Unlike ordinary ASK or OOK transmitter, which only has one frequency, also known as frequency where peak transmitting power is achieved. FSK transmitter has two frequencies, one lower frequency and one slightly higher frequency.
In hardware programming such as Si4010 chip, the frequency set is actually the higher frequency – not the center frequency, which only exist in theory.
And the lower frequency is determined by factor called FSK deviation, which in setting should be higher freq. – center freq. or ( higher freq. – lower freq. ) / 2
So if we need to use FSK transmitter Si4010 to set transmitting frequency using 433.87 and 433.97, then the settings would be as follows.
FSK freq. equals 433.97MHz – the higher frequency.
FSK deviation equals 50kHz – half of actual low-to-high deviation.
And better check and verify the actual transmitting frequency and power with a spectrum analyzer to confirm.