From: Chris Trayner <C.Trayner@leeds.ac.uk>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
Date: Fri, 14 Jan 2011 00:01:07 +0000
Thread-Topic: LF: Re: Re: FET RDS
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Dear All,

Stefan raises an interesting issue.

On 2011 Jan 13, at 21:36, Stefan Sch=E4fer 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 numb=
ers 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 ou=
tput 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 vol=
tage 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 measure=
ments (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 ob=
vious 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