Coherent frequency shift key demodulator
OwnerJohn W. McCorkleinventor
Life of this patent
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Abstract
Disclosed is a frequency shift key demodulator for the coherent detection of both mark and space tones. Cross coupled phase locked loops lock onto and track the difference in frequency between translated mark and space tones. An additional set of cross coupled phase locked loops locks onto and tracks overall frequency drift of the mark and space tones. Signals are generated within the first set of cross coupled loops allowing for coherent detection thereby permitting detection during signal to noise ratios of less than one. Loop bandwidths can be independently controlled and are independent of the data rate thus permitting wide system flexibility.
Claims10
A frequency shift key detector for demodulating mark and space tone frequency shifts from a carrier frequency comprising:
- first phase locked loop means for locking onto and tracking the difference in frequency between said mark and space tones, said first phase locked loop means including:
- a first space tone sub-loop comprising:
- first phase detector means for receiving space tones and comparing them with a space tone loop signal;
- a first summer having multiple inputs, one positive input of which is coupled to the output of said first phase detector;
- first filter means coupled to the output of said first summer;
- a first voltage controlled oscillator having its control input coupled to the output of said first filter means;
- and
- first divider means coupled to the output of said first voltage controlled oscillator and providing said space tone loop signal coupled to said first phase detector means;
- a first mark tone sub-loop sharing in common with said first space tone sub-loop said first summer means, said first filter means, and said first voltage controlled oscillator and in addition to said elements shared in common further including:
- second divider means coupled to the output of said first voltage controlled oscillator for generating a mark tone loop signal;
- and
- second phase detector means for receiving mark tones and comparing them with said mark tone loop signal, wherein the output of said second phase detector means is coupled to a negative input of said first summer;
- second phase locked loop means for locking onto and tracking the overall frequency drift of said mark and space tones, said second phase locked loop means including:
- a second space tone sub-loop sharing said first phase detector means with said first space tone sub-loop and further comprising:
- a second summer having multiple inputs, wherein one positive input is coupled to the output of said first phase detector means and another positive input is coupled to the output of said second phase detector means;
- a second filter means coupled to the output of said second summer;
- a second voltage controlled oscillator having its control input coupled to the output of said second filter means;
- and
- a mixer having a first input for receiving said carrier, mark and space frequencies and a second input for receiving an input mix signal and an output providing frequency translated mark and space tones, wherein said second input of said mixer is coupled to the output of said second voltage controlled oscillator and wherein the output of said mixer is coupled to said first phase detector for supplying said mark and space tones thereto;
- a second mark tone sub-loop, sharing in common with said second space tone sub-loop said mixer, said second summer, said second filter means, and said second voltage controlled oscillator and sharing in common with said first mark tone sub-loop said second phase detector means wherein said second phase detector means is coupled to the output of said mixer.
Dependencies are inferred from the claim text.