[Elecraft] SWR

[Wes (N7WS)]

I guess it's semantics.  If an undesired output event causes a corrective action 
to be taken in a previous stage isn't this a closed loop?  I believe it is, even 
if it's a "man-in-the-loop" system, i.e. I see on the oscilloscope that I'm flat 
topping so I turn the audio gain down to correct it.

On 2/12/2015 9:59 PM, Don Wilhelm wrote:
> Wes,
>
> An analog open loop ALC is one in which the drive is reduced in response to an 
> overload condition (high SWR, power level that exceeds the maximum set 
> level).  The response is applied only after the offending condition occurs - 
> that is an open loop system.
> With the K3, KX3 and K2 power control system being a closed loop system, that 
> condition does not exist.  The overshoot possibility is detected early and the 
> power is reduced accordingly without PA overshoot and the PA going into a 
> distortion region.
>
> If you have any understanding about closed loop control systems, you will then 
> understand 'damping' conditions.  Underdamped systems will oscillate near the 
> 'set point' while overdamped systems will never come up to the set point.  
> Critically damped systems will quickly settle to the set point and maintain 
> the proper condition (power level, temperature, you 'name it' condition).  In 
> the case in point, the power level is the 'set point' that is considered.  An 
> open loop system is totally undamped and will only respond 'after the fact' to 
> conditions that exceed certain thresholds.  Closed loop systems approach those 
> thresholds with actions that will result in critical damping of the response 
> mechanism.
>
> 73,
> Don W3FPR
>