[hpsdr] Pennywhistle bias adjustment

[Graham / KE9H]

Geoff wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Hi Guys,
> 	I'm presently trying to set the bias on my recently assembled pennywhistle. I 
> am getting a "basic" current of only 8.2mA 
About right. That is the 6 Volt regulator, and the load from the two pots.
> and I'm finding the reaction to 
> pot movement a little odd. For the first 30 odd degrees of pot movement i get 
> no reaction at all.
Also normal.  The FETs do not conduct until you reach their "threshold" 
around 2.5 Volts on the gate,
then they start to conduct.  500 ma will typically be with around 4.8 to 
5.1 Volts on the gate, and
6 Volts on the gate will be several amperes.  You can look at the 
manufacturer's spec sheets
to see the typical bias curve.

>  After that I get a nominal reaction from both pots but it 
> is a bit touchy. How precise (how even?) do you need to be in setting the 
> bias with this little beast? :-).
>   
If you are within five (or ten) percent of the desired values, it will 
work fine.  The current will also
rise up to another ten percent with a hot heatsink.

--- Graham / KE9H



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