[Elecraft] APF

[Sam Morgan]

I'll copy and paste from my notes, hope it helps clear it up for you.
sm

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APF on the K3
The APF is an easy tool to use. However, one of the main concerns people 
are talking about now is the fact that the filter seems to be a bit 
tight, and tends to ring. This, of course, is normal for an APF (IIR 
type filter), but not everyone wants to hear ringing on their CW signal 
when they use it. Others just can’t copy the CW signal when the ringing 
is at the same exact frequency as their CW tone. For this reason I have 
written this little tutorial on how to remove the ringing from your APF 
filter while still getting the most out of it.

Let’s start by centering your SHIFT (before you turn on the APF) and 
make sure you see the * next to your PITCH/center IF frequency. Next, 
spot the CW signal in. i.e., hit the SPOT button when you’re trying to 
copy someone, or use the VFO to center the line on the CWT display. Now 
you can be sure that the CW station is in the center of your IF 
passband. Next, adjust the width setting to your liking. It almost 
doesn’t matter once the APF is on what you choose. However, what it will 
help with is blocking noise from the sides of the signal you’re 
listening to. This can be helpful if you have other stations that are 
very close to the CW signal you’re trying to peak. The lower you set 
your Width filter the more you will reject to the sides. Any signals 
(including noise) that are left over on the sides will be compounded 
into the center of your IF passband and cause more ringing noise. So I 
would suggest that you keep your WIDTH from 250 Hz to 100 Hz setting. 
You can play more with the WIDTH control once you have the APF on.

Turn on the RIT control (tap the RIT button). Next, lower the RIT 
frequency -20 Hz. (You must have CONFIG:PB CTRL in the config menu set 
to .01 to move in 10 Hz steps). If your pitch is set to 500 Hz, then the 
tone of the CW should now be 480 Hz. Next, turn on the APF by HOLDING 
the DUAL PB button. Adjust the APF using the SHIFT Fc Knob (now APF 
control). Move the APF down -20 Hz to match the CW signal. Once you have 
the signal in the center of the new 480 Hz tone you should hear that the 
CW signal pops up about 5 or 6 dB.

What you should also notice is that CW tone is now centered at a 
different frequency than your IF passband center. This also allows you 
to avoid peaking the ringing sounds that live in the center of your 
selected IF area. Your brain is now free to discriminate the two 
different tones instead of having to copy code at the same frequency as 
all the ringing noise.

If and When the K3 has a variable Q adjustment you will then be able to 
avoid the ringing by opening up the tightness of the PEAK Filter and 
allowing a little more signal to pass. Of course you also compromise the 
“sharpness” of the current APF operation, but if you feel that you don’t 
need as much peaking on the signal then you will be able to control it a 
little more accurately. Having a Variable Q control would be best used 
on a signal that is ‘less’ weak than those hard to copy signals in the 
noise floor.

Another great feature of the APF is being able to use it on either/or 
filter position I or II. If you like, you can set filter I for APF off, 
and filter II to APF on. Switching between the two filters is as easy as 
HOLDING the SHIFT knob in. Once you set either filter position with the 
APF on or off the K3 will remember how you have it set.

Dusty K3WC

================
Jun 26, 2012; 1:19pm
K3/KX3 Operating Tip: APF (audio peaking filter) for weak-signal CW work
	
Both the K3 and KX3 have an APF control (audio peaking filter). APF
can be extremely effective when you're trying to copy weak CW signals
buried in noise.

Most DSP filters have very steep skirts; they're often referred to as
"brick-wall" filters for this reason. This is the type of filtering
associated with the WIDTH control (K3) or PBT control (KX3).

In contrast, the APF is a filter with a very narrow peak (about 30 Hz
at the -3 dB points) and very broad skirts. The narrow peak brings up
the signal amplitude slightly, while the broad skirts prevent noise
from being amplified and delayed in such a way as to dominate the
signal. The APF function used in the K3 and KX3 is a modern DSP
reinterpretation of the hardware APF circuit found on some legacy
transceivers, which was very effective.

On the KX3, the APF switch is labeled "APF" (tap), while on the K3,
it's labeled "DUAL PB" (hold). The K3 actually provides two different
special filter functions; use the DUAL PB menu entry to set up the
switch for APF.

When you turn on APF, the DSP graphic changes to remind you of the
nature of APF, with its narrow peak and broad skirts.

I suggest setting the passband width to about 250-300 Hz when using
APF. The DSP graphic still shows an approximate representation of the
width even with APF turned on.

73,
Wayne
N6KR
====================

On 11/30/2013 11:45 AM, todd ruby wrote:
> Can someone explain how to engage and use the Audio Peak Filter on the K3?
>
> thanks in advance
>
> todd
>
> WB2ZAB

-- 

GB & 73
K5OAI
Sam Morgan