Patent GB-552683-A (GB)
Title: Improvements relating to electrical frequency measuring or discriminating apparatus
IPC: G01R23/00
Filed: 1941-10-16
Abstract: 552,683. Frequency and capacitance measurements. STERLING, Sir L. Oct. 16, 1941, No. 13319. Convention date, Oct. 19, 1940. [Class 37] [Also in Group XL] Apparatus for indicating deviations of either the frequency of an alternating voltage or the resonant frequency of a network from a predetermined value comprises a valve having one control grid influenced by the frequency to be examined and another control grid connected to a resonant impedance, the steady anode current being compensated and an instrument in the anode circuit indicating change of anode current consequent on frequency change. An input voltage of predetermined frequency is applied from terminals a, b, Fig. 1, between the first control grid 12 and the cathode 11 of a hexode 10. A steady positive potential is applied to the screen 14 through a resistance 20, the screen grid being byepassed to earth for A.C. by a condenser 21. The second control grid 13 is connected to earth through a piezo-electric crystal 17 byepassed for D.C. by a resistor 18. A load resistor 22 is connected in the circuit of the anode 15, which is byepassed to earth for A.C. by a condenser 23. The crystal 17 is ground to the resonant frequency of the input voltage or a harmonic thereof. If the input potential is of the same frequency as the resonant frequency of the crystal 17 the voltages on the grids. 12, 13 will be in quadrature. Over a narrow band of frequencies about the resonant frequency of the crystal the phase of the voltage developed on the grid 13 varies rapidly and substantially linearly in either direction from its normal quadrature position with change of input frequency and causes a corresponding change in the anode current. The steady anode current of the valve 10 and also changes due to power supply fluctuations are balanced out by a similar valve 10<SP>1</SP> having an anode load resistance 221 equal to the resistance 22 and adjusted by means of a resistor 27 so that the anode current is equal to that of the valve 10. Hence the potential at the anode 15<SP>1</SP> of the valve 101 is the same as'that of the anode 15 of the valve 10 when no input voltage is applied. A centre-zero voltmeter 25 indicates zero under these conditions and also when an applied voltage at the terminals a, b is of the same frequency as the crystal 17. Any small deviation of this frequency causes a corresponding change in the meter deflection to one side or the other from the zero position. Fig. 2 shows another arrangement for balancing out the steady anode curent. The valves 10, 10<SP>1</SP> are connected in push-pull and the voltage across a, b is applied through a transformer 28 to the grids 12, 13<SP>1</SP> in opposite phase. The steady anode current of the valves 10, 10<SP>1</SP> is balanced by adjusting the variable screen potential by means of a resistor 20<SP>1</SP>. In modifications the steady anode current is balanced out by means of the screen grid current or by passing an equal and opposite current through the meter 25. Fig. 5 shows an arrangement for measuring the capacitance connected across terminals x. The signal source is a triode oscillator 30 of the tuned anode type, the tank circuit of which comprises a calibrated condenser 31, a zero-adjusting condenser 32, and an auxiliary trimmer condenser 33. With the condensers 31, 32, 33 set to their maximum values the oscillator is tuned to the frequency of the quartz crystal 17 or 17<SP>1</SP>. The output of the oscillator is fed to a buffer amplifier stage 34 having a tuned output circuit 35. The positive terminal of the oscillator power supply battery 36 is earthed to remove any D.C. potential difference between the shield or chassis 37 and the unknown condenser. The output from the amplifier 34 is fed to a frequency deviation meter having two crystals 17, 171 of different sensitivities, either of which may be connected by means of a switch 38. The meter 25 is connected between the anodes 15, 151 in series with a resistor 26, a portion of which may be disconnected by a switch 39 ganged with the switch 38 to change the sensitivity. The unknown condenser is connected between the terminals x after the system has been balanced, and the capacity of the condenser 31 is decreased until zero reading is again obtained. The condenser 31 is calibrated reversely so that it reads the unknown capacity directly. The vernier condenser 32 is similarly calibrated. In a modification, Fig. 6, the crystal is replaced by a resonant circuit comprising an inductance 44 and condenser 45, the coil being shielded by a metallic screen 46. The resonant frequency of the circuit 44, 45 is varied and the input frequency, supplied by a crystal oscillator, is held at a fixed value. The condenser to be tested is connected between the terminals c, d across the tuned circuit 44, 45. The steady anode current is balanced by placing the load resistance 22 between the anode 15 and earth and connecting the H.T. supply and one terminal of the meter 25 so that a compensating reverse current flows through the meter. The crystal oscillator may be incorporated in the valve 10. A resistance may be included in the anode circuit to develop a frequency discriminating voltage for use in automatic frequency or tuning control, demodulating a frequency modulated radio signal &c.