[hpsdr] Pre-Distortion

[Shirley Márquez Dúlcey]

> At the risk of showing myself to be a complete plebeian, is there a discernible improvement that one can hear when using predistortion; or is this more of an esoteric exercise in design excellence? In other words is this an improvement that one can only detect using test equipment.

For simple modes like SSB, the person listening to you probably won't
hear the improvement from pre-distortion. (It is possible that the
result will sound subjectively better but the change will have no
effect on the intelligibility of the signal.) But people sharing the
band may notice.

In a typical amateur HF SSB radio, the third order intermodulation
(IMD) products from a two-tone signal are about 30dB below the PEP
power. 30dB is five S units in theory (in theory because S meters are
wildly variable). That means that if you are trying to listen to a
weak station that is 5 S units down from a strong station, the
distortion products from the strong station would be equal in
magnitude to the desired signal. The weak station will be difficult to
copy. The usual solution is to move farther away from the strong
signal, but that may be difficult to do on a crowded band. This is
especially important for QRP operators because their 5W signal is
already 13dB below a 100W signal and about 25dB below a maximum legal
power signal, so it doesn't take much more to push the QRP signal down
below the distortion products of a higher power station.

Now suppose the owner of the stronger station buys an improved
transmitter where the IMD products are 40dB below PEP. Now the
distortion products are 10dB down from the desired signal. This isn't
enough to make it armchair copy but is a huge improvement; the weak
station will be much easier to copy.

There is absolutely nothing that you, the receiving station, can do to
improve this situation by improving your receiver design. (Changing
your antenna to reduce the strength of the unwanted strong signal is
another matter.) The unwanted signal is in the air, and it's just as
strong as the one you want. But the transmitting station can improve
it.

For complex digital modes, predistortion may be important to people
receiving the signal with the predistortion included. Distortion
products inside the receiver passband may be strong enough to
interfere with decoding of the digital signal; lowering the level of
transmitter distortion improves the internal signal to noise ratio of
the received signal and contributes to reliable and accurate decoding.

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