There are two basic feedback methods using the technique outlined.
1. Polar loop.
First described by the late Vlad Petrovic of Bath University, this uses
fedback from the output to adjust the phase and amplitude to cancel
distortion. It gets round AM-PM conversion problems in the PA.
2. Caretsian Loop.
This does the feedback by converting the output to baseband and subtracting
it in the modulation process. It does lea to somewht higher out of band
emissions unless the dynamic range of the mixers is very low. It is used
extensively commercially in the 220MHz mobile radio narrowband equipment
sold by Midland in the US, and has a number of patents on it for the modern
implementations. One implementation may well have unwittingly used other
patented technology.
73
Peter G3RZP
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