Improvements in high-frequency harmonic analysers

摘要

606,310. Current and voltage measurements. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., EL SHISHINI, M., and EL SAID, M. A. H. Jan. 11, 1946, No. 1112. Addition to 585,936. [Class 37] A high-frequency wave analyser includes circuits for producing a logarithmic output from a potential wave to be analysed and from a sinusoidal oscillatory potential wave, these outputs being applied to a common load circuit so that they are added or subtracted and the resultant applied to a circuit which produces a final output having an exponential relationship to the sum or difference product, the final output being applied to an indicator. The complex wave to be analysed from a source WT, Fig. 1, is applied to a logarithmic element LET, the output from which has an instantaneous logarithmic relationship to the complex wave. A locally produced sinusoidal oscillatory wave from a source WO is fed to a second logarithmic element LEO, the output from which has an instantaneous logarithmic relationship to its input wave. The outputs from the elements LET, LEO are applied to a common load resistor CLR in which they are added linearly, the potential drop across this resistor being applied to an anti-logarithmic element ALE, the output of which has an instantaneous exponential relationship to the input. This output contains two beat frequencies which are products of the multiplication of the complex wave and the oscillatory wave from the sources WT, WO. When the frequency of the local oscillator WO is adjusted with respect to any of the component frequencies of the tested complex wave, the instrument selects and beats with this component giving low-frequency terms, the amplitudes of which are proportional to the product of the amplitudes of the local oscillator signal and the desired component frequency. The beat frequency component is selected, amplified and indicated by an A.C. instrument I. The elements LET, LEO consist of thermionic valves connected as diodes. The anti-logarithmic element ALE may include a diode polarized to operate on the exponential portion of its characteristic, or it may include a variable-mu valve. The selective amplifier, Fig. 3, consists of two sections connected in cascade. The first section includes a threeelectrode piezo-electric crystal PE1, resonant at the beat frequency, and a valve V2 having its anode circuit tuned to the beat frequency. The beat frequency output from the antilogarithmic element V1 is coupled to the grid. of the valve V2 by the crystal PE1. The second section includes a three-electrode piezo-electric crystal PE2, also resonant at the beat frequency, and a valve V3 having its anode circuit tuned to the beat frequency. The output of the valve V3 is connected to a valve voltmeter unit VM. The acorn diodes D1, D2 included in the elements LET, LEO and the variable-mu valve V1 of the element ALE may be assembled in a single housing and the connection between the local oscillator circuit WO and the valve D2 of its corresponding logarithmic element may be made through a plug-in coaxial cable CC. A calibrated variable attenuator AC is provided at the input of the amplifier and a calibrated variable-gain control GC1 is introduced into the cathode circuit of one of the variable-mu amplifiers. The local oscillator is provided with a variable-mu buffer amplifier BA having a variable-gain control GC2 which increases the output of the local oscillator when the tested signal is relatively weak and vice versa.