[Elecraft] K3:New APF what happened?

[Bill W4ZV]

N2TK wrote:
> 
> John,
> Maybe the impact of the APF is a moving target influenced by the band
> conditions at the time?
> Yesterday morning 160M was very quiet. I could not hear the VK6 without
> the
> APF. When I turned on the APF the VK6 popped out of the band noise and was
> easily copyable. There was no hint of ringing even with the bandwidth down
> to 50HZ.
> 
> Last night as usual in the evenings here 160M was noisy. When I turned on
> the APF it still did its magic. But there was almost a trace of "ringing".
> 
> I need to play more with it to see if it is a definite pattern comparing
> between quiet and noisy band conditions.
> 

Ringing is a function of how much impulse noise (i.e. lightning induced,
electric fencer "ticks", key clicks, etc) hits the filter.  No impulse noise
= no ringing,  lots of impulse noise = continuous ringing.  As you said,
yesterday morning 160m was stone quiet and the background noise was
non-impulsive or white noise.  Last night a strong cold front moved across
the Southeastern states which had lightning associated with the front.  The
lightning strikes created impulse noise which caused the filter to ring more
than in the morning.  This effect is present in any narrow filter, whether
analog or digital, and there's really not much that can be done to avoid it. 
Reducing AF Gain is about the only solution to attenuate the continuous
ringing sound but that also attenuates the signal you're trying to hear.

73,  Bill
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