[Elecraft] Power measurement with a 'scope

[Ron D' Eau Claire]

> Now, it's true, if you don't know whether your 5% voltage error
> is high or
> low, then the resulting power measurement could be 10% low or 10% high,
> giving, in some sense, a possible 20% error spread.  But, of course, then
> the voltage measurement would, in the same sense, have been a 10% error,
> not a 5% error, and, again, the power error would be twice that of the
> voltage.
>
> dave belsley, w1euy


Thanks, Dave. I wasn't being rigorous in my example and I apologize.

The point I was making is that because the voltage is squared in the process
of calculating the power, the accuracy of the voltage measurement is much
more important to the final result than one might realize at first.

For example, if you are making exactly 5 watts out and measure 14.24 volts
instead of 15 volts rms (reading 5% low) or 15.75 volts instead of 15 volts
rms(reading 5% high) then the power calculation at 50 ohms load would be
4.06 watts at the lower side and 4.96 watts on the high side. That's a total
range of error of just about 20%. That's pretty bad when one starts out
thinking that 5% accuracy is surely adequate for a good, precise power
measurement.

Ron AC7AC
K2 # 1289