The bit period in the baseband signal is T = 1/200000 seconds, and thebaseband pulses are rectangular. The bandwidth of the baseband signal (to thefirst null) is given by B = 1/(2T ). For the RF components, 2 d f = 150 kHz.The bandwidth of the FSK signal is therefore2 d f + 2B = 150 kHz + 200 kHz = 350 kHz.
When only a short time duration dT is available, the uncertainty theorem puts a constraint on the accuracy of recognizing the difference between the two frequencies df. dF x dT >= 2 x PI();For example, the telephone channel is less than 4 KHz wide, and so a rather large separation would be df x 2 KHz. This implies that telephone line FSK information transfer rates will not exceed around 300 b/s.