[hpsdr] Pure Signal RF Sampler

[Richard Grubb]

I am wondering why the documentation on Pure Signal implies the use of a 
directional coupler as necessary to obtain the required sample of the 
transmitted RF. I can understand why that could be desirable when the 
feedback was not adaptive, or where the sampler is remote from the final 
amplifier.. However when the predistortion is being generated adaptively 
I do not understand why a simple resistive non directional coupler would 
not be perfectly satisfactory and much easier to implement.  The antenna 
system should be quite linear and the voltage sensed adjacent to the 
final amplifier output should surely always be a valid measurement of 
the transmitter transfer function. Of course a directional coupler is 
always useful to measure VSWR and is usually already present in modern 
amplifiers, although often not readily accessible for use with Pure 
Signal.

I have implemented Pure Signal on an ANAN 10, Juma 1000 PA combination 
and used both the published directional coupler design and a simple 
resistive pick off at the PA output with equally excellent results.

What am I missing?

Dick, W0QM, G3FNL
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