[Elecraft] Why predistortion ?

[E.H. Russell]

Well put.

TKS,
73 ED W2RF




-----Original Message-----
From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On Behalf Of Wayne Burdick
Sent: Sunday, September 13, 2020 6:03 PM
To: JR <jrichards at k8jhr.com>
Cc: elecraft at mailman.qth.net
Subject: Re: [Elecraft] Why predistortion ?

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

> On Sep 13, 2020, at 2:34 PM, JR <jrichards at k8jhr.com> wrote:
> 
> Why are so many ops obsessed with predistortion?
> 
> The Elecraft K-Line already produces some of the cleanest, purest transmit signals around.   Do we need predistortion ?  If so, why, and how much better is it than what we already have?  Should I wait to buy a new rig until that is available?
> 
> Thanks and Happy Trails to all.   K8JHR
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