Remember, the task is to generate a frequency of 3.5999MHz, using cheap and available components.
And here it is:
- generate 6.400MHz
- generate 8.000MHz
- mix! => 14.400MHz & 1.600MHz
- filter ...
- divide by four (FlipFlops) => 14.400MHz/4=3.600MHz
- filter ...
FSK by pulling on one of the crystals.
In my design, I would probably pull one crystal for modulation and the other for the TX frequency.
Now, that was not that hard, or was it?