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Re: [TenTec] Voltages on 50 ohm Dummy Load

 To: Discussion of Ten-Tec Equipment Re: [TenTec] Voltages on 50 ohm Dummy Load Ken Brown Discussion of Ten-Tec Equipment Wed, 08 Jun 2005 18:47:47 -1000
 ```Hi Barry, You want to use the RMS voltage value to calculate the power using Power equals Voltage squared divided by resistance. However measuring the peak to peak AC voltage with an oscilloscope is easier. So so divide the peak to peak voltage by two, to get the peak voltage, and then multiply that by the reciprocal of the square root of two (about 0.707) to convert the peak voltage to the RMS voltage. The resulting calculated RMS voltage is what you use in the power equation to determine CW RF Power, which is what a "regular" RF wattmeter should tell you. Getting an accurate AC voltage measurement requires careful attention to a few details. To protect the front end of your oscilloscope I suggest you use a X10 scope probe. Using a 10X probe also reduces the resistive and capacitive loading on the circuit you are measuring, in this case a 50 ohm noninductive resistor. In order for the measurement to be accurate the 10X probe has to be compensated properly. This means that the capacitive voltage division accomplished by the capacitance of the probe and the input capacitance of the scope is 10:1 as well as the resistive voltage division being 10:1. If that is not the case, you can get accurate DC and low frequency AC voltage measurements with the 10X probe, but not accurate high frequency voltage measurements. Just because the scope is a 200 MHz scope, does not mean that you can necessarily make accurate voltage measurements up to 200 MHz. I would say that you can expect to make good, accurate AC voltage measurements on all the HF bands with a 200 MHz scope, IF it is properly calibrated AND the probe you use is properly compensated. The usual way to get the 10 X probe properly compensated is to look at a real good square wave using the probe, and adjust the little trimmer in the probe so the square wave has a flat top and a good front corner without overshoot or hooking. Most good scopes have a built in "calibrator" square wave generator that can be used for this. DE N6KB Williams, Barry wrote: >Hello All, > >I need to calibrate the watt meters on my Titan, Omni VI+ and 253 Tuner. >I have a Drake 2 kw PEP dummy load and a nice 200 mHz storage scope. In >order to measure output, should I measure RMS, or Peak voltage, (if I >use a sine wave input), and what are the voltages for 100 watts and the >legal limit? > >My guess for a sine wave would be that for 1500 watts PEP out, I should >see a peak voltage of about 274 volts, and 70 volts for 100 watts, if >the dummy load is truly 50 ohms. Is this correct? > >Thanks, >Barry > > >_______________________________________________ >TenTec mailing list >TenTec@contesting.com >http://lists.contesting.com/mailman/listinfo/tentec > > > _______________________________________________ TenTec mailing list TenTec@contesting.com http://lists.contesting.com/mailman/listinfo/tentec ```