Good morning Grace - I would recommend method #1 and sample detector provided you meet the required span/points criteria ... and you are correct, the sample detector will give you a lower reading.
15.247 does satet "Peak Power" but the FCC's measurement procedure allows you to use the sample detector and power averaging - in which case you have to make the limit for the spurious emissions outside of restricted bands -30dBc rather than -20dBc. As for power averaging - older analyzers may not have this function. When setting this up make sure you have seleceted the correct averaging type. It must be power averaging. It is not video averaging and it is not voltage averaging. On the Agilent PSA you select the RMS Power averaging under averaging type. On the R&S ESI you select linear averaging and make sure you have units set for dBm (or mW, either work) - but check the manual because it does explain how to do it. 100 sweeps is sweep count. You do not want max hold enabled, that would not result in any averaging ... I am sure that when you turn on the averaging function the trace will automatically set up in a clear-write mode with a sweep counter. Once it completes the 100 sweeps it will either stop (if in single sweep mode) or continue to sweep, adding the next sweep into the average calculation and throwing out the first sweep. Once again, good luck! Mark From: Grace Lin [mailto:[email protected]] Sent: Thursday, August 27, 2009 4:39 AM To: Mark Briggs Cc: [email protected] Subject: Re: Power Meter Good Morning Mark, Thank you very much for writing to me. As a matter of fact, I thought about to write to you yesterday. I read your paper and re-read yesterday. We are able to configure the EUT, an IEEE 802.11 a/b/g device, for continuous transmitting. I assume Method #1 applies. There are two detectors mentioned in the Method #1. The EUT qualifies for using sample detector. However, there is about 8-10 dB difference between using sample detector and peak detector (using Channel Power function of R&S ESU receiver, channel bandwidth=20MHz, span=30MHz, RBW=1MHz, VBW=3MHz, SWP=Auto). Since 15.247 requires peak output power, I assume peak detector is the right choice. Do you agree with this assumption? In addition, could you please interprete the following sentence (step 6 of Method #1): "Trace average 100 traces in power averaging mode." My questions are: 1. What does "power averaging mode" refer to? Does it refer to set detector as Average? 2. Does "trace average" refer to set trace as Average (not clear write or peak, etc)? 3. Does "100 traces" refer to set sweep count=100? I sincerely appreciate your help. Best regards, Grace On 8/26/09, Mark Briggs <[email protected]> wrote: > Grace - > > Do you have a problem using the FCC's methods of measuring the output power > with a spectrum analyzer (for peak power you would use method 3 in the FCC > procedure outlined in their document DA 02-2138 - the method that uses max > hold)? > > Typically for 802.11 devices you are stuck with FCC method 1 (if the device > transmits continuously or your analyzer can be gated to only sweep when the > device transmits) or FCC method 3 (if device does not transmit continuously > and the analyzer gating does not work). Method 3 will give you a higher > value (by as much as 8dB) than method 1 for the same device because it is > measuring peak power and not average power. > > If you are not going to use one of the three methods (that use the analyzer > to make the power measurement) then you would need to measure the peak > power. I have used a R&S Peak power sensor (envelope power) and the results > compared favourably to the method 3 results. An alternative would be to > use a diode detector and o'scope to determine the peak power based on the > maximum voltage recorded by the diode detector. > > Feel free to look at this paper that compares measurements using method #3, > an average power sensor, a peak power sensor and a diode detector: > http://www.elliottlabs.com/documents/OFDM.pdf, but what is missing is the > use of FCC method #1 (which would give results similar to the average power > sensor). > > Good luck, > > > Mark > ________________________________________ > From: [email protected] [mailto:[email protected]] On Behalf Of Grace Lin > Sent: Tuesday, August 25, 2009 5:02 AM > To: [email protected] > Subject: Power Meter > > Dear Members, > > I am in an urgent need for a power meter and would like to hear your > comments. My purpose is to measure peak output power per FCC 15.247 and > 15.407 for IEEE 802.11 a/b/g/n devices. > > There are several vendors available on the market (Agilent, Anritsu, Booton, > Gigatronics, Rohde & Schwartz, etc.). Is there any particular vendor and > model you recommend? You may wish to reply it off-line. > > What is your comment on USB power sensors (eliminate a power meter and > connect to a PC)? Examples are Agilent > U200A http://www.home.agilent.com/agilent/product.jspx?cc=US&lc=eng&pageMode > =OV&pid=1079918&ct=PRODUCT&id=1079918 (measureing average power only) and > R&S NRP-Z81 > http://www2.rohde-schwarz.com/en/products/test_and_measurement/power_volt_me > ter/power_meters/NRPZ81.html. Do USB power sensors have the same quality as > traditional power meters with sensors? > > Does "envelope power" refer to peak power? What is the diference between > "true average" and "average" power? > > Is there any instrument specification that I have to focus on? > > Thank you very much for your time and look forward to hearing from you. > > Best regards, > Grace Lin > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. 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