance, if the signal lasted one tenth of a filter time constant, then the potential (Volts) that the filter charges to is one tenth of the peak level that was fed into the filter. This is very important when the test instrumentation uses a different bandwidth than the real world victim protected by the requirement. Take broadcast TV as an example. FCC/CISPR requires a 120 kHz bandwidth, but TV uses 6 MHz. An interference signal lasting 1 us with a 10% duty cycle (on 1 us, off 9 us) would be averaged by the CISPR measurement and would appear at roughly 10% the level it would present to the TV receiver. In this case, the pulse desensitization factor is calculated as 20 log (duty cycle), because we are talking about a coherent signal where the voltage is proportional to the duty cycle, There is one last very important concept here and that is duty cycle itself. Duty cycle is not an absolute but is relative to the filter time constant. We could imagine a signal lasting 10 us and having a 10% duty cycle (10 us on, 90 us off) and the CISPR receiver and the TV would report exactly the same level. Duty cycle is relative to receiver bandwidth.
---------- From: "Ken Javor" <ken.ja...@emccompliance.com> To: umbdenst...@sensormatic.com , emc-p...@majordomo.ieee.org , stu...@timcoengr.com Subject: Re: duty cycle correction factors List-Post: emc-pstc@listserv.ieee.org Date: Thu, Oct 18, 2001, 2:37 PM I wasn't going to weigh in on this but... what was presented by Mr. Umbdenstock is equivalent to saying that since 2 + 2 = 4, then 2 x 2 = 4. It is tautological. The decibel scale is a power ratio. If a signal has a particular duty cycle then it is the total power that is affected by the duty cycle ratio. If something is on 100% and then you reduce the on-time to 50%, clearly you consume half the previous POWER. dB = 10 log (P1/P2) Let "a" be the duty cycle ratio, with 0<a<1, so that P1 = aP2. Then dB = 10 log (aP2/P2) = 10 log (a). QED. ---------- >From: umbdenst...@sensormatic.com >To: emc-p...@majordomo.ieee.org, stu...@timcoengr.com >Subject: RE: duty cycle correction factors >Date: Thu, Oct 18, 2001, 12:26 PM > > > Stuart, > > Duty cycle in 15.231 is related to a voltage ratio, therefore 20 log(duty > cycle) will provide the correct factor. > > Demonstrate it to yourself. Start with a given value (say 100V), multiply > this by some duty cycle (say 15% or .15). Convert the result to dB. This > is your reference result. Now take 20 log of a duty cycle (.15). Convert > your given value (100V) to dB. Add the numbers together, duty cycle dBs to > the given value dBs, and behold -- the same answer as the reference result. > > Best regards, > > Don > >> ---------- >> From: Stuart Lopata[SMTP:stu...@timcoengr.com] >> Reply To: Stuart Lopata >> Sent: Thursday, October 18, 2001 12:00 PM >> To: emc >> Subject: duty cycle correction factors >> >> >> Part 15.231 devices use a duty cycle correction factor to adjust peak >> readings. The duty cycle represents the fractional on-time over a given >> period of time (that must be under some limit). Anyways, given this >> fractional time, d, how do you make the conversion to dB? >> >> 10log(d) or 20log(d)? >> >> There have been some misinterpretations, since the readings are made at a >> span of zero hertz (voltage readings). Normally, a reduction in voltage >> would use the 20log scale. However, since the duty cycle does not >> represent >> a scale down (it represents the off-time versus on-time), the 10log scale >> seems more appropriate. >> >> I have seen conflicting documents, so would like your professional >> opinions! >> >> Thanks, >> >> Stuart Lopata >> >> >> ------------------------------------------- >> This message is from the IEEE EMC Society Product Safety >> Technical Committee emc-pstc discussion list. >> >> Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ >> >> To cancel your subscription, send mail to: >> majord...@ieee.org >> with the single line: >> unsubscribe emc-pstc >> >> For help, send mail to the list administrators: >> Michael Garretson: pstc_ad...@garretson.org >> Dave Heald davehe...@mediaone.net >> >> For policy questions, send mail to: >> Richard Nute: ri...@ieee.org >> Jim Bacher: j.bac...@ieee.org >> >> All emc-pstc postings are archived and searchable on the web at: >> No longer online until our new server is brought online and the old >> messages are imported into the new server. >> > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Michael Garretson: pstc_ad...@garretson.org > Dave Heald davehe...@mediaone.net > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > All emc-pstc postings are archived and searchable on the web at: > No longer online until our new server is brought online and the old > messages are imported into the new server. > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
