Patent GB-594766-A (GB)
Title: Frequency modulated radio altimeter or distance indicator
IPC: G01S13/34
Filed: 1944-02-23
Abstract: 594,766. Radiolocation ; valve circuits; electric resonators. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb. 23, 1944, No. 3414. Convention date,June 4, 1942. [Classes 40 (iii) and 40 (v)] A radio receiver for detecting the frequency difference between two radio frequency waves, particularly applicable to a frequency mediated radio altimeter of the type described in Specification 317,068, [Class 40 (v)], comprises two rectifiers in a balanced circuit, the relative phases of the waves supplied to each rectifier being such that in the combined output obtained from the anode of one rectifier and the cathode of the other, only the beat frequency appears, any amplitude modulation of either input wave being balanced out. In the radio altimeter of Fig. 1, carrier from transmitter 10, frequency modulated by device 12, is radiated from aerial 11 and also fed in push-pull through transformer 17 to the anodes of diodes 13, 14. The reflected signals picked up by aerial 23 are supplied in parallel by way of transformer 21 to the anodes of diodes 13, 14. The push-pull detector input circuit is tuned to the midfrequency of the transmitted signal by means of capacitor 28, while the two parallel input circuits to the diodes are tuned to the same frequency by choice of the inductance values, particularly for the secondary winding of transformer 21, which resonate in part with the diode capacities 29, 30. Beat frequency signals are in antiphase at the anodes of the diodes while signals at the same points due to amplitude modulation of either or both input radio frequency waves are in phase. The beat frequency output is therefore taken from the anode of diode 13 and the cathode of diode 14, a blocking capacitor 22 being inserted between the cathode of diode 14 and earth. The beat frequency is applied through R.F. chokes .31, 32 to the opposite ends of a balancing resistor 33, the output being obtained from a variable tapping 36, adjustment of which enables any undesired amplitude modulation to be balanced out. In an embodiment shovm in Fig. 2, a balanced diode detector comprises diodes 76, 77, the circuit feeding their anodes being formed of a tuned line 81 which includes two coaxial conductors. 82, 83, the inner and outer conductors being connected for radio frequencies by condensers 84, 86, 87 and 88. Cathode 85 of diode 77 is earthed for radio frequencies by capacitor 92 while cathode 80 of diode 76 is conductively connected to earth through a small resistor 89, and capacitor 91 for purposes of symmetry. An adjustable shorting bar 96a, 96b tunes the two circuits formed by lines 82, 83 and the centre conductor 94 to the midfrequency of received signals, while a capacitor 97 in adjustable contact with conductor 82, 83 produces a third circuit resonant at the same frequency, which provides push-pull voltages at the diode anodes. The heterodyning signal from the transmitter is applied to tuned circuit 81 in push-pull by means of a loop 111 in a plane parallel to the plane containing conductors 82, 83 and separated from them by an electrostatic screen 116, while the received signals are fed in parallel to the diode anodes by a loop 113 which may be terminated by a tuning condenser 114 and is in a plane at right angles to the plane of the first loop and separated from conductors 82, 83 by a screen 117, Fig. 3 (not shown). The beat frequency signal is taken from the anode of diode 76 by means of the inner conductor 101 of line 82 and resistor 102 and from the cathode of diode 77 through a filter resistor 106 and capacitor 108 and resistor 107, the output being obtained from the tapping on the balancing resistor 103. The apparatus may be employed to measure the Doppler effect for the purpose, for example, of determining the ground speed of an aeroplane, by transmitting only constant frequency carrier wave.