Patent GB-616157-A (GB)
Title: Improvements in and relating to testing equipment for electrical lines and apparatus
IPC: G01R27/32
Filed: 1945-02-14
Abstract: 616,157. Telephone and like transmission measurements; current, voltage, and impedance measurements. BRITISH BROADCASTING CORPORATION, RANTZEN, H. B., RENDALL, A. R. A., and PEACHEY, F. A. Feb. 14, 1945, No. 3747. [Class 37] [Also in Group XL (c)] Apparatus for testing electrical equipment such as transmission lines and apparatus comprises means for generating a finite plurality of sinusoidal components of fixed frequencies and predetermined constant amplitudes distributed over a relatively wide band of acoustic frequencies, means for applying to equipment to be tested a spectrum formed by combining these sinusoidal components, and frequency selective means for measuring the amplitude of each sinusoidal component independently after transmission through the equipment, and for measuring intermodulation components in a relatively narrow part of the spectrum containing none of the sinusoidal components. The spectrum may comprise frequencies of 50, 100, 200, 400, 600, 800, 1500, 2500, 3500, 4500, 5500, and 6500 cycles per second. A band from 1000 to 1200 cycles per second is used for measuring intermodulation components, the band from 800 to 1500 cycles per second being left free for this purpose, summation products being produced from frequencies below and difference products from those above this band. The spectrum may be produced from the mains frequency by using a saturated iron core reactor or alternatively may be generated directly by means of a rotating template cut to the required contour. Such a template, may be used to generate oscillations by placing it in the path of a beam of light falling on a photo-electric cell or by using it as one plate of a condenser whose capacity is varied in accordance with its contour. A separate sinusoidal template may be used for each component, each such template being used to generate an oscillation. All these oscillations are mixed and applied to produce a photographic marking on the complex template, which is then cut to the shape indicated by the marking. Alternatively the separate sinusoidal templates may be mounted on a common shaft and used to generate the spectrum directly. As shown in Fig. 1, the output from a circuit 10 under test, and through which a spectrum of oscillations is transmitted, is applied to a calibrated local oscillator 11 so that the frequency of all the components in turn is changed to an intermediate frequency lying outside the spectrum. As each component is brought to this intermediate frequency the gain of an intermediate frequency amplifier 12 is adjusted to bring the indication of a meter 13 in the output to a standard value. A gain control 14 is calibrated so that the setting for a standard reading is a measure of the attenuation of the circuit 10, the linearity of which may thus be determined. In an alternative arrangement, Fig. 2, a series of filters 15, 16, 17 are employed. Each filter passes one of the sinusoidal components and one passes the signals in the band 1000-1200 cycles per second. The outputs of the filters are applied in turn by means of a switch 18 to a variable gain amplifier 19 having a meter 20 in its output circuit. In the modification shown in Fig. 3, the spectrum is divided into two bands which are successively applied to the input of the equipment. The bands are produced by two generators 21, 22 and are fed alternately to the input of the equipment 10 under test by means of a switch 23. High and low-pass filters 24, 25, each having a cut-off at about 900 cycles per second, are connected to the output of the equipment 10. A relay having an operating coil 26 is connected to the output of the filter 24 through a rectifier 27 and is operated when the lower band is transmitted through the circuit 10. When the upper band is in use the relay is in the unoperated condition shown and the output of the high pass filter 25 is fed through contacts 28, 29 to a measuring device 30 for measuring the individual sinusoidal components. At the same time the output of the low-pass filter 24 is connected through relay contacts 31, 32 to a measuring device 33 for measuring the intermodulation components. When the lower band is in use the relay is operated and the output of the filter 24 is connected through contacts 34, 29 to the measuring device 30 and the output of the filter 25 is connected through contacts 35, 32 to the measuring device 33. Specification 471,632 is referred to.