[Elecraft] [K2] KNB2 vs. electric fence.

[Don Wilhelm]

Steve,

I suggest that you build the KNB2 first and see if it is effective for 
the noise you have to deal with.  Normally, the gating level is not the 
problem, but the blanking pulse width can be varied (G3ULB mod) to 
handle other noise pulses.

The stock KNB2 is quite effective for impulse noise of the automotive 
ignition type.  Yes, if you have other types of noise sources that you 
must deal with, try either (or both) of these mods to see if it helps in 
your situation.  Similarly, your proposed change for Q3 may make a 
difference too, so try it out.  There is nothing wrong with a bit of 
experimentation.  What I am encouraging you to do is build the KNB2 as 
"stock" first, and then experiment from there.  The actual beta of 
several 2N2222As that I have measured is in the range of 100, so your 
proposed transistor change may make no difference.

73,
Don W3FPR

NU7B wrote:
> Joe:
>
> I just ordered a KNB2 and looked at DL7MAJ's modification. In looking at the
> schematic, it seems like the shortest time constant for the one-shot is
> above 10 µs. The DL7 mod noted that Q2 was being loaded by the base of Q3 -
> possibly there is a load transient associated with charging C11,12. The
> 2222A has a fairly low beta, and Q3 has no RE, so its base current could be
> fairly high. Raising the beta seems like it would reduce the loading on Q2.
>
> I'll measure the beta on what comes with the KNB2. I've got 2n3904 (B=100)
> and 2n5089 (B=400), so I may substitute. The 5089 has a lower fT of 50 MHz,
> but for a 10µs pulsewidth, it should be fine.
>
> Steve NU7B
>   
>