Demonstration of sliding frequency changes
A fun and useful demonstration of the capabilities of QDX and the Input Analysis application, is to
investigate the FT8 tone change transition. We know according to the WSJT-X documentation that
to avoid splatter onto adjacent frequencies, WSJT-X makes a smooth transition between one tone
symbol and the next. It’s nice to be able to check this using the Input Analysis screen.
Here the screen width is 80Hz so each horizontal column is a 1 Hz bucket. The FT8 symbol
change event is 2 tones (a frequency change of 12.5 Hz since the FT8 tone spacing is 6.25 Hz.
The scroll rate is sped up the fastest rate, 10 ms per horizontal row (100 frequency measurements
per second).
We see clearly that at the moment the FT8 tone changes, the measured frequency smoothly
changes in the expected way from one tone to the next.
Note that an even smoother transition could be made, by setting the “Minimum samples”
parameter to 240 samples – this would result in slightly lower accuracy measurement but a
smoother transition in tones.
We’re talking here about tradeoffs in different aspects of performance; either way the performance
is excellent and QDX keeps up with the changes in input frequency! So this is a nice confirmation
of the excellent performance characteristics of QDX.
QDX operating manual; firmware 1_04
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