Apparatus for comparing the amplitude of two signals of different frequencies

Claim 1

IN AN I.L.S. SYSTEM IN WHICH TWO SIGNALS OF DIFFERENT FREQUENCIES ARE RADIATED AS MODULATIONS ON A COMMON CARRIER, A RECEIVER FOR COMPARING THE DEPTHS OF MODULATION OF THE TWO SIGNALS COMPRISING MEANS FOR RECEIVING AND DETECTING THE RADIATED SIGNALS TO PRODUCE A COMBINED INPUT SIGNAL CONTAINING THE TWO MODULATION FREQUENCIES, MEANS FOR DERIVING, FROM SAID INPUT SIGNAL, A FIRST REFERENCE SIGNAL OF A FREQUENCY EQUAL TO THE MEAN OF THE MODULATION FREQUENCIES, MEANS FOR DERIVING FROM SAID INPUT SIGNAL, A SECOND REFERENCE SIGNAL OF A FREQUENCY EQUAL TO HALF THE DIFFERENCE OF THE TWO MODULATION FREQUENCIES, A BALANCED MODULATOR FOR AMPLITUDE MODULATING THE FIRST REFERENCE SIGNAL WITH THE COMBINED INPUT SIGNAL, AND SYNCHRONOUS DETECTING MEANS CONTROLLED BY SAID SECOND REFERENCE SIGNAL FOR DETECTING THE OUTPUT FROM THE BALANCED MODULATOR AT A FREQUENCY EQUAL TO HALF THE DIFFERENCE BETWEEN THE MODULATION FREQUENCIES.