Patent GB-975600-A (GB)
Title: Apparatus for the determination of attenuation in waveguides
Inventor: CRAVEN GEORGE FREDERICK
IPC: G01R27/28
Filed: 1961-09-07
Abstract: 975,600. Measuring attenuation-frequency characteristics. STANDARD TELEPHONES & CABLES Ltd. July 9,1962 [Sept. 7, 1961], No. 32202/61. Heading G1U. An apparatus for the determination of attenuation in a wave-guide at different frequencies includes a means for applying a series of signal pulses of continuously increasing frequency to the waveguide, a means for automatically comparing the amplitudes of successive pulse reflections at each signal frequency and a means for recording the relative amplitudes of succeeding pulse reflections. A signal from a backward wave oscillator (BWO Fig. 1) is modulated to provide both a signal frequency source and the local oscillator frequency for an I. F. amplifier. These two frequencies are passed via a hybrid T network to a transducer which applies the signal frequency to the waveguide under test through an inductive diaphragm which ensures loose coupling to the guide. The diaphragm reflects the local oscillator frequency back to the mixer, while the power from the signal frequency, which has been applied to and extracted from the waveguide through the diaphragm, passes along the same path. The reflections are blocked from returning to the BWO by an isolator stage, and pass through the mixer, I.F. amplifier and detector to a comparator network, where two successive pulses are selected and the ratio between their amplitudes is determined. This ratio is recorded by a pen recorder. The backward wave oscillator is modulated by applying to its frequency sensitive electrode a line voltage, which as it is raised increases the oscillator frequency, this frequency being the local oscillator frequency. Short rectangular pulses of a radio frequency voltage are superimposed on the line voltage, so as to alter the oscillator frequency for the duration of each pulse to provide the signal frequency. The differerence between the local oscillator and signal frequencies must be maintained constant so that the correct I.F. is maintained for subsequent amplification, and thus the amplitude of the modulating voltage must be controlled through a circuit connected indirectly to the line voltage supply. Such a circuit may be as indicated in Fig. 2, the pulse modulating voltage being applied to the B.W.O. cathode across a resistor R4 from the anode of valve V 3 . The grid of V 3  is normally biased well below cut off, except when the modulation input is applied, but this bias is changed in accordance with the line voltage from a power supply which generates between 70'and 200 volts in accordance with an A.C. output through a transformer from the line voltage circuit. The line voltage itself varies between 700 and 8000V and is applied negatively to the other end of R4. Because of the non-linearity between the BWO frequency and the applied voltage, it must be arranged that the ratio between the modulating pulse amplitude and the line voltage increases as the BWO frequency increases, and this is brought about by the inclusion of the pentode V 1  in the biasing circuit of V 3 . Part of the bias of V 1  is derived from its anode circuit so that the bias applied to V 3  is not linearly related to the line voltage.With the further aid of the diode V 2  the desired relation may be achieved. The attenuation of the waveguide at different frequencies is determined by comparing the relative amplitudes of succeeding signal pulses in the received train. This may be done by the use of a commercial ratiometer designed to accept signals at 1000 c/s within an amplitude range 0. 3- 10 mV, if the signals from the waveguide are converted so as to be suitable for use with such a ratiometer. The pulse signals are therefore fed to two identical gating circuits (see Fig. 4), which select successive pulses in the two channels. These pulses are stretched to give approximately level D.C. voltages, which are applied to the grids of identical variable Á valve stages in series with the output of a 1 kc/s oscillator. Thus the amplitudes of the 1 kc/s outputs from the stages, which are applied to the ratiometer, are functions of the pulse amplitude inputs. An alternative method of coupling the apparatus to the waveguides is described (Fig. 5 not shown) which utilizes a fast microwave switch operated by the modulating pulse to direct the signal frequency and the local oscillator frequency along their appropriate paths.