[Elecraft] APF (audio peaking filter) on the K3/K3S/KX3

[Ronnie Hull]

I went from a TenTec Orion to my K3 ( whose box I opened almost the very day the K3S was announced which is total BS) and I really mis the SAF feature in the Orion. The APF in my K3 doesn't touch it.

W5SUM

Sent from Ronnie's IPhone

> On Feb 5, 2016, at 09:56, dave <ho13dave at gmail.com> wrote:
> 
> 
> I'd agree. My experience with APF is that it will indeed pop a signal that is already copyable, if a bit weak, out of the noise. Makes for easier copy. However the ringing is so bad on a really weak signal that it is of no help. I guess it depends on what you view as 'weak' and your noise level. I was hoping it would make previously un-copyable signals copyable, but that does not happen. Instead it improves the copyability of weak but already copyable signals.
> 
> I find that simply narrowing the filters to 50, 100, or 150 Hz will give essentially identical results as using the APF and is much easier to do.
> 
> As for the super critical tuning, I do not find it to be all that critical. The signal can be improved somewhat by going to the 'fine' tuning, but it makes only a small difference. 10 Hz steps are OK and much quicker. The APF has a BW of about 30 Hz so tuning cannot be all that critical. +/- 5 Hz out of 30 is not that big of a deal.
> 
> This has been one of my biggest disappointments with the K3. After reading all the hype about APF I was hoping for a real improvement. But does not happen. OTOH, the narrow filters of the K3 are outstanding.
> 
> 73 de dave
> ab9ca/4
> 
> 
> 
>> On 2/5/16 8:53 AM, David Kuechenmeister wrote:
>> I tried this the other day with no success. That is, the weak
>> signal that was barely detectable, became so buried in ringing that
>> I couldn't detect it at all. I think I followed the procedure
>> outlined in Wayne's email below... Noise was around S5 on 40m and
>> the K3 was configured with APF active. I had my 500 Hz filter
>> selected and varied the DSP WIDTH from 500 to 300 without
>> substantial improvement. FINE tuning selected. What am I
>> forgetting? vy 73,Dave N4KD
>> 
>> On Tuesday, February 2, 2016 9:36 AM, Wayne Burdick
>> <n6kr at elecraft.com> wrote:
>> 
>> 
>> Hi all,
>> 
>> This morning I was hearing lots of very weak signals in the
>> 40-meter CW band, right at today's noise floor of about S-5. (Among
>> others, I copied BG4GOV in Shanghai.)
>> 
>> Many of the signals were not copyable until I turned on APF (audio
>> peaking filter). This filter's center peak is very narrow (around
>> 30 Hz), but the response broadens out quickly below the 3-dB points
>> in the curve. This brings up the desired signal without allowing
>> noise to cause ringing--something often heard with narrow filters
>> of the "brick-wall" variety. On a noisy band, the effect is just
>> short of magic.
>> 
>> APF is very narrow, so you'll need to tune in 1-Hz increments when
>> it is turned on. Also, APF works best when used with a DSP/crystal
>> filter WIDTH setting of 300-500 Hz.
>> 
>> Use of APF differs for the three transceivers.
>> 
>> KX3: 1-Hz tuning is selected automatically on the when the APF
>> switch is pressed.
>> 
>> K3/K3S: APF is accessed via a hold of the XFIL switch. This
>> function is labeled "APF" on a K3S, or "DUAL PB" on the K3). Use
>> CONFIG:DUAL PB to make sure the selected filter function is APF.
>> See the K3/K3S owner's manual for a description of the alternate
>> setting. 1-Hz tuning can be selected by tapping FINE.
>> 
>> You might want to give APF a try if you haven't already.
>> 
>> 73, Wayne N6KR
>> 
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