Patent GB-518447-A (GB)
Title: Improvements in and relating to frequency measuring instruments
IPC: G01R23/09
Filed: 1938-09-28
Abstract: 518,447. Frequency measurements. BRITISH THOMSON-HOUSTON CO., Ltd. Sept. 29, 1938, No. 28336. Convention date, Sept. 29, 1937. [Class 37] Frequency measuring apparatus comprises a condenser which is charged to a constant voltage once per cycle and then discharged through an ammeter, unidirectional conducting means permitting this sequence and a second condenser shunted by a resistance included in the discharge circuit in order to limit the ammeter deflection at high frequencies. As shown in Fig. 1 the alternating voltage e, the frequency of which is to be measured, is applied between the grid and cathode of a triode 11 of high mutual conductance, the voltage e being suffcient to bias the valve 11 to zero on the positive half-wave and beyond cut-off on the negative half-wave. A resistor 9 limits grid current flow on positive voltage swings. The plate circuit of the valve 11 contains a resistor 12, the voltage across which is applied to the control grid 18 of a power pentode 14, the plate circuit of which includes resistances 15, 16. The voltage across the resistance 15 is applied to the grid of a triode 20 and gives sufficient negative bias to hold the valve at cut-off. The resistor 16 is connected in the circuit of a diode 21 and condenser 22 so that this condenser is charged to the voltage across the resistor 16 when current flows therein. This occurs during the period when the valve '20 is biased to cut off. Different values of the condenser 22 may be used for different frequency ranges. In series with the condenser 22 and the valve 20 is an ammeter 23 having an adjustable shunt 24. The pentode 14 is biased to cut 'off during that portion of the input cycle when a voltage exists across the resistance 12 and has zero bias for another portion of each cycle. During the period of cut-off no current flows in the resistors 15, 16 and there is no bias on the grid of the valve 20, which allows the condenser 22 to discharge through the ammeter 23, the diode 21 preventing discharge through the resistor 16. Also contained in the discharge circuit of the condenser 22 is a larger condenser 24, shunted by an adjustable resistor 26, for reducing the value of the discharge current impulses through the ammeter circuit as their number per second increases with frequency, so that the deflection of the ammeter 23 increases with frequency but tends to approach a constant maximum deflection at very high frequencies. A simplified arrangement is shown in Fig. 6 in which a power pentode 27 has its control grid connected, in series with a limiting resistor 28, across the terminals 10 to which the voltage whose frequency is to be measured is applied. This voltage must be sufficient to drive the grid from a positive value to a value beyond plate current cut off. The plate circuit includes a resistor 30 through which current flows during the portion of each cycle when the control grid of the pentode 27 is positive. Across the resistor 30 a voltage drop develops which is constant for different voltages across the terminals 10 and is nearly equal to the voltage of the battery 29. Connected across the resistor 30 is a circuit containing a condenser 31 and diode 32, through which the condenser 31 is charged to the peak voltage across the resistance 30. The discharge circuit of the condenser 31 includes an ammeter 23 with an adjustable shunt 34, a diode 35, limiting condenser 36 with its calibrating shunt 37, and the resistor 30. The diodes 32, 35 may be contained in the same envelope. The ammeter circuit may be arranged on the opposite side of the diode circuit and may be earthed.