Patent GB-1382960-A (GB)
Title: Vor receiver with adaptive filters and phase shifter for improved accuracy
IPC: G01S1/02
Filed: 1973-10-01
Abstract: 1382960 Radio navigation; automatic tuning BENDIX CORP 1 Oct 1973 [21 Nov 1972] 45694/73 Headings H4D and H3A A VOR receiver comprises means for separating from the signals received from a VOR beacon the reference signal of fixed phase and the signal whose relative phase varies in accordance with the bearing of the receiver from the beacon, individual filtering means for each of the two signals, means for determining the phase difference between the two signals, and means for indicating that phase difference. The phase difference determining means comprises a voltage controlled phase shifter 19, Fig. 1A to which the reference and variable signals as appearing at the outputs of the filtering means are applied and which includes a clock oscillator having an output frequency (F) higher than the frequency (f) of the reference and variable signals, said output frequency (F) being variable according to a control voltage applied to said oscillator. The output from the clock oscillator is applied to a frequency divider which provides a first output signal of frequency (f) and a pair of phase opposed output signals which are respectively in negative and positive quadrature with said first output signals. A phase detector 16 receives the signal of variable phase, a signal in phase opposition therewith, said pair of phase opposed output signals from the frequency divider, and provides an error signal. Said error signal is integrated by a means 18 and is then applied to the clock oscillator to control its frequency. As described, a VHF receiver 10 produces a conventional composite output signal with the variable phase signal as a 30 Hz AM modulation and the reference phase signal as a 30 Hz FM modulation of a 9960 Hz subcarrier, which is recovered by a demodulator 11. The two signals are passed through respective low pass filters 12, 14 and tracking filters 13, 15, the filter 15 being connected to an inverter to provide a second output in phase opposition to its first output. The filters 13, 15 have narrow band-pass characteristics, and are self-tuning to counter the effects of frequency variations of the VOR signals, temperature changes at the receiver &c. In each filter, sine wave input at a junction 31, Fig. 2, is applied to a voltage controlled selective filter 32 and to the drain of a FET33, which serves as a quadrature chopper demodulator. The filter 32 is an operational amplifier with an input circuit comprising a capacitor 34 and a resistor 35 and with a T- connected feed-back network comprising a resistor 36 and a FET 37. The filter band-pass characteristic is determined by the parallel impedance of the resistor 36 and the FET 37, the latter impedance being controlled by the voltage applied to the gate. The output, at a junction 38 is of sine wave form, normally shifted in phase by 90 degrees from the input at the junction 31. Said output is converted to square wave form by a cross-over detector 39 and is fed back to the gate of the FET 33. When the square wave from the detector 39 is in phase quadrature with the input to the junction 31, a voltage corresponding to that portion of said input between 90 degrees and 270 degrees of its cycle is developed in the drain of the FET 33. This is averaged in an operational amplifier 41 and is applied as control voltage to the FET 37. With the normal phase quadrature relationship said averaged voltage is zero, but any shift therefrom results in a change of characteristic of the filter 32 such that its pass band is centered on the frequency of the input at the junction 31. The phase shifter 19, Fig. 1A, produces a square wave output which is in phase with the signal of variable phase from the filter 15 and two square wave outputs, respectively leading and lagging said signal by 90 degrees, for application to the phase detector 16. The said inphase output is applied to a synchronous modulator 21, and the + 90 degree output is applied to a synchronous modulator 22. The mdoulators 21, 22 also have inputs from the filter 13, and they produce D.C. outputs respectively proportional to cos # and sin #, where # is the VOR bearing angle. These outputs are modulated at 400 Hz in choppers 23, 24 and, after amplification, may be applied to a radio magnetic indicator or RMI (Fig. 1B, not shown). Alternatively, they may be used for manual VOR operation, in a conventional arrangement (Fig. 1C, not shown). Reference phase signal from the filter 13, and variable phase signal from the filter 15 are applied to respectively open and close a control gate 28. Clock signals, from the oscillator in the phase shifter 19, are conducted through said gate to a counter 29, and provide a digital value of the VOR bearing, which is transferred to a register 30 during each period of the 30 Hz VOR signal. Operation of voltage controlled phase shifter.- The 30 Hz square wave output of the crossover detector 39, Fig. 2, of the reference phase signal tracking filter is inverted and is applied to trigger a 50 ms. one-shot multivibrator 51, Fig. 3, at a 15 Hz rate. A ramp generator 52 is connected to the multivibrator 51 to be reset each time it fires. The sawtooth wave from the ramp generator 52 is applied to a sample and hold circuit 53, which produces a constant voltage output equal to the voltage attained by the linearly rising sawtooth at the time a sampling pulse appears from a differentiator 54. The output from the circuit 53 is applied as one input to an amplifier 55, whichprovides a control voltage for a 245À76 kHz clock oscillator 56. Output from the VCO 56 is divided in frequency by a factor 8192 in a divider 57 to produce a first 30 Hz signal, on a line 60, which is further divided in frequency by a factor 2 in a divider 58 to produce a 15 Hz signal, which is connected as input to the differentiator 54. The divider 57 also produces 30 Hz signals on lines 61, 62 which have respective phase differences of +90 degrees and - 90 degrees with the signal on the line 60, and which are applied as inputs to a phase detector 63. The signal of variable phase from the tracking filter 15 (Fig. 1A) is applied directly as a third input to the phase detector 63 and through a phase inverter 64 as a fourth input thereto. In a state of equilibrium, the signal on the line 60 is in phase with the signal. from the filter 15. Otherwise there is an error signal output from the phase detector 63 which is passed through a rate limiter amplifier 62, then integrated in a loop integrator 66 to provide a second input to the amplifier 55. When the reference and variable signals are in phase (i.e., a bearing of 0 degrees) the sampling pulse from the differentiator 54 is arranged to be midway along the sawtooth voltage waveform from the generator 53 so that the control voltage from the amplifier 55 holds the VCO 56 at 245À76. An error signal from the phase detector 63 changes the control voltage from the amplifier 55, and there is a transitory change in the frequency of the VCO 56 which continues until the output on the line 60 is again in phase with the variable signal. Simultaneously the 15 Hz input to the differentiator 54 is changing its time relationship with the sawtooth output from the generator 52 so that the output from the sample and hold circuit 53 tends to balance the output from the loop integrator 66. In the final stable condition the signal on the line 60 is in phase with the 30 Hz variable phase signal input to the phase detector 63, and the sampling pulse is shifted along the sawtooth by an amount which is equivalent to the phase difference between the variable and reference signals. The rate limiter 65 includes an amplifier 67 with inputs from the phase detector 63 and from a feedback circuit comprising a buffer amplifier 69, a multiplier 70, and a threshold detector 71. The multiplier 70 produces an output which is proportional to the product of the output of the amplifier 67 and of a analogue voltage corresponding to the distance of the receiver from the VOR station. An override circuit 68 is provided to open the feedback circuit of the rate limiter 65 under certain conditions, i.e., during acquisition, when there is a considerable phase difference between the two signals. The circuit 68 is operated by a null detector 75 which receives a square wave variable phase signal from the filter 15 (Fig. 1A) and a square wave signal from the divider 57 which is 180 degrees out of phase with the signal on the line 60. The detector 75 also operates a warning circuit 76. An integrator reset circuit 77 is a threshold circuit which is triggered whenever the level of output voltage from the integrator 66 is equivalent to a phase change of 270 degrees. It also operates the override circuit 68 and a freeze circuit 78, which temporarily disconnects inputs to the modulators 21, 22 (Fig. 1A).