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8. PureSignal Adaptive pre-distortion information
PureSignal Predistortion
PureSignal document repository
Linearity of an amplifier implies that the output increases linearly with increases of the input signal. i.e., if an
input of 5W produces an output of 50W, then an input of 10W should produce an output of 100W.
Unfortunately, the analog amplifiers in our power amplifier stages are not so perfect. A real amplifier might
behave more like the figure below.
The unfortunate side-effect of this NON-linearity is Inter-Modulation Distortion [IMD], some of which results in
“splatter.” Consequently, on the Panadapters of our SDRs it’s easy to see signals that extend outside a normal
transmit bandwidth. This phenomenon is more evident on strong signals as the IMD may be 25dB or more
down from the peak of the signal. This IMD pollutes our bands and raises the interference and noise levels for
amateurs that may be operating many kilohertz away from the offending transmitters. Some of the IMD is less
visible on Panadapters because it falls IN-BAND, i.e., within the bandwidth of the transmitted signal. This adds
distortion to the signal.
Fortunately, several techniques have been developed to improve the linearity of analog amplifiers and thereby
reduce the IMD levels that they generate. One such method is “Predistortion”—making the amplifier’s output
more linear by “predistorting” the input signal in such a way as to offset the distortion that the amplifier will
produce. The output of the amplifier can therefore be corrected to behave very much like the ideal case shown
above.
Editorial note: IMD robs you of the value of an expensive Linear Amplifier by needlessly pumping out bad
Energy in opposite sideband and IMD splatter.
Copyright Apache Labs © page 41 26 February 2019