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 > > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > > -- View this message in context: http://elecraft.365791.n2.nabble.com/Tips-for-using-APF-tp5964773p5965975.html Sent from the [K3] mailing list archive at Nabble.com. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
