Patent GB-1011513-A (GB)
Title: Improvements in or relating to amplitude modulation measuring apparatus
IPC: G01R29/06
Filed: 1963-01-16
Abstract: 1,011,513. Navigational instruments. WAYNE KERR LABORATORIES Ltd., and G. G. GOURIET. Jan. 3, 1964 [Jan. 16, 1963], No. 2025/63. Heading B7W. [Also in Divisions G1 and H3] The amplitudes of two signals of different frequencies modulating a single carrier are compared without separating the signals by amplitude modulating a reference signal having a frequency equal to the mean of the first two frequencies with the combined signals and then detecting the amplitude of the output signal whose frequency is half the difference frequency. In aircraft instrument landing systems guidance in the azimuth plane is given by two radio beams having the same carrier frequency one modulated at 90 c/s. and the other at 150 c/s. The two beams overlap, and equal strength signals are received by the aircraft receiver 11 when it is on the correct path. With the invention an accuracy of comparison of 1 part in 5 to 10,000 required for a blind landing system is attained. The mean frequency (120 c/s.) signal is generated in the reference generator 12 and combined in a balanced modulator 16 with the guidance signals. As explained below two 120 c/s. signals in quadrature are provided, and a selector switch 15 normally connects the quadrature 120 c/s. signal to the modulator. It is shown by mathematical analysis of the modulator that its output will contain a signal at 30 c/s. (half the difference frequency) whose amplitude is proportional to the difference of the amplitudes being compared if the 120 c/s. reference signal has the correct phase. The phase of the reference signal is first adjusted manually (by a signal applied at 17) and then automatically by the circuit 30 which detects any phase deviation of the 30 c/s. modulator output from the 30 c/s. in phase reference signal and adjusts the 120 c/s. reference generator accordingly. The meter 21 giving the required comparison is fed through a phase sensitive circuit 22. Both phase sensitive circuits receive phase reference signals in quadrature from the reference generator through a frequency divider 23, dividing by four. The quadrature component of the 30 c/s. modulator output is indicated on a meter 31. To measure the depth of modulation the switch 15 is reversed, and an output obtained at meter 31 proportional to the sum of the original 90 and 150 c/s. signals, and hence to the depth of modulation. This may be used in a check receiver on the ground (Fig. 3, not shown) which controls the radiated signals. A convenient method is to connect a null indicator (36 in Fig. 2, not shown) in series with a calibrated attenuator to compare the output of the phase sensitive detector 30 with a signal proportional to the carrier signal at the receiver 11. The reference signal may be obtained in a variety of ways (Figs. 4, 5, 6, not shown). The 90 c/s. and 150 c/s. signals may be fed into a square law circuit, and a 240 c/s. signal extracted and passed through a frequency divider. The 99 c/s. and 150 c/s. may be separated by filters, fed into a balanced modulator and the 240 c/s. extracted. A local 240 c/s. oscillator may be used synchronized to the sum of the two input frequencies. For a ground check receiver the 120 c/s. signal may be derived from an 1800 c/s. oscillator which also, by dividing circuits, provides the 90 and 150 c/s. signals. For aircraft check receivers a 120 c/s. signal may be radiated.