The Demod page

Tuning the Demodulator should be the second task you perform. This is a subjective process, so be warned! There is a way to do it by using the output from the XR2211 FSK demodulator, but I'm still trying to figure out how to do it.

To do this procedure, you will need to find a strong, stable RTTY signal on the air. I suggest you attempt this during a contest weekend, or you can use the ARRL RTTY bulletins, or a maritime station such as FUF which runs RYRY test tapes almost constantly.

The first step is to find the station and tune it in by using an audio spectrum analyzer such as Freq v5.1. Tune it so that the low tone occurs at 2125 kHz. At this point close the analyzer program and run your TNC software. At this point text should be scrolling across your screen.

Now turn the trimpot R113 counterclockwise. This trimpot is the far right pot in the row of four located above the eight modulator trimpots.

At some point, the received signal will begin to show errors and then stop. Do not continue turning the pot counterclockwise. Make a note of the pots position. Now turn the pot clockwise, paying attention to the exact number of turns. You should see the text begin to scroll, become intelligible, begin getting errors, then stop again. Do not continue past this point.

Now, how many turns did you make? Take that number and divide by 2. Now turn the pot counterclockwise that amount of turns. The text should be clear again. Your demodulator is now properly aligned. (As a side note, it took my pot 3 1/2 turns to lose the signal--which mean I had to turn the pot 1 3/4 turns counterclockwise to regain the signal.)

Now what did we do? In a nutshell the FSK demodulator is a Phase Locked Loop (PLL) demodulator. What we have done is forced the free running PLL frequency from a "locked" to "unlocked" state at both extremes of its operating range, and taken the average. This should (I think!) place the free running PLL frequency at 2210 Hz...midway between the "Mark" and "Space" tones.