Patent GB-623025-A (GB)
Title: Improvements in or relating to electric frequency measuring circuits
IPC: G01R23/09
Filed: 1946-06-20
Abstract: 623,025. Frequency measurements. STANDARD TELEPHONES & CABLES, Ltd., and DUNN, R. H. June 20, 1946, No. 18556. [Class 37] A frequency meter comprises means for comparing the frequency of an A.C. to be measured, or a frequency proportional thereto, with a standard frequency, means for producing impulses at a repetition rate equal to or proportional to the difference between the frequency of the A.C. to be measured and the standard frequency means for translating the repetition rate of the impulses into a quantity proportional thereto, and means for measuring that quantity. Fig. 1 shows a circuit in which impulses are produced of a lower repetition frequency than that of the supply system to be measured, but proportional thereto. The supply frequency 4 and a standard frequency 5 are applied to comparing means 6 to produce a beat frequency 7, which is converted into impulses by a relay in an impulser 8 and applied to an integrator 9, the output of which is measured by a meter 10. Fig. 2 shows a modification for measuring at a central station 14 the frequency of the supply at a substation 12. The supply circuit 11 is applied through a communication circuit 13 to comparing means 16 together with a standard frequency 5 to produce a beat frequency 7, which is converted into impulses by a relay, in an impulser 8. These are applied to an integrator 9, the output of which is measured by a meter 10. Where no communication circuit capable of transmitting the supply frequency is available a circuit for transmitting impulses of frequencies below the audible range may be used. Impulses are derived at the substation from the supply frequency at repetition rates below the audible range, transmitted to the central station, and applied to an integrator directly or they may be compared with a standard frequency to produce impulses of a still lower beat frequency which are applied to an integrator. In the arrangement shown in Fig. 5, two standard frequencies 16, 17, one -above and one below the normal frequency to be measured, are compared with the reduced supply frequency in separate comparers 19, 20, the outputs of which are applied to separate intogrators 21, 22. The outputs of these are measured by meters 23, 24, and are also combined and passed through one winding of a relay AR, the other winding of which is energized by a potential 25 through an impedance 26. When the relay AR is operated an alarm 27 is energized over contacts ar1. An additional relay may short-circuit or disconnect one of the two meters in order to prevent ambiguity as to the direction of the variation in frequency and separate " high " and " low " alarms may also be operated. Fig. 8 shows an arrangement for reducing the frequency of the supply and producing impulses at the lower frequency. The supply circuit 30 is connected through a transformer 31 to a rectifier 32 and resistance 33, the halfwave rectified voltage across which is passed to a differentiation circuit comprising a condenser 34 and resistance 35, producing sharp positive and negative peaks. The negative peaks are removed by a rectifying circuit 36, 37, so as to produce short positive pulses, one per cycle of the supply frequency. These pulses are applied to a discharge tube counter 38 in which the firing of one tube primes the adjoining tube by raising the voltage of a control electrode to such a value that the next pulse causes a discharge in the control gap and causes the main gap to fire. The tubes fire in sequence, the first tube being primed on the firing of the last. The anodes of one half of the tubes are connected to one winding of a polarised relay 1R and those of the other half are connected to the other winding. The relay contacts er1 connect a source of potential to output terminals 39 when the relay is operated in one direction and disconnect it or alter its polarity when operated in the other direction, so that one impulse is delivered to the terminals 39 in every complete firing cycle of the tubes, i.e. the frequency of the impulses is onesixth that of the supply. Fig. 10 shows means for obtaining a standard frequency, or two standard frequencies, from the standard Post Office 1000 c.p.s. signal. This is amplified in an amplifier 44, converted to square wave-form in a squarer 45, then passed through a differentiator and rectifier 46, from which the remaining positive peaks are passed through high-speed counters 47, 48, which reduce the frequency 16 times. A counter 49 divides the frequency by 8 and a second counter 50 divides the frequency by 7, so that two different standard frequencies are obtained. Fig. 11 shows a modulating circuit for use as the comparing means 6, Figs. 2 and 3. The supply 53 is connected to the primary of a transformer 54, the secondary of which is connected through rectifiers 55, 56 to the two windings of a relay RL. This is bridged by a condenser 57 and the junction of the windings is connected through one winding of a transformer 58 to a tapping on the secondary of the transformer 54. Another winding of the transformer 58 is connected to the standard frequency supply 59. The contacts rl1 of the relay RL pass impulses to an integrator. In the arrangement shown in Fig. 12 the supply frequency is applied through a transformer 53 to a rectifier circuit comprising a rectifier 60 and resistance 61 and the rectified half-waves are applied through a resistance 62 to the control electrodes of a gaseous discharge tube 63. In the anode circuit is a self-extinguishing circuit consisting of a condenser 65 and resistance 66. The positive peaks of the supply current cause the control electrodes to fire and ionise the main gap of the tube 63 which also fires and immediately extinguishes itself due to the voltage drop in the resistance peak to be applied to a resistance 67 through a transformer 64, and thence through a condenser 68 to the control electrodes of two gaseous discharge tubes 69, 70. The anodes of these tubes are supplied from the standard frequency 58 applied to the primary of a transformer 71, the secondary of which is centre-tapped and the halves connected to half-wave rectifiers formed of rectifiers 74, 75 and resistances 72, 73. These are connected to the anodes of the tubes 69, 70 through resistances 76, 77 and the windings of a balanced relay RL, which are bridged by condensers 78, 79. Each peak voltage from the transformer 64 fires the control gaps and ionises the tubes 69, 70 so that a discharge takes place across the main gaps when adequate potential is present. Anode potential is only applied to the tubes 69, 70 during peaks of the standard frequency 58, which is rectified so as to provide anode potential to one tube on the positive half-cycle and to the other tube on the negative halfcycle. The relay RL operates in step with the beat frequency between the supply frequency 53 and the standard frequency 58. Specifications 577,961, 629,245 and 629,246, [Group XIX], are referred to.