Patent GB-672509-A (GB)
Title: Improvements in and relating to frequency reponse systems
IPC: H03L7/04
Filed: 1950-07-18
Abstract: 672,509. Frequency measurements. BRITISH THOMSON-HOUSTON CO., Ltd. July 18, 1950 [July 26, 1949], No. 17946/50. Class 37. [Also in Groups XXXV and XL (c)] A frequency response system comprises a source of standing waves, and adjustable probe means 'coupled with the source for detecting a change in frequency of the energy causing the standing waves, a change in frequency of the energy causing a shift of the standing wave pattern with respect to the probe means. A tubular conductor 8, Fig. 2, is terminated at its ends by conductors 9, 10. A conductive tuning member 11 with fingers 12 is movable along the tube 8 by a screw 13. An impedance member 10a having a wave impedance equal to the surge impedance of the tube 8 is located at a quarter of the wavelength at the centre frequency of operation from the end conductor 10 to prevent reflection from this end. Energy is coupled into the tube 8, from a source which varies in frequency, by a section of transmission line 15 terminated in a loop 16. A shift in the standing wave pattern along the tube 8 caused by a change in frequency of a signal supplied through the loop 16 is detected by probes 29, 30 carried by stub transmission lines 19, 20. These are formed with flanges 27, 28 which are slidably held between members 25, 26 and the outer surface of the tube 8. Connected across the stub transmission lines 19, 20 are output circuits comprising transmission lines having tubular outer conductors 37, 38 and inner conductors 39, 40. Crystal rectifiers 46, 50 are connected between the inner conductors 39, 40 and conductors 57, 58, which are by-passed to earth by capacitors comprising plates 47, 59 and dielectric 61 and plates 48, 60 and dielectric 62. Load resistors 53, 54 and filter capacitors 55, 56 are connected across the output circuits. The output of the frequency response system is taken between the conductors 57, 58, the distance apart of the probes 29, 30 and the position of the conductor 11 being adjusted so that at the centre frequency the voltage between the points 57, 58 is zero. If the frequency varies a unidirectional voltage is developed between these points, the polarity and magnitude depending upon the direction and magnitude of the frequency departure. In a modification, Fig. 3, the conductors 8, 8a comprise a short section of coaxial transmission line into which the probes 29, 30 and the coupling loop 16 project. At one end of this section a length of flexible coaxial transmission line 62 of low loss shorted at the far end is attached, while at the other end a length of lossy coaxial transmission line 64 is connected. The flexible transmission lines 62, 64 are coiled up to save space. The chamber comprising tube 8, Fig. 2, or members 8, 8a, Fig. 3, may be used as mixing chambers for signals of different frequencies. Fig. 4 shows a frequency standard having means for controlling its temperature in order to maintain the dimensions of the apparatus constant so that a uniform response is obtained for a given frequency. The section of wave-guide between the probes 29, 30 and the short-circuited end is rolled into a spiral, forming a block 66. The apparatus is enclosed in a container 67 and is maintained in a fixed position by supporting blocks 75a, 75b, 75c, 75d, 75e. The temperature of the wave-guide and the atmosphere inside the container 67 is maintained constant by means of a heater 68, thermostat 69, and electric supply source 74: A D.C. voltmeter 75 is connected between the points 57, 58 and may be calibrated to indicate the extent of frequency departure from a standard.