[Elecraft] APF For K3

[Ron D'Eau Claire]

According to Icom (who has it on some of their rigs): 

"The APF changes the audio frequency response by
boosting a particular frequency to enhance a desired
CW signal... The peak frequency can be adjusted"

So this is an adjustable frequency audio filter such as we've used for the
past 3/4 century.

As Doug, KR2Q, noted, the K3 is capable of a 50 Hz bandpass. That's about as
narrow as a CW signal being keyed at 20 or 30 wpm can be fit into. Narrower
will attenuate the CW sidebands so the dots and dashes start "slurring"
together. 

I've used an audio filter many times and found them of very limited value to
me if the receiver has a decent I.F. bandpass characteristic. 

That said, if it it's reasonable to implement, people want it, and it can be
switched out, I'm all for it. 

Ron AC7AC