Patent GB-1359392-A (GB)
Title: Aircraft navigation receiver apparatus
IPC: H03H11/12
Filed: 1972-03-17
Abstract: 1359392 Radio navigation; FET limiters; automatic gain control UNITED AIRCRAFT CORP 17 March 1972 12634/72 Headings H3Q H3T and H4D An aircraft navigation receiver in which first and second data signals radiated from a navigation station are compared, comprises two channels containing active filters for translating the first and second data signals into forms suitable for comparison, and means for comparing said translated first and second signals to determine the orientation of the aircraft relative to a reference. As described the invention is applied to VOR systems although ILS is mentioned. The use of active filtering removes extraneous signals but does not introduce any phase errors into the signal channels which may occur with the use of passive filters. In the VOR receiver shown in Fig. 1, detected signals are separated into the reference 9960 Hz sub-carrier modulated with the 30 Hz reference signal, and the variable phase 30 Hz signal by high- and lowpass filters 33, 45 (which may be passive). In the reference channel 31 the signals are clipped and fed to a discriminator 35 (which is described in Specification 3,024,419) which produces the 30 Hz reference modulation at its output (Fig. 4, not shown). This is then limited and fed to an active filtering means comprising two integrators 37, 38 which use integrated circuit amplifiers 40, 43 and capacitive feedback. The direction dependent 30 Hz signal from the detector 28 passes through filter 45 and resolver 71 (where it is given a phase shift dependent on the desired course) to limiter 46. Thereafter it is subjected to active filtering as in the reference channel 31. The two 30 Hz signals are then compared in phase in a conventional utilization means 63 to provide a "deviation from course" indication on meter 94. In a second embodiment (Fig. 5, not shown) the integrators forming the active filters utilize constant current generators, which are also used with series and shunt capacitors to form the high and low pass filters 33, 45, respectively. The generators, Fig. 8, comprise two field-effect transistors 181, 191 connected in opposition with resistive feedback 188, 198 and a balancing potentiometer network. If the input at 182 is positive, transistor 181 is reverse biased and the current flow is limited by the resistor chain and kept constant by feedback resistor 188. Transistor 191 is forward biased and thus a constant current is provided at the output, which is integrated by capacitor 199. If the input is negative the conditions are reversed and a constant negative current is produced at the output. Active filtering, Figs. 9, 11.-Various forms of active filter are described, the simplest being shown in Fig. 9 where the input signal is limited and applied to one input of a differential amplifier 203, the output of which feeds a follower amplifier 204 driving a series resonance circuit tuned nominally to 30 Hz. The output of the resonance circuit forms the filter output 161 and provides the second input to the differential amplifier 203. If the input signal remains at 30 Hz the amplifier 203 output is zero and the filter output 161 is the unaltered input signal. Any change in component values effectively detunes the resonance circuit producing a phase-shift at 209. This is detected by the differential amplifier 203 whose output retunes the resonant circuit. This arrangement is stated to be immune from the effects of component ageing, drift, noise and amplitude modulation. Fig. 11 shows a further embodiment of active filter which provides a constant charging rate for capacitor 428. A step voltage input to the integrated circuit amplifier 421 produces a corresponding output which charges capacitor 428 through resistor 429. The voltage across resistor 429 is fed back to the inverting input to the amplifier. Under these conditions the step function is transformed to a straight line whose slope is a function of the impedance of resistor 429 and capacitor 428. Diodes 432, 433 provide a threshold value for the feedback such that slowly varying signals are not transformed and the amplifier merely has unity gain. It can be seen that the filter acts in the same way to a negative step function and thus will transform a square wave into a triangular wave, and a triangular wave into a sine wave as above. A modification of this circuit (Fig. 14, not shown) involves connecting the feedback from the charging rate sensing resistor (429) to the inverting input which also receives the input signal through a series resistor (435). The non- inverting input is in this case grounded. Figs. 15-17 (not shown) show further modifications to these basic active filters involving the use of extra integrated circuit amplifiers in the feedback paths from the rate sensing resistor (Fig. 15), a high pass filter (Fig. 16) and compensating for the voltage drop across the rate limiting diodes (Fig. 17). Automatic gain control, Fig. 10 (not shown).- The output of the R.F. amplifier 22, Fig. 1, is maintained constant by using an R.F. amplifier with resistive feedback to maintain a constant gain and using the detected output therefrom to control the impedance of three varactor diodes in a capacitive potential divider network connected to the amplifier input.