Patent GB-831647-A (GB)
Title: Improvements in and relating to frequency measuring apparatus
IPC: G01R23/00
Filed: 1956-04-30
Abstract: 831,647. Frequency measurement. GENERAL ELECTRIC CO. April 30, 1956 [April 29, 1955], No. 13201/56. Class 37. [Also in Group XL (c)] Apparatus measuring the frequency of an input signal comprises an oscillator whose frequency is variable by a reactance valve in its tuned circuit whose anode current is in quadrature with the anode voltage, and a phase comparator developing a unidirectional voltage representing a measure of frequency deviation between the input and oscillator signals, which is applied to the control grid of the reactance valve; and a current-responsive instrument deflected by an operative current of the reactance valve as a function of the reactance thereof to indicate the frequency of the input signal. In Fig. 2, a Hartley oscillator 10 is tuned by variable capacitance 20, inductance 17 and reactance pentode 35 whose anode current and voltage are in quadrature, with its cathode positively biased to a preset value determined by voltage regulator tube 31, and its anode circuit coupled to its control grid over capacitance 30 and resistance 27, the capacitance 26 shunting the grid to earth in parallel with capacitance 25 being adjusted so that the grid voltage is in quadrature with the anode voltage; while the grid is biased to control the frequency of oscillator 15, whose output is coupled by capacitance 44 to the anodes of a pair of phase comparator diodes 40, 41 which are earthed over resistance 45; while an amplitude-modulated input signal is capacitance coupled to cathode resistance 42 of diode 41, and a load resistance 46 interconnects the anode and cathode of diode 41. The diodes operate as conventional peak rectifiers with a short charging, long discharging time constant, and develop across resistance 45 a unidirectional voltage proportional to the peak value of the vector difference of the local oscillator and input amplitude-modulated signals, while across resistance 46 a voltage appears proportional to the peak value of the local oscillator signal. The difference voltage appears across capacitance 47 and varies independently of the modulation from positive through zero to negative for phase differences between the local and input signals varying from zero through 90 degrees to 180 degrees, so that the difference voltage varies sinusoidally as the input phase difference varies through 360 degrees, and is amplified by triode 13 to vary the control grid voltage of reactance pentode 35 so that the local oscillator frequency is adjusted to equality with the input signal frequency irrespective of the modulation thereof. The difference between such adjusted frequency and that set by the oscillator tuned circuit is indicated by centre zero galvanometer 51 in series with calibration resistance 52 between the slider of potentiometer 32 and the screen of pentode 35 energized over series resistance 36. The meter reading may be calibrated as a measure of this frequency deviation, enabling the input signal frequency to be determined. Deviation outside the range of variation is indicated by an audible or visual alarm device 48 across the phase comparator output.