ZL2AFP CMSK
MSK (Minimum Shift Keying) is very similar to PSK, but instead of changing the phase to signal the data bits, the frequency is advanced or retarded a very small amount (exactly half the symbol rate), sufficient to exactly achieve a 180° phase shift in one bit period. Because the resulting phase change is produced smoothly, without any sudden changes in phase, the signal does not require raised cosine (amplitude) modulation or other means of spectrum management. For PSK modes, such modulation must be employed to drop the output to zero at the phase change, in order to reduce the keying sidebands. The MSK spectrum is very similar to PSK, but the phase relationship between the carrier and the data is different. MSK is little used on HF, but has been widely used on LF, notably (at 100 baud and 200 baud) for DGPS beacons, and (at 50 baud and 100 baud) for VLF submarine communications.
The HUGE advantage of MSK over PSK is that because there is no amplitude information on the signal, the transmitting amplifier need not be linear. The transmitter duty cycle is always 100%, not reduced by the AM modulation. In other respects the mode is similar to PSK, and in fact the same receiver demodulator can be used, although a different means of recovering symbol sync is required.
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