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Re: [TenTec] Voltages on 50 ohm Dummy Load

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] Voltages on 50 ohm Dummy Load
From: Ken Brown <ken.d.brown@verizon.net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Wed, 08 Jun 2005 18:47:47 -1000
List-post: <mailto:tentec@contesting.com>
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.


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?
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