[Amps] Measuring IMD

[peter chadwick]

A very simple method which is adequate for most amateur use is to couple diode detectors to the input and output of the amplifier. The input detector goes to the X input of an oscilloscope and the output to the Y input. Adjust the coupling to get more or less equal voltages. The oscilloscope only needs to work at audio so an old tube Heathkit scope will do! When the line on the screen as you talk is straight, the linearity is adequate. With a 2 tone test, you can see by the curvature of the line such things as too low a standing plate current, poor grid circuit regulation and overdrive, and by making voltage measurements on the graticule, you can calculate 3rd and 5th order distortion. See the 1964 edition of 'Single Sideband Principles and Circuits' by Pappenfus, Bruene (who died the other month) and Schoenike.

With a solid state PA, a spectrum analyser is really needed because you have to consider the much worse high order IMD products (7,9,11) which tend not to be negligible. Even with LDMOS, which is as bad as bipolar on linearity.

73

Peter G3RZP