Hi Simon, Richard, and all,
As Bill stated, the phase of an FT8 signal is continuous. The first
derivative of phase (i.e., the frequency) has discontinuous steps at
symbol boundaries.
The attached plot shows the average spectrum of ten simulated FT8
signals carrying different messages. Small details will be different
for different messages, but the broad outline is always the same.
The ITU occupied bandwidth containing 99% of the power is 52 Hz. Skirts
extend farther outward, of course. The signal is 65 Hz wide at -20 dB,
102 Hz at -30 dB, and 195 Hz at -40 dB.
We could, of course, apply a digital filter to the generated audio
signal to reduce power in the wings of the spectrum. The filtered
signal would no longer have a constant envelope.
-- 73, Joe, K1JT
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