From SEDSWiki

FSK vs PSK

In our initial research when we tried to determine the specifications of the system, our conclusion was that BPSK would be the best modulation scheme to use due to the improved error performance for an equivalent signal to noise ratio.

However, during the design of the modulator it became apparent that FSK is significantly simpler to implement, both in the transmitter and in the receiver. For example, a simple FSK modulator is shown in the figure below. For the waveforms in the top half of the picture, the green line shows the input binary data - just a series of 1's and 0's - and blue line shows a FSK modulated signal. The circuit used to generate this is shown in the bottom half of the picture. It is very simple, just an op-amp, a switch (NMOS transistor), three resistors and two capacitors.

Similarly, a simple FSK demodulator is shown below. We just bandpass filter at the two FSK frequencies, convert the outputs of the two bandpass filters to a DC level, and then use a comparator to choose which level is higher ie. whether we have a high frequency or a low frequency tone.