Patent GB-943470-A (GB)
Title: Improvements in variable capacitance electrical information transmitting means and transmitting devices
Inventor: BURR KENNETH JOHN
Filed: 1959-02-19
Abstract: 943,470. Bridge measurement. BAILEY METERS & CONTROLS Ltd. Feb. 11,1960 [Feb. 19,1959], No. 5764/59. Heading G1U. Electrical information transmitting device comprises an input signal responsive device controlled by a physical variable and providing variations of electrical capacitance, and a transmitting device comprising a semiconductor diode whose capacitance is variable by applied bias; with a control device varying such bias according to changes of an output signal from the transmitting device, and comparison means sensitive to the values of both capacitances to detect changes in a predetermined relationship thereof and thereupon to control the output signal and the capacitance of the semiconductor diode variable capacitor so as to restore the predetermined relationship; the change in the output signal then being a measure of the change in a capacitance of the input signal device. A capacitance C1 variable e.g. by steam pressure or temperature is connected in series with a comparatively large capacitance C2 across trimmer capacitances C3, C4 and silicon diode D2 (back biased to operate as a capacitance) in series with capacitances C5 and C6,C7; as an arm of a bridge completed by capacitance C9 and inductance L tapped at L1, L2. The base of transistor T1 is connected to the junction of C1 and C9 and to its collector over resistance R3; both being returned to the negative side of a first diode stabilized full wave rectified transformer fed power supply whose positive is returned to the junction of C1 and L while the transistor emitter is connected to inductance tap L1 to operate as an oscillator energizing the bridge at radio frequency. Tap L2 is connected to the base of transistor T2 whose collector is connected to the negative of the first supply while its emitter is connected to the positive over a parallel resistance and capacitance and directly to the base of a transistor T3 whose emitter is returned to the positive and collector to negative supply, the latter being also connected over resistance R12 to the base of transistor T4 whose emitter is returned to positive supply and whose collector is earthed; the transistor base being negatively biased over resistance R15 from the capacitance smoothed output of a diode rectifier D3 fed from one side of the first power supply transformer. A second transformer fed fullwave rectified power supply has its positive applied to develop a standing bias for diode D2 across variable resistance R1 shunting capacitances C6,C7, while its negative is connected over resistance R16 to an output terminal; returned to earth and its opposite terminal through large capacitance C13. Further bias is applied to the other side of D2 from a potentiometer across the negative and positive outputs of the first power supply comprising resistance R8 in series with variable resistance R7, whose slider is electrically and mechanically coupled to that of a potentiometer R5 in series with a temperature compensating nickel wire resistance R4 across a resistance R6. In operation, the oscillatory output from the bridge is applied to transistor T2 for amplification and rectification to a signal which is amplified by transistor T3 to bias transistor T4 to develop a corresponding current in a D.C. measuring instrument across the output terminals 30. When the ratio of the capacitances in adjacent bridge arms equals the ratio of the inductances in the remaining arms, zero r.f. voltage is developed between the base and emitter of transistor T1, when the capacitance comprising C1 is less than that of C9 the base emitter voltage maintains positive feedback across the transistor, and when the capacitance comprising C1 exceeds that of C9 there is negative feedback. Thus the resultant output current of transistor T4 varies in accordance with the oscillatory amplitude and develops a corresponding variable bias across R1 which is applied to D2 to adjust its effective capacitance so as to maintain constant the sum of the capacitance combination of C1 in series with C2 and the combination of C5 in series with D2 together with C6 in parallel with C7. The output current value accordingly represents variations of the input capacitance C1. Temperature variations are compensated by variations of the value of the nickel wire resistance R4 correspondingly varying the bias applied to D2 and thus correcting for increase of its capacitance and that of transistor.T1 with temperature; the ganged variable resistance R7 and potentiometer R5 varying the temperature coefficient of the correction.