Patent GB-656293-A (GB)
Title: Improvements in systems for distance measuring, and the like, by the use of wave energy
IPC: G01S13/34
Filed: 1947-12-30
Abstract: 656,293. Frequency measuring. GUANELLA, G. Dec. 30, 1947, No. 34893. Convention date, May 12, 1940. [Class 37] [Also in Groups XXXVIII and XL(c)] In a distance-measuring or path-difference direction-finding or beacon system of the type in which frequency-modulated waves arriving at a receiving station over two paths of different length are heterodyned to give a beat frequency (see Groups XXXVIII and XL (c)), two phasedisplaced A.C. voltages are derived from the beat frequency which actuate or control an electro-mechanical device, e.g. a relay or motor, to give an indication of the value of the beat frequency. Due to the inertia of the device the indication is insensitive to transient interference. In Fig. 1 (not shown), the beat frequency obtained by heterodyning at a receiver the signals radiated from two spaced transmitters is applied directly and via a 90 degree phase shift circuit to the two stator windings of a two-phase induction motor driving a D.C. generator whose output is applied to a voltmeter calibrated in terms of frequency. Fig. 3 (not shown) illustrates a similar type of indicator in a radio altimeter having two receiving aerials spaced vertically #/4 apart and a separate transmitting aerial so that the beat frequencies obtained by heterodyning the direct and reflected signals received by each receiving aerial are in phase quadrature and in Fig. 4 (not shown) the signals received at two spaced receiving aerials in a path-difference direction-finding system are (a) heterodyned directly, and (b) heterodyned after subjecting one signal to a 90 degree phase shift, to give two phase-quadrature beat frequencies. In the latter system the direction of rotation of the motor depends upon the position of the transmitter relative to the perpendicular bisector of the receiver base-line and hence the system indicates the sense and magnitude of the bearing angle. Since it is difficult to provide a circuit giving a 90 degree phase shift over a wide range of frequencies, the system of Fig. 1 may be modified by increasing the beat frequency h1 by a constant amount h2 before applying it to the phase shifter. The induction motor then rotates at a speed (h1+h2) and drives one input of a differential gear, the other input of which is driven at a speed h2 by a second induction motor energized by a voltage of frequency h2 so that the speed of the differential output is h1. Fig. 5 (not shown) illustrates this type of indicator applied to a radar system. In Fig. 6 the tuning of a bandpass filter N is automatically controlled so that the mid-point of the frequency band is equal to the beat frequency obtained by heterodyning in D5 the signals radiated from T and picked up by the spaced receivers R1, R2 in a path-difference direction-finding system. One output from D5 is applied directly to one stator winding F1 of a two-phase induction motor M3 and another output is applied through the filter N and a 90 degree phase-shift circuit W8, C8 to the other stator winding F2, the rotor of the motor being mechanically coupled to the tuning condensers of the filter. When the filter is tuned to the beat frequency it introduces a further 90 degree phase-shift, the voltages in F1 and F2 are then in phase opposition and there is no torque on the motor but when the filter is detuned with respect to the beat frequency, the phase shift introduced by the filter differs from 90 degrees and the motor is rotated to tune the filter. The angular position of the tuning condensers and hence the value of the frequency is indicated on a scale Z. In Fig. 7 the beat frequency h1 obtained by heterodyning in D1 the direct and reflected signals in an acoustic distance-measuring system is further heterodyned in D5 with an auxiliary voltage of frequency h2 produced by generator G5 which is driven by a D.C. motor M5. The difference frequency (h2 - h1) selected by the amplifier A3 is applied directly to the movable coil F1 of a wattmeter relay K and through the filter N to the fixed coils F2. When the difference frequency (h2 - h1) coincides with the mean pass frequency (h3) of the filter N, the voltages applied to F1 and F2 will be in phase quadrature and there will be no torque on the movable coil F1 but when (h2-h1) differs from h3 the coil F1 experiences a torque, closes a corresponding circuit and energizes motor M4 which adjusts a regulating resistance W9 controlling the speed of motor M5 so that the frequency h2 is varied until h3=h2-h1. A pointer Z on the speed adjustment member reads against a scale calibrated to indicate the frequency h1 and hence the distance d to the reflecting surface X. In Fig. 8 the frequency h4 of an auxiliary oscillator G4 is automatically made equal to the beat frequency h1 produced by heterodyning in a receiver R the signals radiated from two spaced transmitters T1, T2 in a path-difference beacon system. The output from G4 is applied in phase quadrature to two detectors D3 and D4 to which the beat frequency h1 is also applied so that the difference frequency (h1 - h4) outputs from D3 and D4 are in phase quadrature and these are applied through filters N3, N4 and amplifiers A3, A4 to the two stators F1, F2 of a two-phase induction motor M3, the rotor of which is mechanically coupled to the tuning condenser of G4 which is thus automatically adjusted to reduce the difference frequency to zero. The frequency of C4 is indicated by a pointer Z coupled to the tuning condenser drive.