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APF Primer?

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kevinr@coho.net

APF Primer?

I have used APF a number of times. Sometimes it worked, but often the
ringing makes it hard to hear anything intelligible.

I have a two families of questions to hopefully remedy my problem.

1) What are the best band conditions to try APF? Does it work under
medium QSB against medium to high noise levels? Is it better on 80, 40,
or 20 meters?

2) What are the procedures to use APF on your K3, KX2, KX3, etc.? How
should I set the bandwidth of the filter I'm already using? How do I
peak the APF performance? Does the set up change on a per band basis or
does one setting fit all cases?

As I mentioned, I have gotten it to work well a few times, but usually
the signal sounds buried in a hard walled well. Then I get emails
telling me how well it worked during ECN. My interest is renewed, hence
my questions.

Help?

Kevin. KD5ONS

-

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k6dgw

Re: APF Primer?

Kevin:

1) For a K3, APF seems to be very dependent on current AGC action. If
there are very strong signals close in and the AGC is doing its thing,
APF doesn't seem to work. You can adjust the AGC parameters, however my
experience is that I use APF infrequently enough that it's better to
adjust the AGC to non-APF situations.

2) The APF will ring, it is very narrow. With APF on, using a wider
DSP BW [400 or 500 Hz] will help some. Tuning just off the peak, if
possible, can also help although even at 1 Hz tuning, sometimes that's hard.

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 8/24/2020 7:33 PM, kevinr wrote:

> I have used APF a number of times. Sometimes it worked, but often the
> ringing makes it hard to hear anything intelligible.
>
> I have a two families of questions to hopefully remedy my problem.
>
> 1) What are the best band conditions to try APF? Does it work under
> medium QSB against medium to high noise levels? Is it better on 80,
> 40, or 20 meters?
>
> 2) What are the procedures to use APF on your K3, KX2, KX3, etc.? How
> should I set the bandwidth of the filter I'm already using? How do I
> peak the APF performance? Does the set up change on a per band basis
> or does one setting fit all cases?
>
> As I mentioned, I have gotten it to work well a few times, but usually
> the signal sounds buried in a hard walled well. Then I get emails
> telling me how well it worked during ECN. My interest is renewed,
> hence my questions.
>
> Help?
>
> Kevin. KD5ONS

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Brian Hunt

Re: APF Primer?

In reply to this post by kevinr@coho.net

Kevin -

Here's my take on the APF. I use it a lot during QRP Fox Hunts and I just used it to pull G3SED out of the weeds on 20.

Best conditions are when only random band noise is in the passband for my technique although it can be used in conjunction with the NB (the IF blanker, not the DSP option). The passband should be QRM free.

I set the DSP passband to about 700 Hz and turn on NR set to NRm5-3 or so. This is fairly light noise reduction with a level of unprocessed noise+signal mixed in. Since there is only random noise in the passband that level of NR works well. I use this to tune around looking for the Fox.

When I find a signal of interest, I turn on the APF and peak the signal in the very narrow passband using FINE tuning. I also frequently use an audio spectrum display on the PC to center the signal on the marker which is set to the CW pitch frequency. Although the APF is very narrow: ~30 Hz at -3 dB, the sharp peak is more like 5 Hz wide so FINE tuning is needed and the marker helps because my ears aren't that good at matching pitch. You can set the DSP filter narrower than 700 Hz to eliminate some QRM but that seems to be the sweet spot for the NR to work well.

To your other questions ...
Make sure the noise level is not activating the AGC or it will be working against you.
Really no band preference but it's better not to have big static crashes to deal with. Set AGC to fast if there are.

It took me a while to figure this out. Have fun!

73,
Brian, K0DTJ

-----Original Message-----
From: "kevinr" <[hidden email]>
Sent: Monday, August 24, 2020 19:33
To: "Elecraft Reflector" <[hidden email]>
Subject: [Elecraft] APF Primer?

1) What are the best band conditions to try APF? Does it work under
medium QSB against medium to high noise levels? Is it better on 80, 40,
or 20 meters?

2) What are the procedures to use APF on your K3, KX2, KX3, etc.? How
should I set the bandwidth of the filter I'm already using? How do I
peak the APF performance? Does the set up change on a per band basis or
does one setting fit all cases?

______________________________________________________________
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Help: http://mailman.qth.net/mmfaq.htm
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Vic Rosenthal

Re: APF Primer?

In reply to this post by k6dgw

The K3 APF has a very sharp peak (about 30 Hz @ 3 dB) but broader
shoulders (about 300 Hz wide). It is not in the AGC loop, so any signals
inside the bandwidth set by the bandwidth control may activate the AGC.

Therefore, if you have nearby strong signals, you should reduce the
bandwidth to keep them from affecting the signal you are peaking with
the APF.

But if you reduce the bandwidth below 300 Hz, then you will cut into the
shoulders. This starts to have a negative effect on CW intelligibility.
For some biological reason that I don't understand, the "context"
provided by the shoulders helps overcome the ringing effect of the very
narrow peak.

High noise levels also negatively impact the usefulness of the sharp
peak, because of the ringing.

73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
On 25/08/2020 23:49, Fred Jensen wrote:

> Kevin:
>
> 1) For a K3, APF seems to be very dependent on current AGC action. If
> there are very strong signals close in and the AGC is doing its thing,
> APF doesn't seem to work. You can adjust the AGC parameters, however my
> experience is that I use APF infrequently enough that it's better to
> adjust the AGC to non-APF situations.
>
> 2) The APF will ring, it is very narrow. With APF on, using a wider
> DSP BW [400 or 500 Hz] will help some. Tuning just off the peak, if
> possible, can also help although even at 1 Hz tuning, sometimes that's
> hard.
>
> 73,
>
> Fred ["Skip"] K6DGW
> Sparks NV DM09dn
> Washoe County
>
> On 8/24/2020 7:33 PM, kevinr wrote:
>> I have used APF a number of times. Sometimes it worked, but often the
>> ringing makes it hard to hear anything intelligible.
>>
>> I have a two families of questions to hopefully remedy my problem.
>>
>> 1) What are the best band conditions to try APF? Does it work under
>> medium QSB against medium to high noise levels? Is it better on 80,
>> 40, or 20 meters?
>>
>> 2) What are the procedures to use APF on your K3, KX2, KX3, etc.? How
>> should I set the bandwidth of the filter I'm already using? How do I
>> peak the APF performance? Does the set up change on a per band basis
>> or does one setting fit all cases?
>>
>> As I mentioned, I have gotten it to work well a few times, but usually
>> the signal sounds buried in a hard walled well. Then I get emails
>> telling me how well it worked during ECN. My interest is renewed,
>> hence my questions.
>>
>> Help?
>>
>> Kevin. KD5ONS