Monday, September 24, 2012

PSK vs. FSK/MSK

In thinking about BPSK vs. FSK or more accurately, MSK (Minimum Shift Keying) I found myself pondering whether they are functionally equivalent.

MSK is basically FSK with the shift set to ½ the baud rate.  Realistically this is the smallest shift you can use without trading off transmission speed.  At 31.25 baud in MSK your keying frequency shifts are 7.8125 either side of your transmit center frequency.  If you think of the phase of a carrier that is 7.8125 below the transmit center frequency, it will lag by 90 degrees after 32  mS, and at 7.1825 above, it will lead by 90 degrees after 32 mS, so MSK appears to be functionally equivalent to BPSK while using +90 and -90 degree shifts instead of 0 and 180.

I like MSK/FSK when it comes to transmittter design as there is no amplitude modulated component to the signal, greatly simplifying transmitter design as non-linear techniques can be utilized.  I wonder however if there is a price to be paid on the receiver side when trying to decode the signal at low signal levels and in trying to keep simple receiver designs locked onto the signal.

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