Patent GB-679505-A (GB)
Title: Improvements relating to electric circuit arrangements for measuring radio frequency power
Inventor: CROWLEY-MILLING MICHAEL CROWLE
IPC: G01R21/00
Filed: 1950-03-20
Abstract: 679,505. Power measurements. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Jan. 1, 1951 [March 20, 1950], No. 6929/50. Class 37. A circuit arrangement for measuring the peak values of pulse modulated R.F. power comprises a bridge circuit having in one arm a thermistor or bolometer heated by the R.F. power to be measured, means for feeding a control current to one diagonal of the bridge, means for controlling this current in accordance with the out-of-balance current so as to tend to restore the balance, and means for measuring the control current during the pulse periods to determine the peak power value of the R.F. power. The arm D, B of a bridge circuit A, B, C, D is constituted by a thermistor M. Control current for balancing the bridge is applied across the points C, D and is controlled by a valve V1. The anode circuits of differential valves V2, V3 are connected across the primary of a centre-tapped transformer T, the secondary of which is connected to an amplifier AMP. The output of this is applied to the grid of the valve V1. The points B, A are connected to the control grids of the valves V2, V3. Positive voltage pulses, synchronized with the R.F. pulses applied to the thermistor M and of pulse length proportional to the R.F. pulse length, are applied to the terminal P and thence to the suppressor grids of the valves V2, V3 so as to render them conductive during the pulse periods. If the bridge is in balance the points A, B are at the same potential and the anode currents in the valves V2, V3 neutralize each other in the transformer T so that no voltage is applied to the grid of VI. Normally the current through V1 is adjusted by varying the screen grid voltage by means of a potential divider R1 to bring the bridge into balance when no R.F. is applied. If R.F. power is applied to the thermistor, its temperature and resistance vary and the bridge is unbalanced. Voltage pulses proportional to the bridge outof-balance current and synchronized with the R.F. power are fed through the transformer T and amplifier AMP to V1 so as to reduce the control current to the bridge during the pulse periods and bring it back into balance by reducing the operating current through the thermistor M to compensate for the R.F. current supplied to it. The amplitude of the negative-going pulses of current through V1 is an indication of the amplitude of the pulses of R.F. power and may be measured by measuring the pulse voltage developed across a resistance R2. The voltmeter shown is of the cascade peak diode type. During the period of negative pulses applied to the grid of V1 the point X becomes more positive, as also does the junction point Y of two diodes D1, D2. The diode D2 conducts and charges a condenser C2 to the peak value of the pulse. When the pulse ends the diode D1 restores the point Y to the potential HT+. The voltage across the condenser C2 is measured by a high resistance voltmeter V, the time constant of R, C2 being long compared with the pulse recurrence frequency.