Coherent frequency difference detector having direct-current output independent of modulation

Claim 1

A COHERENT FREQUENCY DIFFERENCE DETECTOR CIRCUIT COMPRISING, A SIGNAL INPUT CIRCUIT FOR RECEIVING FREQUENCY-MODULATED SIGNAL, A SOURCE OF REFERENCE FREQUENCY FOR DEVELOPING FIRST AND SECOND REFERENCE SIGNALS DIFFERING IN PHASE BY 90 DEGREES, MEANS FOR MIXING SAID INPUT SIGNAL WITH SAID FIRST AND WITH SAID SECOND REFERENCE SIGNALS SEPARATELY TO DEVELOP FIRST AND SECOND BEAT FREQUENCIES RESPECTIVELY, FIRST AND SECOND DIFFERENTIATINIG MEANS FOR DIFFERENTIATED SAID FIRST AND SECOND BEAT FREQUENCIES RESPECTIVELY, MEANS FOR MIXING SAID FIRST BEAT FREQUENCY AND SAID DIFFERENTIATED SECOND BEAT FREQUENCY TO DEVELOP A THIRD BEAT FREQUENCY, MEANS FOR MIXING SAID SECOND BEAT FREQUENCY AND SAID DIFFERENTIATED FIRST BEAT FREQUENCY TO DEVELOP A FOURTH BEAT FREQUENCY, AND SUBTRACTING MEANS HAVING AN OUTPUT CIRCUIT FOR SUBTRACTING SAID THIRD AND FOURTH BEAT FREQUENCIES AND FOR FILTERING THE DIFFERENCE THEREOF TO PRODUCE IN SAID OUTPUT CIRCUIT A DIRECT-CURRENT OUTPUT CONTROL VOTLAGE HAVING AN AMPLITUDE DEPENDENT UPON THE FREQUENCIES OF THE CARRIER OF THE SIGNAL APPLIED TO SAID INPUT RELATIVE TO THE FREQUENCY OF SAID SOURCE BUT INDEPENDENT OF THE FREQUENCY MODULATION OF SAID CARRIER.