Patent GB-657109-A (GB)
Title: Improvements in or relating to electrical mixing circuit arrangements
IPC: G01R21/00
Filed: 1948-04-09
Abstract: 657,109. Phase angle, power, modulation, current, and voltage measuring. EL-SAID, M. A. H. April 9, 1948, No. 10003. [Class 37] [Also in Group XIX] A thermionic circuit for averaging the instantaneous product of two alternating voltages comprises a multigrid tube, operated with automatic grid-leak bias under initial velocity conditions, so that the plate current is proportional to the product of a linear function of plate voltage and an exponential function of control grid voltage, and the control grid current is proportional to another independent exponential function of the control grid voltage, wherein the two alternating voltages are applied respectively to the plate and the control grid and the change in the average component of plate current is proportional to the average value of their instantaneous product. In Fig. 1 a pentode P with suppressor connected to cathode and earth, screen connected to a positive source, grid returned to cathode through a resistance R1 and anode connected to an H.T. source through a resistance R2, operates according to a family of exponential IA/VG curves whose exponential indices are constant for plate voltage variations, an exponential IG/VG curve of greater exponential index, and a substantially linear VA/IA curve. A sinusoidal voltage e1=E1 sin #t+# is applied to the control grid and R1 is adjusted until grid current sufficiently biasses the grid relative to the cathode for there to be no change in the anode current and thus no change in the D.C. voltage across C3 when the A.C. voltage is applied. A sinusoidal voltage e2=E2 sin #t applied to the anode (in the absence of the grid signal) also causes no change in anode current since the IA/VA characteristic is linear, but when anode and grid voltages are applied together the D.C. voltage across C3 varies proportionately to E1E2 cos #. Incorrect adjustment of R1 introduces an A.C. component. The tube may be a tetrode or other multi-grid type and functions as a watt-meter if the input signals are respectively proportional to voltage and current. The D.C. output may be measured by a vacuum tube, voltmeter. Fig. 3 shows a watt-meter in which a multiplying pentode P as above is enclosed in a screened measuring head and fed with two A.C. voltages proportional to the supply voltage of a source W and the voltage drop across a resistance in series with a load L. The D.C. output is connected to the grid of onehalf of a double triode degenerative valve voltmeter TT, and the output of a similar dummy pentode P1 with its control grid and anode byepassed to earth is connected to the grid of the other half. A microammeter is connected between the cathodes of the voltmeter triodes through switchable series resistances and a reversing switch S, to indicate the mean wattage dissipated in the load, and the dummy pentode operates to cancel errors due to supply voltage variations. Switching may be provided whereby the voltage or current elements of the watt-meter may function separately. The dummy pentode may be in the measuring head with the voltage signal fed to its plate (Fig. 4, not shown) to balance out errors due to non- linearity in the characteristics of the multiplying pentode, and errors in the adjustment of the grid resistance may be corrected by feeding the current signal to the grid circuit of the dummy pentode and byepassing its anode to earth (Fig. 5, not shown). The dummy and multiplying pentodes may be connected in push-pull with the voltage signal connected to both anodes and anti-phase current signals connected to the respective grids, to correct characteristic non-linearity and inaccurate adjustment of the grid resistances. The current signals are derived from centrally-earthed series resistors (Fig. 6, not shown) or from a centretapped current transformer (Fig. 7, not shown). The device is operable for phase angle measurement by introducing a variable phase-shift, into either input circuit and adjusting until the wattmeter indication is zero, or by amplitude limiting the inputs, so that the watt-meter indication is proportional to cos #. Modulation depth is measurable by utilizing the known relation between the percentage modulation and the modulated and unmodulated carrier powers.