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Tips for using APF

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wayne burdick

Tips for using APF

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A couple of notes from the Elecraft lab:

1. When using APF, it's critical to tune the signal in very
accurately. (Otherwise it may seem like APF isn't working.) For best
results, I recommend using 1-Hz VFO tuning. An alternative is to
adjust the APF center pitch, which can be moved in 5-Hz increments
using the SHIFT control. Note that only 10-Hz increments are displayed.

2. There are two filter "Presets" (I and II, accessed by HOLDing the
HI/WIDTH knob). You can turn APF on for one preset and off for the
other. This control is closer to where the action is than DUAL PB, and
I find it more convenient to use.

3. You can of course hear the effect of APF by ear, but if you want to
quantify it, try using the built-in AF voltmeter on a very weak
constant carrier buried in noise. Here's the procedure I use:

a. Switch to a noisy band/antenna (80 m at night, let's say, with a
vertical or inverted-V).
b. Turn APF off.
c. Set WIDTH to 300 Hz, with no SHIFT.
d. Hunt for an extremely weak on-air carrier -- one you can just
barely detect. A carrier that's caused by a local computer or TV is
ideal because there won't be much fading during the test.
e. Tap DISP and rotate VFO B until you get to AFV (audio volt
meter). Let the reading stabilize.
f. Rotate VFO B one more tic clockwise to dBV. The number will
bounce around 0.0 by maybe +/- 1 dB. Call this the reference level.
g. Now tune well off the carrier so that you hear only noise. The
dBV number will indicate how many dB the signal has dropped. If it
drops 3 dB, your S+N/N ratio is 3 dB.
h. Tune the signal back in and make sure the level is back to
around 0.0.
i. Turn on APF. Using FINE VFO tuning (1-Hz steps), make sure the
carrier is peaked as indicated by ear or on the dBV indication.
j. Use VFO B to go back to AFV for a few seconds to capture a new
relative signal level. Then go back to dBV, which should again show
about 0.0 (bouncing a little).
k. Tune well off the carrier so you again hear only noise.
l. Note the new (negative) dB reading.

I just repeated this little test on 80 m with persistent S-6 noise.
Here's what I got:

Without APF: 1 dB S+N/N
With APF: 6 dB S+N/N

In fact, having turned APF on, I was now able to find a lot more
little carriers :) The secret is to tune slowly. Try hunting for DX
with APF on at the low end of 40 or 80 m when the sun is just setting.

73,
Wayne
N6KR

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Bruce Beford-2

Re: Tips for using APF

Nice write-up on tips for using the APF function, Wayne.
Would you please consider adding this to the K3 Operating Tips page on the
Elecraft website? (and the manual, as I am sure this will be a recurring
area of inquiry...)
73,
Bruce, N1RX

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Bill W4ZV

Re: Tips for using APF

In reply to this post by wayne burdick

Very nice summary Wayne! I'd add a couple of comments:

1. I find an even wider WIDTH works better for me (400-450 Hz using a 500 Hz 8-pole). If I set WIDTH too narrow (e.g. 50-100 Hz) I get too much ambient ringing.
2. Tuning is indeed very critical. This is why I only use APF for *extremely* weak signals buried in noise. I never leave it on continuously and it would be unusable in a fast-paced activity like a contest.
3. I reduce my AF Gain from its normal setting. APF compensates with Gain of ~9 dB at its exact center so the signal will "pop up" when you get the critical tuning correct.
4. Indeed the effectiveness of APF seems to vary with the type of noise. I find it most useful in mild atmospheric noise (lightning from a long distance) and less useful in galactic or white noise.

73, Bill W4ZV

wayne burdick wrote

A couple of notes from the Elecraft lab:

1. When using APF, it's critical to tune the signal in very
accurately. (Otherwise it may seem like APF isn't working.) For best
results, I recommend using 1-Hz VFO tuning. An alternative is to
adjust the APF center pitch, which can be moved in 5-Hz increments
using the SHIFT control. Note that only 10-Hz increments are displayed.

2. There are two filter "Presets" (I and II, accessed by HOLDing the
HI/WIDTH knob). You can turn APF on for one preset and off for the
other. This control is closer to where the action is than DUAL PB, and
I find it more convenient to use.

3. You can of course hear the effect of APF by ear, but if you want to
quantify it, try using the built-in AF voltmeter on a very weak
constant carrier buried in noise. Here's the procedure I use:

a. Switch to a noisy band/antenna (80 m at night, let's say, with a
vertical or inverted-V).
b. Turn APF off.
c. Set WIDTH to 300 Hz, with no SHIFT.
d. Hunt for an extremely weak on-air carrier -- one you can just
barely detect. A carrier that's caused by a local computer or TV is
ideal because there won't be much fading during the test.
e. Tap DISP and rotate VFO B until you get to AFV (audio volt
meter). Let the reading stabilize.
f. Rotate VFO B one more tic clockwise to dBV. The number will
bounce around 0.0 by maybe +/- 1 dB. Call this the reference level.
g. Now tune well off the carrier so that you hear only noise. The
dBV number will indicate how many dB the signal has dropped. If it
drops 3 dB, your S+N/N ratio is 3 dB.
h. Tune the signal back in and make sure the level is back to
around 0.0.
i. Turn on APF. Using FINE VFO tuning (1-Hz steps), make sure the
carrier is peaked as indicated by ear or on the dBV indication.
j. Use VFO B to go back to AFV for a few seconds to capture a new
relative signal level. Then go back to dBV, which should again show
about 0.0 (bouncing a little).
k. Tune well off the carrier so you again hear only noise.
l. Note the new (negative) dB reading.

I just repeated this little test on 80 m with persistent S-6 noise.
Here's what I got:

Without APF: 1 dB S+N/N
With APF: 6 dB S+N/N

In fact, having turned APF on, I was now able to find a lot more
little carriers :) The secret is to tune slowly. Try hunting for DX
with APF on at the low end of 40 or 80 m when the sun is just setting.

73,
Wayne
N6KR

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Greg - AB7R

Re: Tips for using APF

In reply to this post by Bruce Beford-2

Bruce,

I already plan to post this there this weekend. Thanks!

73
Greg
AB7R

On Thu, Jan 27, 2011 at 1:42 AM, Bruce Beford
<[hidden email]>wrote:

> Nice write-up on tips for using the APF function, Wayne.
> Would you please consider adding this to the K3 Operating Tips page on the
> Elecraft website? (and the manual, as I am sure this will be a recurring
> area of inquiry...)
> 73,
> Bruce, N1RX
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

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Ken Chandler

Re: Tips for using APF

In reply to this post by Bruce Beford-2

And added to the IPhone K3 App please!!!

Ken..G0ORH

Sent from my iPhone

On 27 Jan 2011, at 09:42, "Bruce Beford" <[hidden email]> wrote:

Mike KS7D

Re: Tips for using APF

Absolutely! It will be in the next update, version 1.2

73,
Mike ks7d

On Jan 27, 2011, at 2:31 PM, Ken Chandler wrote:

> And added to the IPhone K3 App please!!!
>
> Ken..G0ORH
>
> Sent from my iPhone
>
>
>
>
> On 27 Jan 2011, at 09:42, "Bruce Beford" <[hidden email]> wrote:
>
>> Nice write-up on tips for using the APF function, Wayne.
>> Would you please consider adding this to the K3 Operating Tips page on the
>> Elecraft website? (and the manual, as I am sure this will be a recurring
>> area of inquiry...)
>> 73,
>> Bruce, N1RX
>>
>>
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

Jim-2

Re: Tips for using APF

In reply to this post by Ken Chandler

Ken, that iPhone app is nice isn't it?

73 de KE4WY Jim

Sent from my Starship

On Jan 27, 2011, at 2:31 PM, Ken Chandler <[hidden email]> wrote:

Ken Chandler

Re: Tips for using APF

Yes it is, an excellent App indeed.
I have all 3 Elecraft manual Apps, wouldn't be without then.
I can actually take my k3 to field day, without the worry of my mates nicking my manuals for further study in the privy!!

Ken..G0ORH

Sent from my iPhone

On 27 Jan 2011, at 20:35, Jim <[hidden email]> wrote:

> Ken, that iPhone app is nice isn't it?
>
> 73 de KE4WY Jim
>
> Sent from my Starship
>
> On Jan 27, 2011, at 2:31 PM, Ken Chandler <[hidden email]> wrote:
>
>> And added to the IPhone K3 App please!!!
>>
>> Ken..G0ORH
>>
>> Sent from my iPhone
>>
>>
>>
>>
>> On 27 Jan 2011, at 09:42, "Bruce Beford" <[hidden email]> wrote:
>>
>>> Nice write-up on tips for using the APF function, Wayne.
>>> Would you please consider adding this to the K3 Operating Tips page on the
>>> Elecraft website? (and the manual, as I am sure this will be a recurring
>>> area of inquiry...)
>>> 73,
>>> Bruce, N1RX
>>>
>>>
>>> ______________________________________________________________
>>> Elecraft mailing list
>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>>> Help: http://mailman.qth.net/mmfaq.htm
>>> Post: mailto:[hidden email]
>>>
>>> This list hosted by: http://www.qsl.net
>>> Please help support this email list: http://www.qsl.net/donate.html
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html