On 9/27/20 10:22 AM, David Eckhardt wrote:
This method is highly likely more accurate than the Bird Watt Meters at 5%
of full scale and other wattmeters offered into the amateur market.
Excellent procedure!
Assuming the dummy load is 50 ohms (viz all that discussion on the list about the loads for the NanoVNA)
Most oscilloscopes don't have very tight tolerances on their amplitude accuracy.
A Rigol DS1000 (a popular sub kilobuck scope) says:
DC gain accuracy is +/- 3%
(and a bunch of other stats that are basically related to that DC spec) It is, after all, an 8 bit ADC.
A decent analog scope like the venerable Tek 465B is also 3%
Then you have the input impedance: 1 Meg +/- 2% in parallel with 13 pF +/- 3pF.
13 pF is 122 ohms at 10 MHz - so you use a 10x probe, but then the 2% of the 1Meg against the series Z of the probe starts to be a problem.
5% is about what you'd probably wind up with using the scope + load approach
Dave - W?LEV
On Sun, Sep 27, 2020 at 5:04 PM Howard Fidel <howard@...> wrote:
I measure power into my 50 ohm dummy load by transmitting in CW and
measuring the RMS voltage on my scope. P = V*V/50.
