Power calibration

Another issue that I've been contemplating for a while is power meter calibration. The amplifier module has forward and reverse VSWR outputs that can potentially provide power and SWR indications but as always with these things it ain't that simple. There are two principal issues to resolve:

1. The non-linear relationship between sensor output and RF power, as seen in the chart below;
2. The electrically short length of the SWR bridge, resulting in apparently higher SWR indications than  are the case in practice.
   

Forward sensor pin value vs.RF power

The non-linearity is difficult to resolve mathematically (at least for my brain that last worried about such things over 50 years ago) so I decided to implement a simple lookup table instead. 

Twenty arbitrary data points define forward sensor pin values and corresponding RF output power. These can be user defined and the results are stored in EEPROM. Power calculations are done by linear interpolation between the nearest two data points.

The table from which this graph derives is based on empirical measurements at the power points marked by vertical drop lines. The graph was then smoothed by eye to produce best guestimate table values. 

I still need to get access to an accurate power meter to validate these values, especially as I would like to implement an overall amplifier efficiency algorithm. That will only make sense if the input values are accurate.

The high SWR indication is interesting. Using my antenna as a dummy load (!) my SWR bridge says that the SWR is 1.2:1. In my software I convert forward and reverse sensor outputs to equivalent RF power and then calculate the SWR using the power SWR formula (1 + sqrt(Wr / Wf)) / (1 - sqrt(Wr / Wf)), where Wf=forward watts and Wr=reverse watts. This results in a reading of around 1.6:1.

It seems that the electrically rather short SWR bridge is prone to a certain amount of leakage and as the reverse power is relatively low, this effect is noticeable as an apparent increase in SWR. 

I am still considering a fix for this but my thinking is that as my sensor pin vs RF power table has removed the non-linearity of the actual sensors then it is a simple matter of scaling the reverse sensor power value to match measured SWR to actual SWR. That is the next thing on my software development task list.