显示摘要
722,530. Wattmeters. GENERAL ELECTRIC CO., Ltd., and HUNT, G. J. Jan. 5, 1953 [Jan. 3, 1952], No. 292/52. Class 37. An R.F. wattmeter comprises a pair of identical bridges, each with one arm a thermistor element. The first bridge is in the feed-back line of an oscillator and varies the output thereof with temperature, thereby balancing both bridges in the absence of R.F. The thermistor in the other bridge is fed with the R.F. power and D.C., the bridge being automatically balanced by alteration of the D.C. current in accordance with the out-of-balance output of the bridge. This change in D.C. current, which is proportional to the R.F. power dissipated is measured indirectly. The oscillator 3 supplies current to the two identical bridges 1 and 2, the frequency being determined by the value of condenser 48 which only allows the bridge to balance at one particular frequency. If the temperature of thermistor 6 changes, bridge 1 becomes unbalanced and the out-of-balance current is fed back to the oscillator input through transformer 16 causing a change in output current which tends to rebalance the bridges 1 and 2 automatically. When R.F. power is applied to the thermistor 7 bridge 2 is unbalanced, the out-of-balance signal being amplified in valves 21 and 22 and fed to the cathode follower stage 23 which contains the primary 25 of transformer 26 in its cathode circuit. The twin T-network 24 provides negative feed-back except at the oscillator's fundamental frequency, which reduces the harmonic content of the signal. The cathode follower output is compared with the original amplified oscillations, developed across the secondary 37 of transformer 28, in the phase discriminator circuit A which produces a D.C. output voltage across tapping points 36 and 33, dependent on the phase and amplitude of the out-of-balance signal. This voltage alters the D.C. current through bridge 2 so as to balance it automatically. D.C. current is normally supplied to bridge 2 from a voltage stabilized supply via meter 38. The current through 38 is preset by adjustment of resistor 39, and the current through 42 is set at zero in the absence of R.F. by resistor 14. When R.F. power is supplied to thermistor 7 the bridge is unbalanced, and the voltage developed across points 36 and 33 produces a current in meter 42. The current in 38 remains substantially constant so that the current in 42 equals the change in current through bridge 2, which is twice the change through thermistor 7. Since bridge 2 is nearly balanced both with and without the R.F. applied, the resistance, temperature, and total power expended in 7 are unaltered. This difference is the current in meter 42 which can thus be calibrated directly in power. The instrument is independent of changes in ambient temperature. Semi-conductor devices. The two thermistors are mounted together in a brass block, with their associated condensers 8 and 9, to have the same ambient temperature.
