Hi Peter,
Please excuse my attempt at clearing this up.
Rich's off-the-cuff condemnations and your response to them might
confuse others into thinking ALL automatic bias systems are
flawed.
It is absolutely true you do NOT want to switch from out-of-cutoff
into linear bias, or have a bias switching scheme that can not
follow the envelope and that does not switch at very low power (in
the milliwatt range).
But that acknowledgment should not be allowed to stand without
clarification. It might confuse people into wrongly thinking RF bias
switching isn't cheap and easy to implement, and that the
additional distortion is immeasurable when the circuit design is
correct.
There is absolutely no reason for bias to "follow the keying line"
unless the person doing the bias switching design is incapable of
understanding the necessary cures for envelope switching. Proper
RF switching is simple, since rise time is slow in the envelope
(because of filter or transmitter bandwidth) compared to the
potential response of a switching circuit.
From: Peter Chadwick <Peter_Chadwick@mitel.com>
To: "'W8JI@contesting.com'" <W8JI@contesting.com>,
> Tom says:
>
>
> >Bunk. The claims that all kinds of splatter is produced are
> >nonsense, mostly based on not understanding the effects of IMD.
>
> Sorry Tom, you're missing the point. My statement is that if you have a
> signal
> going into the PA and you switch the bias sharply, (we are assuming from
> cut off, by the way), then you'll effectively pulse modulate it, giving a
> sinx/x
>
> spectrum. On CW, we call this 'key clicks'.
>
> Now you're saying that there are better ways to implement electronic bias
> switching than switching to cut off. That is a different argument.
>
> 73
>
> Peter G3RZP
>
>
>
>
>
>
>
>
>
>
>
73, Tom W8JI
w8ji@contesting.com
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