Patent GB-752904-A (GB)
Title: Improvements in or relating to devices for measuring electric power
IPC: G01R21/00
Filed: 1954-02-17
Abstract: 752,904. Voltage, current and power measurement. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Feb. 17, 1954 [Feb. 20, 1953], No. 4643/54. Class 37. A device for measuring electric power, e.g. at high frequency, comprises a self-oscillating circuit wherein a bolometer resistance to which is supplied the power to be measured, damps the oscillations to an extent dependent on such power to correspondingly control the amplitude of the output of the self-oscillatory circuit, which is applied to a second bolometer resistance in a second similar self-oscillating circuit whose output voltage operates an instrument indicating the power supplied to the first bolometer resistance. In Fig. 3 a retroactive triode oscillator 8 generates an output signal at e.g. 20 kc/s. whose amplitude is controlled by a bolometer resistor 1 in series (or alternatively in parallel) with the tuning capacitance which is heated by the oscillatory currents therein to a value at which oscillations are just maintained at a constant level, and the bolometer temperature approximates to the ambient. The dissipation of H.F. power from an external source in the bolometer resistance proportionately raises its temperature to vary the resistance and diminish the oscillation amplitude to a level which is a measure of the H.F. power and at which equilibrium again prevails between the bolometer and ambient temperatures. The oscillator output is connected to the control grid of triode 11 whose anode circuit comprises a parallel resonant circuit 12 tuned to 20 kc/s. and a parallel. resonant circuit 13 tuned e.g. to 2 kc/s. and retroactively coupled to the grid over inductance 15; the two circuits containing a common positive temperature coefficient bolometer resistance 14 which controls the level of oscillation at 2 kc/s. to a value at which the ambient and bolometer temperatures are in equilibrium at the prevailing level of energy supplied to resistance 14 from oscillator 8. An increase in the R.F. energy dissipated in bolometer resistance 1 produces a corresponding diminution in output of oscillator 8 heating bolometer resistance 14, whereby the level of output of oscillator 11 rises to a value which is a measure of the R.F. power input and is indicated by amplifier 6, which may comprise a rectifier and measuring instrument 7. In a modification (Fig. 4, not shown) a single triode is made to oscillate simultaneously at both the above frequencies at amplitudes controlled by the respective bolometer resistances as above, while in a further modification (Fig. 5) the output of oscillator tube 8 at 20 kc/s. is capacitance coupled to the grid of the first of two triodes 17, 18 connected as a resistance capacitance oscillator circuit tuned to 2 kc/s by resonant circuit 19 with cathode-coupled retroaction over bolometer resistance 14, which stabilizes the amplitude of oscillation to a level for equilibrium of the bolometer and ambient temperatures. The input from oscillator triode 8 is amplified by triode 17 and heats the resistance 14, whereby the amplitude of oscillation of triodes 17, 18 is diminished proportionately thereto and an increase of the R.F. energy dissipated in bolometer resistance 1 thus produces a proportionate increase in the output of triode 18 which is indicated by instrument 7 supplied through amplifier 6, which may comprise a rectifier. A variable D.C. may be passed through the first of the two bolometers whereby the deflection of the final indicating instrument may be zeroized in absence of R.F. input. The apparatus may also be utilized as a sensitive R.F. valve voltmeter.