[hpsdr] Pure Signal RF Sampler

[Scott Traurig]

You are not missing anything, Dick. You can develop the signal however you
wish, as long as you provide a signal whose amplitude is within the dynamic
range of the receiver hardware and algorithm.

People have gravitated towards the Xtronic coupler because it provides
ruler flat coupling response from 160M to 6M, which takes some of the
guesswork out of knowing if you are obtaining the proper feedback signal
levels, because the design is bulletproof in terms of power handling, and
because the insertion loss is quite small. Oh, and because they don't have
to build it and characterize it themselves ;-)

There are certainly a few folks who are using their own coupler/sampler
designs, LP-100 sensors, and other, similar solutions. The only solution I
don't recommend is the Clean RF sampler. Bump that knob the wrong way and
it's possible you could fry your receiver, depending on the power levels
you are running and how much additional attenuation you have on the sample
port. Maximum safe input power level to any port on Apache hardware
is +13dBm based on the component data sheets.

73,

Scott/w-u-2-o

On Wed, Sep 5, 2018 at 1:08 PM Richard Grubb <richard.n.grubb at comcast.net>
wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
> 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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