GB534577A - Improvements in or relating to electric measuring devices - Google Patents

Abstract

534,577. Telephone and like transmission measurements ; oscillographs. STANDARD TELEPHONES & CABLES, Ltd. (Brandt W.). Sept. 8, 1939, No. 25429 [Class 37] Apparatus for obtaining a frequency transmission quality curve of a circuit comprises means for applying a voltage proportional to the output of the circuit to produce a displacement of an indicating point in one co-ordinate, means for producing from the output of the circuit a voltage proportional to the frequency of the output and independent of its amplitude and for producing from this last mentioned voltage a displacement of the indicating point in the other co-ordinate. A current of a frequency continuously varied throughout the required range and of constant amplitude is produced by means of a modulator M. composed of dry contact rectifiers connected as shown. Such modulators have the property that the side band output is substantially independent of the carrier frequency input provided that this is several times larger than the modulating frequency input. The modulating frequency for the modulator M is derived from an oscillator OSC2 of fixed frequency and high stability, whilst the carrier frequency input is derived from a variable frequency oscillator OSC1. A constant amplitude of variable frequency is obtained at the output of the modulator M. The upper side band is suppressed by a low pass filter LPF and the lower side band is passed through an amplifier AMP1 to the circuit X under test, which is assumed to be the receiving terminal of a carrier system and thus to include a de-modulator. The output, which is the voice frequency band, is passed through an amplifier AMP2 and part thereof is taken through a transformer TR1. rectified by a rectifier R1 with a smoothing condenser C1 across its output, and applied to the vertical plates, PY of a cathode-ray oscillograph CRO. A potentiometer P1 across a DC source allows the spot on the screen to be adjusted to a central position and also allows the scale to be adjusted. Another part of the output of the amplifier AMP2 is taken through a transformer TR2 and applied to a modulator DM as a carrier frequency. The modulating frequency is supplied by a fixed oscillator OSC3 and is such as to give an upper side band of the original frequency given by the oscillator OSC1. This upper side band is selected by a band pass filter BPF. The voltage of the variable carrier frequency appliedto the de-modulator is arranged to be sufficiently high compared with that supplied by the local oscillator OSC3 that the output level of the de-modulator is constant and only dependent on the oscillator OSC3. The produced band of changing frequencies but of constant level is sent into a network N having a different attenuation for each frequency and the output from this network is rectified by a rectifier R2 and applied to the second pair of plates PX of the cathode-ray oscillograph CRO. By means of a potentiometer P2 and a D.C. source opposed to the rectifier D.C. voltage the spot is adjusted to be central for the midband frequency. Horizontal deflection of the oscillograph spot takes place in accordance with the changing frequency arriving from the circuit X under test, while its vertical deflection indicates the corresponding transmission characteristic. Specifications 503,893 and 510,714 are referred to.