[Elecraft] APF

[Sam Morgan]

one more fyi on the [XFIL/DUALPB] button
when in RTTY, both AFSK A and FSK D, it's a Dual Passband Filter
when in CW mode it acts like the single peak APF

there's an image of the APF in use on N1EU site:
http://n1eu.com/K3/K3_apf.htm


On 11/30/2013 11:56 AM, Sam Morgan wrote:
> I'll copy and paste from my notes, hope it helps clear it up for you.
> sm
>
> ========================
> 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