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

[Bob McGraw K4TAX]

While I find the APF works really nice, one must also discover the 
artful use of the RF Gain control as well.   Most hams seem to use 
excessive RF Gain which will force noise through the filter and all of 
those undesirable artifacts will be heard.  If in doubt, reduce the RF Gain.

Using this technique, I can tune the band with the APF engaged, 
adjusting the RF Gain a bit, and actually hear signals with this 
configuration which without the APC no signal could be heard.  I view 
this gives one the ability to actually "dig signals out of the noise".

73
Bob, K4TAX

On 2/5/2016 9:56 AM, dave 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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