GB1206161A - Power spectrum analyser - Google Patents

Abstract

1,206,161. Power spectrum analysers; cross-power analysers. SOLARTRON ELECTRONIC GROUP Ltd. Jan.12, 1968 [Jan.17, 1967], No.2489/67. Heading G1U. In a power spectrum analyser a signal x (t) is multiplied by local oscillations sin wt and cos wt (in phase quadrature) in multiplicative mixers 10, 11, the outputs being passed through low-pass filters 13, 14 to an adder circuit 18, squarer 21 and averaging circuit 19, one of the filters being connected to the adder 18 through a reversing switch 20. This switch switches the filter 14 output between positive and negative values thereof and its operating frequency is higher than the cut off frequency of the filters 13, 14, so that any product forms a (t) b(t) at the output of the squarer 21 are reduced to negligible proportions and the squarer output represents the power of x(t) at the frequency of the multiplying sine and cosine signals. The squarer and averager may comprise a thermocouple having its output connected to an RMS meter. In a cross-power spectrum analyser Fig.4 (not shown) for measuring the in-phase and quadrative power between two signals x(t), y(t), the Fig.2 circuit as far as the adder 18 is duplicated, x(t), being fed to two of the multipliers and y(t) to the other two, the outputs of the adder circuits being fed through a further multiplier (25) to an averaging circuit. For measuring in-phase cross-power the reversal switches are connected in series with those multipliers fed with in-phase multiplying signals and for measuring quadrature cross-power one of the reversal switches is connected in series with the other multiplier feeding its respective adder circuit. In a modification of the Fig. 2 circuit, the reversal switch 20 and adder 18 are replaced by a changeover switch 35, Fig.6 (not shown) which alternately connects the output of the filters 13, 14 to the squarer 21. Similar modifications may be made to the circuit of the cross-power analyser.