Patent GB-1157242-A (GB)
Title: VOR Receiver with Self-Calibration to Eliminate Bearing Errors
IPC: G01S1/02
Filed: 1968-01-12
Abstract: 1,157,242. Transistor pulsed phase shift circuits. BENDIX CORP. 12 Jan., 1968 [14 Feb., 1967], No. 1969/68. Headings H3T and H4D. In an omnidirectional radio range system in which the phases of two signals are compared, one of said phases being direction-dependent and the other being invariable, receiving equipment comprises means for periodically applying one of said signals through associated circuitry to both inputs of the phase comparison means, and utilizing any output thereof for operating a compensatory phase shifter means. As described, the output of a receiver 10, Fig. 1, consists of a 9.96 kc./s. sub-carrier which is frequency modulated with a 30 c./s. reference signal, and a variable phase 30 c./s. signal. The former signal is detected in a detector 13 and is connected via a phase-shifting resolver 16, a contact 17 and an arm 19 of a relay 18 (shown in Fig. 1, in the " calibrate " position), and an amplifier 20 to a first input of a phase detector 21. The variable phase 30 c./s. signal is connected via a test key 39, a low-pass filter 15, a voltage-controlled phase shifter 22, and an amplifier 23 to a second input of the phase detector 21. The output of the phase detector 21 is taken via a 400 c./s. chopper 24 and an amplifier 25 to a servo-motor 26, which is on the same shaft as the resolver 16, and which is thereby rotated in such a sense as to reduce the output of the phase detector 21 towards zero. A-bearing indicator 31 on the same shaft then shows the direction of the transmitting station. A tachometer 29, which is also on the same shaft, provides rate feedback for stabilizing the servomechanism. A switch timer 32 periodically cycles the relay 18 from an operate mode to a calibrate mode. In the latter, as shown in Fig. 1, the servomotor 26 is locked by D.C. applied through a contact 38 and an arm 28 of the relay 18, and the 30 c./s. variable phase signal is also applied to the first input of the phase detector 21, via a contact 33 and the arm 19 of the said relay. Any output of the phase detector 21 appearing on the output side of the 400 c./s. amplifier 25 is detected by a synchronous detector 34 and is applied via an arm 35 and a contact 36 of the relay 8 to a storage circuit 37, which operates the voltage controlled phase shifter 22 in a compensatory sense. The timer 32 then releases the relay 18, and the operate mode is resumed. A test switch 39 is provided, by means of which the 30 c./s. reference signal is also connected to the second input of the phase detector 21 when, if all is well, the bearing indicator 31 should return to zero. Circuit Details.-The 30 c./s. variable phase signal on line 12, Fig. 2, is applied through a normally conductive diode 42 and a low-pass filter 15 to the gate of a field effect transistor 49, which acts as a phase splitter, providing oppositely phased signals of equal magnitudes across resistors 52, 53 relative to ground. These signals are applied, respectively, through a capacitor arm 57 and a variable resistance arm 58 to a point 61, which is connected to the input of the amplifier 23, Fig. 1. The voltages across the arms 57, 58, Fig. 2, are in quadrature, and the voltage to ground of the point 61 is therefore constant and equal to the voltage across either the resistor 52 or the resistor 53, and is variable in phase (Fig. 3, not shown). The arm 58 comprises a resistor 59 which is paralleled by a resistor 63 in series with a field effect transistor 62, the conductivity of which is controlled by the voltage developed in a storage circuit 37, comprising a capacitor 64 and a resistor 65. The said voltage is provided by a synchronous detector 34 operating on an output from the 400 c./s. amplifier 25, Fig. 1. The circuits which comprise the field effect transistors 49, 62, Fig. 2, constitute the voltage controlled phase shifter 22, Fig. 1. The switch timer 32, Fig. 1, comprises a bi-stable multi-vibrator 95, Fig. 2, in which a transistor 96 when conductive controls normal operation of the receiving equipment, and a transistor 97 when conductive controls the self-calibration thereof. Pulses from a unijunction transistor 99 trigger the multivibrator 95 and occur asymmetrically owing to the charging of a capacitor 103 either through a resistor 104 alone, or in parallel with a resistor 105. The operate mode may last, for example, four seconds and the calibrate mode one second. Switching of the multi-vibrator 95 is synchronized with the 30 c./s. variable phase signal by shaping that signal in a squaring circuit comprising transistors 108, 109, differentiating in a network 110, and applying to the base of the transistor 99. Provision is made for giving an alarm if the amount of phase shift required exceeds the capacity of the phase shifter 22. A separate synchronous detector 74 operates on another output of the 400 c./s. amplifier 25, Fig. 1, and feeds the gate of a field effect transistor 83, Fig. 2, through diode/capacitor circuits 81, 92 and 82, 93. The transistor 83 is biased by a potentiometer 84 and is normally non- conductive. If the voltage developed from the diodes 81, 82 approaches the bias voltage, transistor 83 commences to conduct and the voltage across a load resistor 85 falls, so allowing a P-type field effect transistor 86 to conduct and operate an alarm through a diode 87.