[Elecraft] K3:New APF what happened?

[Bill VanAlstyne W5WVO]

Very good summary of the technical facts, Bill.

It's IMPULSE noise, everybody, not white noise or pink noise or any other 
kind of noise. Impulse noise is characterized by a very rapid transient 
rise-time producing a large nearly instantaneous spike in amplitude. It's 
literally the electrical equivalent of striking a bell sharply, as opposed 
to rubbing it.

The other thing that can make this feature seem magical (in the absence of 
excessive ringing) is having it properly tuned, and by "properly" I mean 
within a few Hertz. The K3's CWT/SPOT capability is good, but it's often not 
good enough for this degree of accuracy. Mine usually tunes to within +/- 10 
Hz, but that's not always close enough for the APF to work at its best. The 
signal needs to be fine-tuned by ear, IMO, against the sidetone PITCH 
frequency. Most people can hear the difference between two tones down to 1 
Hz or less. (BUT, some people CAN'T. It's a brain thing. Your brain can 
either do this or it can't. My XYL couldn't do this if her life depended on 
it, as she is completely tone-deaf.)

So maybe we can agree that there are a lot of variables at work here that we 
can't control for. I thought at first that the current release was "less 
ringy" than the earlier release; I was fooled by a difference in band noise. 
Since then, I've seen the APF feature behave quite differently on different 
bands at different times. So, if it makes it easier to copy a given signal, 
use it, and if it doesn't, leave it turned off. It's a tool. All tools are 
not appropriate for all jobs all the time. You have to choose your tools and 
when to use them.

Bill W5WVO



-----Original Message----- 
From: Bill W4ZV
Sent: Wednesday, November 17, 2010 15:35
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] K3:New APF what happened?



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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