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.
Williams, Barry wrote:
>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
>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?
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