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: [email protected] >To: [email protected], [email protected], [email protected] >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. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] 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.
