[Elecraft] Tips for using APF

[Wayne Burdick]

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