Thanks Jack for that additional information and caution. I believe that is an accuracy limitation for the probe specs, and not a probe damage rating.
I routinely leave my 'scope probe connected to the dummy load at the workbench and it is subjected routinely to powers up to 180 watts. I use the RF voltage across the dummy load to accurately assess the power output of the KX1, K1 and K2/10 transceivers that I repair. At higher power levels, I use the LP-200 wattmeter which has been calibrated at the 20 watt and the 80 watt levels to make quick decisions on the power output of a transmitter. If I want a quick indication, I just look at the LP-200. It is one of the digital wattmeters that is good to within better than 10% of the actual reading (it is better than 5% at the 20 and 80 watt calibration points). If I need greater accuracy or NIST traceability, I invoke the LP-100 which Larry (N8LP) has calibrated with his NIST traceable standards. I use that wattmeter as the "master" for calibrating all the others at higher power, but at low power levels, I rely on the 'scope peak to peak readings and my 1% 50 ohm dummy load and punch the buttons on my calculator. With this combination of tools, I can commit to better than 5% accuracy in power measurement over the 1 to 100 watt range. I also have accurate power measurement capability for low levels (-75 dBm through 100 watts), with the W7ZOI Power meter, but that is a different subject - if anyone is interested, contact me off-list. 73. Don W3FPR On 12/8/2011 6:40 PM, Jack Smith wrote: > Don: > > I would add a small caution. The maximum safe voltage rating of a 10x > probe is a function of frequency. > > For example, Tektronix's model P6106A, a 250 MHz 10x probe, is rated at > 450V below 1 MHz, but only 60V at 10 MHz and perhaps 50V at 30 MHz (hard > to read the graph accurately). All are RMS values. > > 100 watts is 70.7 V RMS, so the 10 MHz - 30 MHz voltage limits will be > exceeded at that power level. (100V RMS probe limit looks to be around > 6.5 MHz.) > > Of course, I'm sure there's a safety factor in the probe specs, but > before I measured 100 watts output with a 10X probe I would verify the > safe voltage rating in the probe manual. > > Jack K8ZOA > > > On 12/8/2011 6:09 PM, Don Wilhelm wrote: >> Tom, >> >> OK, it seems now is a good time for me to insert my periodic "wattmeter >> accuracy" rant! Sub-titled -- "Technician, know your tools but know >> even better their limitations" OR - "No instrument can be relied on >> until it has proven it is capable of telling the truth under the >> conditions of use". >> >> If you want quite good accuracy at the 5 watt level, I suggest you use >> a 'scope with a 10X probe connected across the dummy load. The peak to >> peak RF voltage can be easily measured by the vertical deflection of the >> 'scope. To convert that peak to peak voltage reading to power (when the >> dummy load is 50 ohms), use your calculator - enter the peak to peak >> voltage, square it, and divide by 400. >> >> If you do not have an open dummy load that makes clipping the 'scope >> probe across it easily, use a Tee adapter at the dummy load end so you >> can gain access to the center conductor. >> >> 40 volts peak to peak is 4 watts - you might want to use that instead of >> 5 watts since it is even and easy to see on the 'scope face. If you >> really need 5 watts, adjust for 44.72 volts peak to peak. >> >> In fact, you can use the 'scope probe across the dummy load up to any >> level permissible by your 'scope and probe. My probes will handle 100 >> volts peak easily, and that is the 100 watt level. The accuracy is only >> limited by the accuracy of your dummy load (mine is within 1%) and your >> ability to interpolate the voltage deflection on the 'scope. I might >> suggest that you can come within 5% of the actual power level, which is >> much better than analog power meters whose accuracy is expressed as a >> percentage of full scale. So a freshly calibrated Bird wattmeter with a >> 100 watt slug is spec'ed for 5% of full scale - OK, that is a 5 watt >> potential error - and that applies anywhere on the scale - so trying to >> read a 5 watt power level with a Bird and a 100 watt slug is an exercise >> in futility - use the 'scope, it is a LOT more accurate. >> OTOH, many recent digital wattmeters will be spec'ed for accuracy >> anywhere within its range rather that as a percentage of full scale used >> with the analog wattmeters. >> >> The (Vp-p)exp2/400 formula (actually [Vp-p]exp2/8R) does not seem to be >> well know. I leave it "to the student" to derive it from standard >> formulas for power, voltage and resistance. Hint - use SQRT2 rather >> than the 1.414 or 0.707 approximations when converting peak to peak >> voltage to RMS - it is an exact formula, the squares and square roots >> cancel. >> >> 73, >> Don W3FPR >> >> On 12/8/2011 5:12 PM, TJ Campie wrote: >>> I finished building a K3 for the radio club at work and I thought I'd take >>> the bull by the horns and attempt to improve the factory setting for the 5w >>> calibration on the 100w Bird watt meter I had. I couldn't get it to go up >>> the the point that looked like 5w on the meter regardless of how much I >>> turned the knob on the K3. The setting ended up at 10 (the minimum) but I >>> think it must have been more correct from the factory. I *think* it was up >>> at about 37 but I don't remember - is there a way to reset it to the >>> factory setting? I don't have access to a 5w accuracy meter. >>> >>> Otherwise, the build went great and was a blast :) >>> >>> TJ W0EA >>> ______________________________________________________________ >>> Elecraft mailing list >>> Home: http://mailman.qth.net/mailman/listinfo/elecraft >>> Help: http://mailman.qth.net/mmfaq.htm >>> Post: mailto:Elecraft@mailman.qth.net >>> >>> This list hosted by: http://www.qsl.net >>> Please help support this email list: http://www.qsl.net/donate.html >>> >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:Elecraft@mailman.qth.net >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
