Patent GB-640084-A (GB)
Title: Improvements in or relating to oscillatory electric detector or measuring circuits or systems
IPC: G01V3/10
Filed: 1945-12-04
Abstract: 640,084. Impedance measurements. STRATTON, A. Dec. 4, 1945. No. 32791. [Class 37] [Also in Group XL (c)] In a detector or measuring system wherein a variable function is applied as a variable impedance in the operating circuit of a self-excited valve oscillator the oscillator is arranged so that with variations of impedance there is derived from the control grid circuit a signal output which is indicative both of the impedance and the function. In the construction shown in Fig. 1 the oscillator in the measuring circuit is of the push-pull type comprising valves V1, V2 having their anodes coupled by the tuned circuit L1, C1 and their grids connected by the tuned circuit L2, C2. The cathodes are connected to earth through a variable resistance R2, and the mid-point of the inductance L2 is connected to earth through a grid leak resistance Rgl and variable resistances R1, R2, across which is connected a balance indicator B1. The impedance RX to be measured is connected either across the inductance L1 or in series at X and its value is measured by the values of the resistances R1, R2 when balance is indicated on the indicator B1. The frequency at which the oscillator is excited is variable by adjustment of the condensers C1, C2. The reactive component of the unknown impedance is obtained by calibrating the condenser C1 and noting the change necessary to bring the frequency back to its previous value before the unknown impedance was inserted. Fig. 2 shows a single-ended oscillator circuit suitable for measurements of impedances which are unbalanced with respect to earth. A valve V1a has a tank circuit L1a, C1a between anode and grid with an interposed grid condenser Cg. The grid circuit includes variable resistances R1, R2, a grid leak resistance Rg1, and radio frequency chokes RF1, RF2, the balance indicator B1 being connected across the resistances R1, R2 at points O1, O2. The variable load impedance RX to be measured may be connected to earth across the whole or either half of the centre fed inductance L1a or in series between the halves of the inductance at X. Instead of a balance indicator the signal across O1-O2 may operate a direct reading indicator appropriately calibrated. For strain gauge measurements or servo pick-up operations a series fed Hartley type oscillator may' be employed comprising a valve V3, Fig. 3, with a tuned tank circuit comprising an inductance TR. variable condenser TC, and grid condenser C3. The grid circuit includes a radio frequency choke RFC2 and a grid leak Rgl2, to the earthy point of which the cathode is connected and led to earth over a variable resistance R3, The output OUT is taken from the junction of RFC2 and Rgl2 over a resistance R4, a voltmeter VT being connected as shown. A servomotor or strain gauge IT feeds its displacement to a converer CR adapted to change displacement into impedance variations, which are then passed to the oscillator circuit. The resistance R3 is adjusted until VT indicates zero at the mean or zero position of the initiator IT, the DC output over the OUT lead then being of positive or negative polarity in accordance with variations of displacement above or below the mean position. The output may operate a follow-up device or a cathode-ray indicator. Fig. 4 shows the application to apparatus for detecting the presence of an object by the change in aerial impedance. Change in aerial impedance resulting from a received reflected signal varies the grid output from a combined transmitter and receiver, the grid output controlling an indicator. A valve V4 is provided with filament radio frequency chokes RFC3, RFC4 and padding condensers Caf, Cgf across the anode-cathode and gridcathode valve capacities. The tank circuit comprises a divided inductance L1, L2, L3 and a grid blocking condenser Cg, the feed to this circuit from the aerial A being taken through a coupling network CN to the junction between the coil L1 and the condenser Cg. The output is taken from the grid circuit at the junction of the coils L2. L3 over a filter resistance FR and a small condenser FC. A grid leak resistance Rgl3 and a positive bias resistance R5 are connected as shown.