synchronous FSK

2L A disadvantage of a number of popular FSK (Frequency Shift Keying) modulators is that the changeover between the 1200 and 2400 Hz frequencies often occurs at unpredictable times. A much better and neater solution would be to switch frequencies when the signal is at zero. If this is done then there is no phase shift in the FSK signal. Generally speaking, this is only possible when there is a definite relationship between the data and the FSK modulator. If there isn"t then the circuit given here should be of some help. The actual FSK signal is obtained by means of the digital sinewave gener- ator described previously (see above). At every zero-crossing the sinewave generator produces sync pulses which are used to clock FF2. The data level present at the "D" input of this flip- flop determines which of the two output frequencies are selected. When a "1" is present at the input, the output frequency will be 38.4 kHz, and when a "0" is present the output frequency will be 19.2 kHz. ...