[hpsdr] Pre-Distortion

[Helmut, DC6NY]

Hi,

just my two 2ct: The typical improvement is far more than 10 db. It can be
around 30 dB(!). I got - 55dBc on 2tone (equal -61 dB PEP) with my 800W
LDMOS/BJT amp. This means the same scale as the HPSDR exciter or no
degradation of spectral purity due to IM after 32 dB amplification.

73, Helmut, DC6NY

-----Ursprüngliche Nachricht-----
Von: hpsdr-bounces at lists.openhpsdr.org
[mailto:hpsdr-bounces at lists.openhpsdr.org] Im Auftrag von Shirley Márquez
Dúlcey
Gesendet: Sonntag, 8. Dezember 2013 06:20
An: hpsdr at lists.openhpsdr.org
Betreff: Re: [hpsdr] Pre-Distortion

***** High Performance Software Defined Radio Discussion List *****

> 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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