Patent GB-567572-A (GB)
Title: Improvements in instruments for measuring the frequency of an electrical oscillation
IPC: G01R23/02
Filed: 1941-12-29
Abstract: 567,572. Frequency measurements. GENERAL ELECTRIC CO., Ltd., and PHILLIPS, F. C. F. Dec. 29, 1941, No. 16742. [Class 37] The frequency of an oscillation is measured by determining the digits separately by beating with a standard oscillation of appropriate frequency and passing the difference frequency to the next stage, the last digits being determined by a direct-reading frequency meter. Fig. 1 shows one stage in which the first digit of a frequency applied to a jack JA or JB is to be determined. If the frequency to be measured is between 50 and 100 kc. it is applied to the jack JA and thence to the primary of a transformer T1, the secondary of which is tuned by a condenser 2 operated by a dial 1. The secondary feeds through an amplifier 17 a detector 18 which can be fed simultaneously through an amplifier 20 from the secondary of a transformer T2, the primary of which is fed by a multi-vibrator 15. The secondary can be tuned by any one of six condensers, selected by a switch 14, so that the fifth to the tenth harmonic of the multi-vibrator can be selected and applied to the amplifier 20. The oscillations are combined with the incoming oscillations in the detector 18 and the difference frequency is passed by a lead 19 to the second stage, where the next digit is determined. If the incoming frequency lies between 10 and 50 kc. it is applied at the jack JB and beaten in a modulator 21 with a frequency of 100 kc. producing a difference frequency of between 50 and 100 kc. which is applied to the primary of the transformer T1. The tuning condenser 2 is operated by a dial 1, resonance being indicated by a tuning indicator 3. The frequency is indicated on a scale 5 against fiduciary marks 6, 4, of which the latter also co-operates with a serrated scale 7 indicating the standard frequency with which the oscillation is to beat., Alternate sections of the scale 7 are placed nearer to and further from the edge of the dial. A mask 8 is moved to that one of two positions in which the numeral on the scale 7 may be seen and moves a switch S to the corresponding one of two positions N, I. A second dial 11 is then adjusted so that the mark corresponding to that indicated on the scale 7 is opposite a mark 13. This operates the switch 14 and selects the correct frequency to bear with the incoming oscillations. The switch S controls relays C, D which cause one of four lamps NL, IL, NL<1>, IL<1> to illuminate a figure on the dial 11 giving the required digit. Succeeding digits with the exception of the last two or three are similarly determined in successive stages. The last digits may be determined by bridge means shown diagrammatically in Fig. 2 (Prov.). One arm of the bridge is formed by a device R equivalent to a condenser and a switch oscillating at the applied frequency, supplied by a source of constant E.M.F. The other arms are formed by constant resistors Q, S and the part of a resistor T between a tapping P and the junction between the resistors T, Q. A galvanometer G is in the diagonal connecting the tapping to the junction R, Q, and includes in its circuit the part, of the resistor T not forming an arm. A constant E.M.F.. V is applied across the other diagonal. The apparatus is shown in Fig. 3, the arms Q, R, S being shown as resistances. The arm T consists of a resistor of 50 ohms in series with five resistors of 10 ohms. Tapping is effected by two arms 30, 31 operated by a dial 32. The bridge is supplied, from H.T. with a voltage held constant by a discharge device N and resistor R8. The galvanometer G is associated with a network R1, R2, R3 so arranged that if a switch 33 is operated the sensitivity of the galvanometer is increased ten times, but the input resistance to the network is unchanged. The galvanometer connections are reversed by operating a relay D in the previous stage, thereby changing over contacts D3, D4. The contact D1 of the same relay selects one of two lamps NL, IL, while D2 changes the connection from one to the other of the tapping arms 30, 31. A lamp SL, operated by a switch 34 gauged with the switch 33, indicates when the reading is to be multiplied by 0.1. The first of the last three digits is indicated on the dial 32, the last two being indicated on the scale of the galvanometer G. Specification 567,571 is referred to.