Synchronous filter apparatus in which pass-band automatically tracks signal, useful for vibration analysis

Claim 1

APPARATUS FOR ATTENUATING ALL INPUT SIGNALS OTHER THAN THOSE OF A PREDETERMINED FREQUENCY, COMPRISING A PLURALITY OF OSCILLATORY VOLTAGE SOURCES ALL HAVING THE SAME FREQUENCY AS SAID PREDETERMINED FREQUENCY AND SHIFTED IN PHASE WITH RESPECT TO EACH OTHER, MEANS FOR SYNCHRONIZING THE FREQUENCY OF AN INPUT SIGNAL APPLIED TO SAID APPARATUS WITH SAID OSCILLATORY VOLTAGE SOURCES, A PLURALITY OF SIGNAL CHANNELS EQUAL IN NUMBER TO THE NUMBER OF OSCILLATORY VOLTAGE SOURCES, CIRCUIT MEANS IN EACH OF SAID CHANNELS RESPONSIVE TO AN ASSOCIATED ONE OF SAID OSCILLATORY VOLTAGE SOURCES AND AN INPUT SIGNAL FOR PRODUCING A DIRECT CURRENT VOLTAGE IN THAT CHANNEL, MEANS FOR MULTIPLYING SAID DIRECT CURRENT VOLTAGE IN EACH CHANNEL BY ITS ASSOCIATED OSCILLATORY VOLTAGE SOURCE, AND MEANS FOR ADDING THE OUTPUTS OF SAID MULTIPLYING MEANS TO DERIVE AN OUTPUT SIGNAL OF SAID PREDETERMINED FREQUENCY ONLY WHEN THE INPUT SIGNAL IS OF SAID PREDETERMINED FREQUENCY.