[Elecraft] K3 SWR Accuracy - reprise

[Wayne Burdick]

Slight clarification. I said:

> The K3's SWR bridge has two sensitivity ranges. If you have power set
> to 12 W or lower, the higher-sensitivity range is used. If you have
> power set above 12 W, the lower-sensitivity range is used. This is
> necessary to prevent heating of the transformers in the bridge.


The transformers in the bridge use the largest coupling ratio  
possible, consistent with not overheating when used at max power  
output (100-110 W). SWR measurement accuracy is better at higher  
power, in general, because it overcomes forward-bias voltage variation  
across the detector diodes. The sensitivity scaling occurs at *DC*,  
after the op-amps that amplify the forward and reflected voltages. The  
higher-sensitivity range, used at 12 W and below, has better noise  
immunity and better granularity feeding the A-to-D converter.

There are two additional factors contributing to SWR measurement  
accuracy. (1) Some interpolation is done in firmware. (2) There's a  
small pre-bias (DC) on the bridge to compensate it at lower power  
levels.

Wayne
N6KR