Patent GB-694291-A (GB)
Title: Improvements in or relating to frequency detectors
IPC: H03D9/04
Filed: 1952-04-15
Abstract: 694,291. Automatic frequency control systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. April 15, 1952 [April 18, 1951], No. 9394/52. Class 38 (iv). [Also in Groups XXXVI and XL (c)] The frequency deviations of an oscillatory wave are detected by applying the wave in push-pull to two delay lines each closed by a resistor equal to the surge impedance. Rectifiers connected to tappings on the resistors have their load impedances joined back to back, and one of the phases of the push-pull input is applied to their junction. The difference voltage taken across the load impedances is a measure of the frequency deviation. As shown, the push-pull input, derived through a band-pass filter 5, 6 from the anode circuit of an amplifier 3, is applied to two concentric lines 8, 9 closed by resistors 10, 11. Tappings 12, 13 on the resistors are connected respectively to rectifiers 14, 15, and condenser-shunted load impedances 18, 19. The wave input to one (e.g. 8) of the lines 8, 9 is applied also to the junction of the load impedances 18, 19 so as to combine vectorially with the delayed waves taken off at the tappings 12, 13, the input to the other line 9 being connected to a balancing network 20. In the vector diagram, Fig. 2, the push-pull input to the lines 8, 9 is represented at 51, 52 and phasedelayed voltages (shown as in quadrature at the mean wave frequency) developed at the tappings 12, 13 are represented at 53, 54. As the frequency deviates the delayed voltages move to 53<SP>1</SP>, 54<SP>1</SP> and their vectorial combination with the vector 52 yield resultants 55<SP>1</SP>, 56<SP>1</SP> whose amplitudes vary in opposite senses. In a modified arrangement, Fig. 3, the lines 8, 9 are of unequal length so that the vectors El, E2 representing the voltages at the tappings 12, 13 are displaced by different phase angles #1, #2 and the taps are so set that their amplitudes are approximately in inverse ratio to the phase angle ratio. As the wave frequency varies, the ends of the vectors E1, E2 lie approximately on a circle 61 of which the vector 52 is a diameter. In a particular example the angles #1, #2 may be 110 degrees and 50 degrees, and the voltage amplitudes E1, E2 in the ratio of 3 to 5. The length of the delay lines 8, 9 may be extended by any desired number of complete wavelengths. The delay lines may comprise Lecher systems, artificial lines, or wave-guides. The detected voltages across the output impedances 18, 19 may be taken through capacities and chokes to resistors 21, 22 which have their junction earthed and their outer ends connected to the grids of push-pull triodes 29, 30 operating as cathode followers with resistors and inductors in the cathode leads. The cathode of the triode 29 is connected to the grid of a further triode 38, and the cathode of 30 to the cathode of a similar triode 39 which has its grid earthed and is contained in the same envelope. The anodes of the two valves 38, 39 are connected together so that the push-pull input is connected to a single-phase output across the anode impedance 44, 45. Ta maintain symmetry, the grid 40 is connected to earth through a resistor 48 and condenser 49 in parallel ; and the cathode of the valve 38 is earthed through a coil 42 and resistor 41 in series, with a resistor 50 in shunt. The frequency discriminator may be used in receivers for frequency modulated waves, and in automatic frequency control systems.