[Elecraft] Why predistortion ?

[Randy Farmer]

I can testify to the effectiveness of predistortion techniques from my 5 
years or so as a CDMA Cellular Base Station engineer with Motorola. The 
first generation of CDMA transmitters had a very specific "spectral 
mask" that had to be certified. As I recall, the transmitter output that 
met the Spectral Mask requirements of something like -60 dBc outside of 
the channel bandwidth showed up with essentially vertical sides (and a 
very flat top) on a spectrum analyzer. Obtaining this performance was 
neither cheap nor easy, and the associated testing was pretty stringent 
as well. Meeting the specs demanded very sophisticated predistortion 
techniques. Be glad that conventional SSB and the fairly simple 
waveforms used in amateur digital comms don't require too much dynamic 
headroom. The first generation CDMA waveform had a roughly 10:1, or 20dB 
peak-to-average power ratio. This meant that the transmitters we 
designed to produce 20W average power were actually capable of 200W 
continuous output power and still met the spectral mask IMD 
requirements. Putting multiple carriers through the PA required either 
higher power capability or derating the power output for each individual 
carrier. I no longer work in the industry, so I don't know what the 
current generation of signals requires, but with the greater bandwidths 
and more complex modulation schemes used now you can bet the 
requirements, and therefore the transmitter design challenge, didn't get 
any easier.

73...
Randy, W8FN

On 9/13/2020 6:03 PM, Wayne Burdick wrote:
> Hi JR,
>
> Virtually all class-AB-biased amplifier stages have worst-case 3rd-order IMD in the range of -30 dBc (ARRL method) on some HF bands. This reflects a limitation in the state of the art. Even with adequate feedback and bias, an amplifier operated in the vicinity of its 1 dB compression point (where it is most efficient) will exhibit such characteristics.
>
> To improve on this you either need to operate class A, which is highly inefficient, or use predistortion.
>
> A small number of commercial transceivers are now providing predistortion, in some cases as an optional/experimental setting (that not everyone uses, or at least not all the time). There are constraints on predistortion power range imposed by the headroom limit of the amplifier stage itself. Go over that point and distortion will be worse than without predistortion.
>
> Predistortion, correctly applied, can reduce interference between adjacent stations on a crowded band. This is a factor at least some fraction of the time during typical amateur radio use. Beyond that, the motivation for a “pure“ signal exemplifies the amateur spirit of cooperation, and is an example of keeping the hobby on the forefront of electrical design.
>
> Wayne,
> N6KR
>
> ----
> elecraft.com