On 2025-02-10 11:34, K9HZ wrote:
In any
case, he does tout using class-A final RF amplifiers in the transmit
stages of radios because they, by shear design, will generate less
IMD.
*** NO thanks. A 100W transmitter the size of a Heathkit Apache?
Or a legal-limit rig the size of a Piper Apache?
Predistortion is interesting. Right now, all implementations that I
know of are closely held commercial secrets. Anybody know of an
open-source attempt to do it?
It doesn't seem THAT complicated. Basically, the power amp's gain gets a
little lower as it approaches its amplitude limit. So you compensate for that
by raising the gain a little - only for the tips of the low-level RF. You
"exaggerate the tips" a little. Sort of a negative feedback situation.
But what about actual negative feedback? Won't work, because there's too much
delay in the RF chain. Guaranteed, at some frequency or frequencies, it would ring
like a bird.
Yet, the commercial systems apparently *do* feed RF back into the exciter. Do they
"SSB-detect" it, turn it back into audio, and compare that to the audio being input?
- Jerry, KF6VB
