>> >On Monday 15 April 2002 06:41, Peter Chadwick wrote:
>> >> Rich said:
>> >> >- Mo' ALC-meter indication is seemingly not mo' betta.
>> >>
>
>We speak of ALC causing the problem - but ALC simply allows the problem to
>occur. The real problem is "flat-topping" during the instant of the first
>pulse - prior to ALC shutting down the amplifier state (whether internal or
>external).
ALC will always be a problem, but it can be ameliorated with RF-NFB and
less than "petal to the metal" mic. gain setting.
>
>If, for example, we build an amplifier that runs 10 kw output cleanly and
>then use ALC to limit the power output to 1.5 kw output, I believe we would
>be in compliance with the FCC rules, even though an original pulse may
>exceed 1.5 kw output prior to the ALC taking effect.
>
? The FCC limit on allowable IMD in the Amateur Radio bands is
indeterminate. The Part-97 FCC rule on 1500W +/- 10% is far from
strictly enforced unless the station is causing interference.
>The problem with ALC is that there is a slight delay between speech pulse
>and application to shut down the amplifier. If that delay occurs after the
>pulse occuring in the amplifier, the amplifier will be overloaded and
>"flat-top". Although the pulse is "immediately" shut off, it is like
>closing the gate after the horse has bolted out.
>
Good analogy.
>I believe ALC can be used safely only if the peak output without the
>application of ALC is distortion free. If there is distortion, the ALC
>will not cure the problem.
Yea, verily,
>
>I understand there are some rigs that use ALC to turn the power down. So
>if you use a 100 watt exciter and crank the output down to 30 watts to
>drive the final, you may still have 100 watts output for a instant. If
>this output causes overdrive of the amplifier, the splatter must occur.
>But if the amplifier is truly linear and has a clean output when driven by
>100 watts (5 kw), then the splatter should not occur.
Provided that the exciter's final unit does not flat-top.
>
>Which is why we build amplifiers that run 5 kw output - so we can use ALC.
>
Chortle
cheers, Colin
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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