Patent GB-578819-A (GB)
Title: Improvements in electrical frequency meters
IPC: G01R23/02
Filed: 1944-05-01
Abstract: 578,819. Frequency measurements. GENERAL ELECTRIC CO., Ltd., and PHILLIPS, F. C. F. May 1, 1944, No. 8135. Addition to 567,571. [Class 37] In a frequency meter according to the parent Specification, in which a condenser C, Fig. 1, is alternately charged and discharged by a pair of discharge devices shown as a switch R which forms one arm of a bridge the other arms of which are fixed resistances Q, S and a variable resistance T, an indicating member operable for varying the resistance T step by step to vary the ratio of the bridge bears numerals 0 to 9 of which at least one is selectively displayed by a display member and also markings corresponding to markings on the scale of the galvanometer G and enabling the indicating member to be set to a position such that the galvanometer will indicate the difference between the input frequency and a frequency which is a multiple or sub-multiple of 10 and corresponds to the indication of the display member. Fig. 2 shows an arrangement suitable for use as the final stage of the instrument described in Specification 578,818 and adapted to measure frequencies of 0.5 to 1 Kc/s and to interpret them as belonging to the alternative ranges 0-0.5 Kc/s and 0.5-1 Kc/s. Leads S1, S2 lead from the reversing switches of that instrument and are alternatively earthed in a preceding stage according to whether inversion is called for or not. The lead M conveys a standard frequency of 1 Kc/s derived from the multivibrator supplying the preceding stage and employed for adjusting the meter. The input oscillation is applied by the lead 19. The valves V2, V3 control the charging and discharging of the condenser C6. The resistor T consists of a section of 50 ohms in series with five sections of 10 ohms and the part included in the arm of the bridge is determined by selector switches SA3, SA4 operated by an indicator dial 32. This dial bears numerals 0 ... 9 adapted to be selectively illuminated by one or other of two lamps NL, IL associated respectively with the normal and inverted conditions. The galvanometer G is provided with a network R1, R2, R3 so arranged that if a switch SA2, also operated by the dial 32, is moved by displacement of the dial from the " off " position shown, the sensitivity of the galvonometer is increased 10 times but the imput resistance of the network is unchanged. A four-position dial 33 operates switches SB1 to SB7. The circuit conditions for standardizing the galvanometer scale in terms of the standard frequency applied by the lead M are provided in the third and fourth positions of the switches SB1, SB2, SB3 and SB7. The first and second positions of all the switches SB, associated respectively with the markings " 0-0.5 kc." and " 0.5-1 kc." on the dial 33 deal with the normal and inverted conditions of indication. The switch SB7 selects the appropriate selector switch SA3 or SA4. Switches SB4, SB5 transpose the connections between the leads S1, S2 and the lamps NL, IL. To prevent a high deflection of the galvonometer during setting of the indicator dial 32, a cut-off relay A is operated from the D.C. component in the resistance R1 through a valve V4. A lead Y is connected to the resistance R1 so that when the input is above a minimum level sufficient negative bias is applied through the resistance R9 to the grid of the valve V4 to cause the release of the relay A, so establishing the H.T. supply to the meter valves through contacts A1. A condenser C7 is connected between the grid and cathode of the valve V4. If this is short-circuited the relay A reoperates quickly, but if the short-circuit is removed some time elapses before the relay A releases. The switch SA1, operated by the indicator dial 32, provides a momentary shortcircuit on the condenser C7 each time its brush moves from one contact to the next, so that the galvanometer deflection is suppressed until after the dial reaches its final position. The switch SB6 similarly controls the galvanometer when the dial 33 is operated. In operation the indicator dial 32 is set to the " off " position and one or other of the words " scale " is illuminated through contact A2 of relay A, indicating that the frequency to be measured and appearing wholly on the galvanometer scale is to be multiplied by 0.1. If a more accurate reading is required the letter on the galvanometer scale opposite the pointer is noted and the dial 32 set with the same letter opposite the reference mark 34. This results in the illumination on the dial of the first digit of the reading, whilst the remainder, shown on the galvanometer scale, is multiplied by 10 and so follows on. Specification 567,572 also is referred to.