[Amps] Measuring IMD

[Karl-Arne Markström]

It is sad to read about the death of Warren Bruene. One of the really great in RF engineering have left us.
 
Regarding the use of linearity monitoring with diode detectors at the input and output, I recall this being a part of the
servo-tuning system used in the AEG-Telefunken 20 kW transmitters  that I worked with in the 80' and part of the 90's.
(Picture at the bottom of this page: http://www.seefunknetz.de/sdj.htm)
 
By first tuning for resonance in all circuits, then loading for the proper load line in the PA and finally adjusting the drive level 
to a point just below where the transfer function started to be curved, a good linearity (at least - 35 dB IM3 relative one tone) was assured.
 
It also was part of the envelope-feedback linearisation system that made the last few dB's improvement of IMD suppression possible.
 
By using this form of linearity monitoring, the most offensive cases of overdrive or misadjustment are possible to identify at a glance.
More accurate measurements require a spectrum analyser or a selective level meter capable of measuring adjacent channel suppression either with multi-tone driving signals or noise.
 
73/
Karl-Arne
SM0AOM
73/
Karl-Arne

----Ursprungligt meddelande----
Från: g8on at fsmail.net
Datum: 2013-11-03 11:28
Till: <amps at contesting.com>
Ärende: Re: [Amps] Measuring IMD

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
_______________________________________________
Amps mailing list
Amps at contesting.com
http://lists.contesting.com/mailman/listinfo/amps