> The problem is setting gain/drive up too high and letting the
> ALC loop control everything. This HAS to result in overshoot
> since there is a time constant in any feedback loop. Proper
> selection of component time constants can reduce this but
> not eliminate the problem.
Unless a separate "Drive" control is accessible to the operator, it takes a
dual-speed ALC loop to completely address the problem. The trick is
designing ALC to respond fast enough to inhibit leading edge power spikes,
while preserving the RF waveform. The effect is most noticeable on
transceivers with inferior ALC design when operating CW. The cheap fix for
the manufacturer to eliminate the power spike issue is to drive ALC harder
and use a faster response time. But this results in waveform distortion.
I am now a firm believer in rigs with a separate Drive control. On rigs
with the control, or on rigs with no Drive control when using SSB, I never
drive the radio into the ALC range -- rather, I find the ALC threshold and
then just slightly advance for a slight indication of ALC activity. On CW,
this will often result is a very clean waveform (notwithstanding other
factors as in the FT-1KMP) and thermal drift shouldn't become problematic.
I could never understand the rationale behind a manufacturer's instructions
where they recommend ALC "anywhere within the red zone" for example. On
many rigs, the upper edge of the red zone results in extreme pre-PA gain and
a lot of RF limiting. Outside of maintaining constant power as a result of
thermal drift, for what other reason is it necessary to drive the ALC so
hard?
-Paul, W9AC
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