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@fsmail.net
Datum: 2013-11-03 11:28
Till: <amps@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
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