[Elecraft] attenuator in amp was Re: K3 and Expert 1K-FA

[Paul Christensen]

> set the power control to the proper drive, and forget the ALC?  ALC
> doesn't fix leading spikes anyway, so what's it good for?

Arguably, ALC helps to function as a time-averaging RF level control that 
can protect against ongoing over-drive but not necessarily peak power busts. 
What I don't like about traditional ALC are the unknown side-effects (e.g., 
non-linearity) that can occur as a result of level-setting and timing 
differences between the transmitter and amp.  Dynamic monitoring for amp SSB 
non-linearity can be accomplished with reasonably good accuracy using a 
scope in trapezoid mode.   However, once transceivers began introducing 
delay as result of DSP, it's become nearly impossible to use the trapezoid 
method to look at system linearity of both the transceiver and amp, and the 
corresponding ALC effect.   Other methods for measurement are probably 
beyond the cost and effort at most stations.

Since the input Z of most solid-state amps is well-controlled at 50-ohms, 
that the input Z should not appreciably change as output Z changes, then 
can't we simply use a feed-forward ALC system instead of feedback ALC?   A 
fast RF voltage peak detector can be bridged across the amp's input Z.  When 
the RF voltage exceeds ~ 70.7Vpk, correction bias is applied to the 
solid-state PA. Simultaneously, a 3dB pad could then be inserted at the amp 
input until average input power drops below 50W.   With the exception of the 
pad, the SPE amp works like this -- only it has a fault summing network that 
looks at half a dozen other parameters, all of which instantly apply MOSFET 
bias through the summing network to cut-off RF, and place the amp in bypass 
when any one of the non-ALC faults occur.

Paul, W9AC