Hmmmm, interesting. Sounds to me like I have a competitive advantage with the blessing of the FCC.
This appears to be a business opportunity . . . think I will open a test lab and offer approvals for less costly designs considering the apparent "relaxation". :-) > ---------- > From: Ken Javor[SMTP:ken.ja...@emccompliance.com] > Sent: Tuesday, October 30, 2001 6:20 PM > To: umbdenst...@sensormatic.com; emc-p...@majordomo.ieee.org; > dmck...@corp.auspex.com > Subject: Re: The Trouble with Convention, The Final Chapter > > Following your logic, "I was just following orders," means that those who > use an average detector on an EMI receiver or who properly use a spectrum > analyzer to average as I demonstrated are at a competitive disadvantage to > those who use bad math and give themselves twice the dB relaxation > warranted. The proper response is to do the job right and so notify the > customer so that he imposes the correct requirements even handedly. > > ---------- > >From: umbdenst...@sensormatic.com > >To: emc-p...@majordomo.ieee.org, dmck...@corp.auspex.com, > ken.ja...@emccompliance.com > >Subject: RE: The Trouble with Convention, The Final Chapter > >Date: Tue, Oct 30, 2001, 4:13 PM > > > > > The point! We have all missed the point! :-) > > > > I do not dispute the science. > > > > The question was not "what is the correct science", rather, "what is > > expected by the FCC (or any other spectrum authority) for successful > > processing of the submittal. It became apparent that the science did > not > > match the requirements. Is the requirement scientifically pure and > > correct? -- no. Is the requirement specified? -- yes. > > > > I noticed no one commented on the impedance of free space in the far > field > > specified for use in the reactive near field given as another example of > > "convention". Again is the convention correct? -- no. Specified? -- > yes. > > > > Submittal convention is not about correct application of science; it's > about > > following specified rules, for whatever the reasons. The reasons > usually > > have to do with simplifying the submittal process while providing > repeatable > > results, this being more important than totally correct science. > Allowances > > have been made for the lack of perfect science. > > > > As a partial explanation of the origin of the current instruction being > > debated, the following is an excerpt from another response regarding > duty > > cycle reporting for a Part 15.209 device: > > > >> the FCC reviewer had explained to me that he had problems > >> > in the past with the interpretation by applicants for using averaging > >> detectors so he preferred to mathematically arrive at > >> > the average voltage. He asked for the peak detector output in units > of > >> uV to be multiplied > >> > by the duty cycle to provide the "mathematical equivalent > >> > of the average detector" in his words. I have been doing this ever > >> since > >> > for various reviewers and no submissions have come back. > >> > > 2.38 mV pk detector, 0.5 duty cycle on an average detector = 1.66 mV > > per vbw averaging per the experiment in this thread below. > > > > 2.38 mV x 0.5 (duty cycle) = 1.19 mV per the mathematical process > > specified. > > > > The 2 methods are clearly not equivalent. But FCC convention requires me > to > > submit "1.19 mV" data, which is also arrived at by the 20 log() > operation > > (hence the confusion about the sanity check earlier). I don't create > the > > convention; I just follow the submittal instructions (requirements). > > > > It is always good to have the correct science at your fingertips, but in > the > > end providing what is requested is what counts. For those still in the > dark > > about 20 log () or 10 log (), my suggestion is to go straight to the > source > > -- ask the FCC what they specify for your situation. > > > > > > Don Umbdenstock > > Sensormatic > > > > > >> ---------- > >> From: Ken Javor[SMTP:ken.ja...@emccompliance.com] > >> Sent: Monday, October 29, 2001 2:41 PM > >> To: umbdenst...@sensormatic.com; emc-p...@majordomo.ieee.org; > >> dmck...@corp.auspex.com > >> Subject: The Trouble with Convention, The Final Chapter > >> > >> In the face of all the responses I and others gave last week showing > the > >> MATHEMATICAL RULES for calculating logarithms and average and peak > power, > >> and the rationale and math behind pulse desensitization calculations, > >> apparently it is still not clear that power is averaged, not potential. > >> In > >> the interest of stopping the flow of incorrect test reports to the FCC > and > >> their apparent approval, I submit the following, "A single test is > worth a > >> thousand expert opinions." For those who are confused and don't know > what > >> to believe, here is the simple test. I have run it and have pix of the > >> spectrum analyzer display I can send to those who are interested (no > >> attachments allowed on general mailings). > >> > >> Tune an rf source and your spectrum analyzer to a common frequency, say > 10 > >> MHz. Set up a baseline rf output, say -40 dBm. Use linear mode on > your > >> analyzer with a reference level of -37 dBm. Measure in peak detect > mode > >> (vbw greater than or equal to rbw). Measure again while averaging, > using > >> a > >> 300 Hz vbw. You will get the same result. Apply a 50% duty cycle > pulse > >> modulation at 1 kHz. Peak detect will show the same -30 dBm, but > video > >> averaging will show a 3 dB decrease to -33 dBm. Try it - I did and it > >> works. On the analyzer I used, linear mode defaults to a millivolt > >> reading. > >> I got 2.38 mV in peak mode, 1.66 mV in average mode - you do the math. > >> > >> > >> > >> ---------- > >> >From: umbdenst...@sensormatic.com > >> >To: emc-p...@majordomo.ieee.org, dmck...@corp.auspex.com, > >> ken.ja...@emccompliance.com > >> >Subject: The Trouble with Convention > >> >Date: Mon, Oct 22, 2001, 4:46 PM > >> > > >> > >> > Similarly, it appears the same issue of convention is the basis of > >> certain > >> > FCC clauses, for example, the reporting of the output of an averaging > >> > detector as called for by 15.209 and other clauses for some frequency > >> bands. > >> > The FCC is looking for field strength, a voltage representing the > output > >> of > >> > the averaging detector. The FCC is aware that there are different > >> > implementations of "averaging" detectors and linearity issues so they > >> > provided instructions to arrive at the reporting level by > mathematical > >> means > >> > for consistency. The instruction was to multiply the peak detector > >> reading > >> > by the duty cycle and report this value in terms of the limit units, > uV, > >> as > >> > the equivalent of the output of the averaging detector. > >> > > >> > So, what is this unit they asked for? It appears to be the function > of > >> > averaging a voltage signal, i.e., if the signal is X and is on for y, > >> 0<y<1, > >> > then the value of the signal to be reported is y*X (uVolts). > >> > > >> > When it is desired to address multiple factors (distance correction, > >> > antenna, cable, preamp, etc.), the process is simplified by > converting > >> to > >> > log terms. By the relationship between P and V, we have developed > the > >> > expression for V in log terms to be 10*log V^2 or 20*log V. > Addressing > >> the > >> > entire expression above, we have 10*log (y*X)^2, or 20*log(y*X). > This > >> can > >> > also be expressed as 20*log (y) + 20*log(X). From this expression we > >> see > >> > that duty cycle is expressed as 20*log (y) for this situation. > >> > > >> > It appears in this case the FCC is looking for average voltage, not > >> average > >> > power. > >> > > >> > Speaking of power, don't forget the power of the sanity check :-) > >> > > >> > Best regards, > >> > > >> > Don Umbdenstock > >> > Sensormatic > >> > > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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