Frequency discriminating circuit

A frequency discriminating circuit comprising:

• a. an input circuit;
• b. an amplitude detecting circuit;
• and
• c. a double tuned circuit connected between said input circuit and said amplitude detecting circuit, said double tuned circuit including,
• i. a transformer having a primary winding and a secondary winding,
• ii. a first variable inductance element connected in series to said primary winding and magnetically shielded from said transformer,
• iii. a first capacitor connected in parallel with a first series connection of said primary winding and said first variable inductance element to form a first resonant circuit together with said first series connection,
• iv. a second variable inductance element connected to one end of said secondary winding, said second variable inductance element being shielded magnetically from said transformer,
• v. a third variable inductance element connected to the other end of said secondary winding, said third variable inductance element being magnetically shielded from said transformer,
• vi. a second capacitor connected in parallel with a second series connection of said secondary winding, said second and third variable inductance elements forming a second resonant circuit together with said second series connection, said second resonant circuit having a quality factor larger than that of said first resonant circuit,
• vii. means for adding an input signal and two signals of opposite phases appearing at the both ends of said secondary winding, and
• viii. means for applying said two added signals to said amplitude detecting circuit.