[AMPS] Bias for SB-220

[Tom Rauch]

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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