GB690292A - Improvements in or relating to electrical frequency indicating meters - Google Patents

Abstract

690,292. Frequency measurements. PEARSON, S. O. Sept. 11, 1951 [Sept. 11, 1950], No. 22314/50. Class 37. A frequency meter comprises two circuits, both supplied from an A.C. source, the frequency of which is to be measured, and constituted so that corresponding alternating currents in the two circuits respond differently to changes of frequency, a pair of full-wave rectifiers arranged to produce two direct currents proportional to the alternating currents, and a D.C. indicating instrument having a single coil through which is directly passed the algebraic difference between the two direct currents or a proportion of that difference so as to give a reading corresponding to the frequency of the source. In the arrangement shown the A.C. whose frequency is to be measured is applied to the terminals B<SP>1</SP>, B<SP>2</SP> of the primary winding A of a transformer having two equal secondary windings A<SP>1</SP>, A<SP>2</SP>. Each secondary winding supplies a resonant circuit D<SP>1</SP>, E<SP>1</SP> or D<SP>2</SP>, E<SP>2</SP> having a rectifier bridge G<SP>1</SP> or G<SP>2</SP> in series. The outputs of the rectifiers are connected in opposition, the difference between the output currents I<SP>1</SP>, I<SP>2</SP> being applied to a D.C. instrument H. Resistances J<SP>1</SP>, J<SP>2</SP> are included to reduce the shunting effect of the rectifiers G<SP>1</SP>, G2 on the instrument H. The inductance and capacitance values are so chosen that one circuit is resonant at a frequency lower than the minimum frequency to be indicated and the other is resonant at a frequency greater than the maximum frequency to be indicated. The rectified currents I<SP>1</SP>, I<SP>2</SP> may be arranged to be equal at a standard frequency so that current then passes through the instrument H. Fre. quencies higher or lower than this produce deflections on either side of this value, and the instrument may be calibrated in terms of frequency. In a modification, the two tuned circuits are replaced by two reactive circuits, of which one has only a condenser in series with the secondary winding and the rectifier and the other has only an inductance similarly connected. Alternatively, one circuit may be reactive while the other includes only an ohmic resistance. A number of indicators H may be connected in series or parallel and situated at widely spaced points. Specification 616,325 is referred to.