Patent GB-1485547-A (GB)
Title: Radio navigation transmission device
IPC: G01S1/50
Filed: 1975-05-02
Abstract: 1485547 VOR transmission; APC systems PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 2 May 1975 [7 May 1974] 18429/75 Headings H4D and H3A A VOR beacon transmitter which generates a static omnidirectional phase reference beam pattern and electronically generates a rotating directional beam pattern by transmitting two double side band suppressed carrier signals having a modulation which is mutually shifted by 90 degrees, comprises means to control the exact phase relationship between the phase reference signal and the double side band signals, which phase relationship may vary due to changes in component values and adversely affect the accuracy of the beacon signal. The figure shows the transmitter circuitry wherein carrier wave generator 1 generates a signal in the 108-118 MHz ICAO band and generator 2 generates the 30 Hz modulation signal, and both signals are fed to phase reference signal generator 3 which is as described in U.S. Specification 3,328,798 and is connected to omnidirectional aerial 4. The double side band signals are generated by double side band modulators 5 and 6, the signals are amplified by transistor amplifiers 23, 24 wherein variations in phase may occur and transmitted from aerials 25, 26. In modulator 6, the carrier signal is applied to phase reversal modulator 8. The 30 Hz modulation signal is applied to - 45 degree phase shifter 10. Zero crossing detector 12 produces a 30 Hz square wave signal from the output of phase shifter 10 and this signal is applied to unit 8 where its edges trigger unit 8 to reverse the phase of the carrier signal. The 30 Hz signal is applied, in addition, to full wave rectifier 20 and amplifier 22 whereupon it is used to 100% synchronously modulate the phase reversed carrier signal in modulator 16 to produce a suppressed carrier double sideband signal. Modulator 15 produces an identical signal phase shifted by 90 degrees. Phase comparison device 27 comprises a non-interfering mixing signal generator 30 at the carrier frequency plug 135 kHz, which is mixed with the transmitted signals a portion of which are obtained from directional couplers 34, 35 and applied to mixers 31 and 33 respectively. The product signals at 135 kHz are applied respectively through filters 36, 38 and then to modulation eliminators 41 and 42 which respectively comprise amplitude eliminators 43, 44 and phase reversal eliminators 45 and 46 which receive the phase reversing control signal from terminals 13 and 14. The outputs of these units are phase compared with the output signal from amplitude eliminator 47 which signal represents the phase of the 30 Hz phase reference signal. Any detected phase differences are applied as D.C. signals to amplitude comparators 54, 55 where they are compared with preset voltages from potentiometers 58 and 59, whereby any phase variation produces control signals from the outputs of comparators 54 and 55 which are supplied to respective phase shifters 28, 29, e.g. varactors, and act as retroactive control signals on phase variations in the double side band signals. It is stated that the VOR signal phase error produced by this arrangement is always less than two degrees.