GB630858A - Improvements in or relating to electric wave analysers - Google Patents

Abstract

630,858. Harmonic analysis ; distortion measurement. STANDARD TELEPHONES & CABLES, Ltd., TACOBSEN, B. B., and PAINE, J. June 28, 1946, No. 19411. [Class 37] Fig. 1 shows a block schematic of an electric waveform or harmonic analyzer applicable to testing the linearity of thermionic amplifiers such as are used in a telephone transmission system. The test amplifier is connected to the analyzer in a self-oscillatory loop circuit (a) comprising a coarsely variable attenuator 1, an impedance network masking the variable input impedance of a band-pass filter 1 which feeds a phase reversing switch X, a finelyvariable attenuator 2 embodying a level stabilizing network containing a metal filament lamp, and a variable attenuator 3. The attenuators adjust the level of loop gain and regeneration. Point P in the output of attenuator 1 is connected to fundamental measuring circuit (b) including fixed attenuator 4 through switch S1 to either the matching dummy load RT1, or to the db. calibrated voltmeter VM and load RL indicating the power level at P, which is adjustable by the attenuators controlling the loop feed-back, the phase angle of which may be reversed by switch X so that oscillations are set up within the pass range of the band-pass filter 1. A distortion component measuring circuit (c) is connected at P and comprises a narrow band-pass filter 2 which accepts the desired harmonic of the fundamental oscillation frequency of the loop circuit and is tunable over a small range, a switch S2 selecting a high or low level output, a calibrated variable gain negative feed-back amplifier, and a selector switch S3 whereby the amplifier output may be alternatively connected to the voltmeter VM or to the matching dummy load RL. Similar distortion measuring circuits such as (d) tuned to other harmonics of the fundamental may be connected to point P. The power level at P may be set up to any required figure within the output capacity of the amplifier under test and measured by the voltmeter through circuit (b), the loss in which is known. Since the gain of the measuring circuit C is also known, the level of the second or other selected harmonic is calculable relative to the power level at P. Fig. 2 (not shown), gives the detailed circuit of a harmonic analyzer of the kind described, which measures the second harmonic of a fundamental frequency of 60 kc/s. The device may embody an oscillator for calibrating the distortion measuring circuits.