Dear All,
Stefan raises an interesting issue.
On 2011 Jan 13, at 21:36, Stefan Schäfer wrote:
> My LF 1 kW class E PA i using a single IRFP360 at > 96 % efficiency
I'm sure you are getting high efficiencies. But measuring it at these numbers
close to 100% actually makes quite interesting demands on the accuracy of the
instrumentation.
You presumably measure DC input power from the PSU as Vin * Iin, and RF output
power as Vout * Iout.
The efficiency is then (Vout * Iout) / (Vin * Iin). The percentage error of the
result (the efficiency) is the sum of the percentage errors of the four
measurements.
You might instead measure the output power into a dummy load as Vout^2/R, but
you still have a percentage error of twice twice the error of the voltage
measurement.
If you measure the output on a scope, personally I wouldn't trust a scope
measurement to better than a couple of percent. The Vout and Iout measurements
(or two Vout measurements) will amount to a few percent error each, so your
efficiency of 96% could really be 96% +/-4% or even 96% +/-6%.
(If you measure Vin and Iin on a multimeter, they could easily have errors of
0.1% or better, and not contribute appreciably to the overall error.)
John G3PAI brought up this interesting problem when he first built a class E
amplifier and measured efficiencies of over 100%. It was immediately obvious to
him that the measurement accuracy was a limitation.
73,
Chris G4OKW
-----------------------
Dr Chris Trayner
School of Electronic & Electrical Engineering,
The University of Leeds,
Leeds LS2 9JT, United Kingdom
Tel: +44 113 34 32053
Fax: +44 113 34 32032
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