GB866975A - Improvements in or relating to electric networks for deriving amplitude modulated signals from frequency modulated signals - Google Patents

Abstract

866,975. Frequency discriminators. GENERAL ELECTRIC CO. Ltd., and EASTER, B. June 25, 1957 [July 4, 1956], No. 20770/56. Class 40(5) [Also in Group XL(b)] An amplitude/frequency converting network has zero output at a real frequency and further critical frequencies defining a pass-band not including the former frequency, so designed that the output amplitude varies linearly with frequency over the said pass-band. The transfer impedance function is so selected that one factor (p-jw 2 ) produces the linear amplitude/frequency response with reference to the real frequency W 2 , and so that the remaining factor represents a maximally flat pass-band, i.e. the first three derivatives with respect to p are zero. A suitable network, as shown, with two poles in the passband, may have shunt capacities 3, 5 across its input and output formed wholly or in part of stray capacitance, its zero being formed by the parallel circuit 6, 4. One other design of network, with three poles in the pass-band, may have an additional shunt capacitor and series inductance between the circuit elements 6, 4 and 7, 5, and an inductance in place of the resistance 8. A detector across the output terminals completes the discriminator. Alternatively two of the three pole networks, each having respectively positive and negative amplitude/frequency slopes and fed by an FM signal may each feed detectors whose outputs are differentially combined. The input shunt inductance of the three pole network may be in the anode circuit of a valve, and a D.C. blocking capacitor may then be in series with the inductor 6. To compensate for the resistance of inductor 6 the capacitor 4 may be replaced by two series capacitors whose junction is connected by a resistance to earth.