Patent GB-641351-A (GB)
Title: Improvements in or relating to alternating current impedance bridges
Inventor: MOODY NORMAN FRANK
IPC: G01R27/02
Filed: 1946-11-15
Abstract: 641,351. Impedance measurements. STANDARD TELEPHONES & CABLES, Ltd. Nov. 15, 1946, No. 33979. [Class 37] A ratio-arm assembly for an impedance bridge comprises two impedances connected in series, and an amplifier, one of the impedances being connected in a feed-back path associated with the amplifier. As shown in Fig. 3 the ratio-arm assembly comprises two impedances 13, 14, either of which may be resistive, reactive, or complex. A balancing amplifier 15 is indicated by the input grid 16 of the first valve and the output anode 17 of the last valve, one or more valve cathodes being indicated by the earthed cathode 18. The impedance 13 is connected to the input grid 16, and the impedance 14 is connected between the anode 17 and the grid 16 so as to feed back a proportion of the output voltage of the amplifier to the input. The oscillator 10 supplying the bridge is connected between the corner 2 and earth. The corner 1 of the bridge is substantially at earth potential as the input impedance of the balancing amplifier 15 is low. Since the output impedance is also extremely low any impedance connecting the corner 4 to earth will be substantially shortcircuited and will not affect the balance. An impedance connecting the corner 2 to earth has no effect since it shunts the oscillator 10. The impedance to be measured is connected between terminals 7, 8 and a known impedance 9 forms the arm 3, 4. Fig. 4 shows a bridge for automatic balancing. The detector is connected by means of a current transformer 19 having two primary windings 20, 21 in series with the arms 2, 3 and 3, 4. The secondary winding 22 is connected to an amplifier 23, the output of which is connected through a transformer 24 to a phase-sensitive detector 25. The windings 20, 21 may have the same number of turns and are wound so that currents I 2 , I 4  flowing towards the corner 3 produce aiding fluxes. The condition for balance is that I 2 +I 4 =O, which produces zero flux in the transformer core and therefore no voltage is applied to the amplifier 23. The range may be changed by changing the turns ratio, e.g. by providing the winding 21 with a number of tapping points or by changing the transformer 19 for another with a different ratio by means of a switch. The balancing amplifier 15 consists of two valves 26, 27 coupled by a blocking condenser 28 and grid leak 29. The anode circuit of the valve 27 includes the primary winding of an output transformer 35 tuned to the test frequency by a condenser 36. The control grid of the valve 26 is connected directly to the corner 1. The transformer 35 is so poled that the feed-back through the impedance 14 is negative. An oscillator 37 supplies a test wave at a frequency such as 800 cycles per second to the primary winding of a transformer 39 having four secondary windings. One secondary winding 40 is connected to the primary winding 42 of a transformer 43 through a phase-changing amplifier 41 which changes the phase by 90 degrees. The transformer 43 has three output windings, one 44 of which has an adjustable tap connected to the corner 2. A phase-sensitive rectifier 25 comprises two similar branches producing at terminals 49A, 49B voltages proportional to the in-phase and quadrature components of the bridge unbalance current. These voltages control the motors that make the balancing adjustments, or may be applied to voltmeters for indicating the balance. As shown, the bridge is arranged for measuring impedances with negative reactance. The impedance 9 is a pure resistance adjustable in relatively large steps for providing various ranges. The impedance 14 is also a pure resistance controlled by one of the motors. The impedance 13 comprises a fixed condenser 55 associated with a resistance 56 controlled by the other motor. This is connected between the corner 1 of the bridge and a tap on an inductance 57 connected between the corner 2 and earth. The impedance to be measured is connected between terminals 7, 8. For measuring an impedance with a positive reactance the impedances in the arms 1, 2 and 1, 4 are interchanged. If a pure resistance is to be measured the impedance 13 is replaced by a fixed resistance connected between the corners 1, 2, the elements 55, 56, 57 being omitted. In order to balance out any small reactive component the transformer 39 is furnished with an output winding 58 with an earthed centre tap and shunted by a potentiometer 59, the slider of which is connected to one contact of a two-way switch 54. When the switch is on this contact for measuring a resistance, the output of the amplifier 23 is supplied with a small quadrature current which may be adjusted by the potentiometer 59 to balance out the quadrature component of the unbalance current. The potentiometer 59 may be driven by the motor that drives the resistance 56, which is disconnected when a resistance is being measured. The amplifier 23 has a low input impedance of about 1 ohm and is shunted by a resistance 53 when the switch 54 is in the position shown. The impedance effectively connected between the terminal 8 and the corner 3 is thus kept very small so that the error produced by an admittance to earth from terminal 8 is negligible. If the unknown impedance has one terminal directly connected to earth the winding 20 and the unknown impedance are interchanged, the earthed terminal of the unknown impedance being connected to the corner 3. The winding 20 is screened to prevent any admittance acting across the unknown impedance. This arrangement is shown in Fig. 5. The winding 20 of the transformer 19 is surrounded by an inner shield 60 which screens it from the windings 21, 22 and from an outer earthed shield 61. A pair of terminals 62, 63 are connected to the winding 20 and the corner 3, terminals 7, 8 being short-circuited. A switch 64 in the position shown connects the inner screen 60 to the corner 2. The admittance between the two screens acts effectively to earth from the corner 2 and therefore has no effect on the balance. To revert to the arrangement shown in Fig. 4, the short-circuit between the terminals 7, 8 is removed, terminals 62, 63 are short-circuited, and the switch 64 is moved to the other position to connect the inner screen 60 to earth. The primary winding 21 of the transformer 19 may be omitted and the impedance 9 connected to the terminal of the winding 20 which is connected to the terminal 8. The transformer is then outside the bridge network and the resistance 53 may be connected across the primary winding 20 instead of across the secondary winding 22. This may then be wound to step up to the input impedance of the amplifier 23. If it is desired to employ unequal currents in the two arms of the bridge, one or more intermediate taps may be provided on the primary winding 20 and the impedance 9 and terminal 8 may be connected to different taps so that the winding acts as an auto-transformer. When a single primary winding is used with one or more taps, the resistance 53 may be connected across that part of the winding which is outside the bridge.