[RTTY] AFSK Tips

[Kok Chen]

On Jan 27, 2013, at 7:57 AM, Dennis wrote:

> Having read some of the issues about AFSK vs FSK, I'm thinking of changing over
> from FSK to AFSK operation.  I'll be using a K3, if that matters. 

Dennis,

By all means engage the K3 AFSK Filter that Andy K0SM/2 described in his article.  This will remove any specter of hum and noise in the audio cable.

Find AFSK TX in the CONFIG MENU and set it to FIL ON.  Remember that the filter only works when you are in AFSK-A, and not DATA-A. 

One thing to be careful of is to make sure that the software modem is putting out tones that are perfectly centered in that AFSK filter.  The AFSK filter will be centered around the tone pair which you have chosen (use the PITCH button to inspect and select the tone pair while you are in AFSK-A mode).  

If your modem tones are not centered within this filter, the AFSK signal that you put out will be harder for someone else to copy.  You want the Mark and Space tones to have the same amplitude after passing through this filter and for the filter to have no group delay.  Elecraft probably used a symmetric FIR filter here, so the group delay should not pose a problem.

In the MAIN MENU, while AFSK-A mode is selected, pick LINE IN from the MIC SEL menu.  The Line Input is controlled by the Mic gain knob.  Because of their line input transformer, Elecraft suggests using a level that is under -6 dB from a typical sound card output, so use about 50% of the sound cards full scale output. In any case, you are mostly protected from horrible things when you use AFSK filter mentioned above.

Then just make sure the "ALC meter" shows 4 to 5 bars by adjusting the Mic gain knob.  The K3 PA ALC does not kick in until you exceed 5 bars.  Before that, the meter indicates the audio drive.  If you have fewer than 3 bars, you might experience the "power creep" problem, where the K3 hunts slowly for the power that you have dialed in -- the first few characters of your transmission can be lost through the "power creep."  At 4 to 5 bars, you have no ALC in the PA and you have no power creep.

The Line Input at the rear panel of the K3 is already transformer coupled.  Best to avoid yet another transformer between the sound card and the radio.

One nice thing about using AFSK on a K3 is that you don't need a separate PTT line to key the transmitter.  You can select the VOX capability.  If you already have PTT set up,  and you are comfortable with it, just keep using PTT.

73
Chen, W7AY