Patent GB-679800-A (GB)
Title: Improvements relating to frequency modulation discriminators
IPC: G01S7/35
Filed: 1950-11-09
Abstract: 679,800. Frequency discrimination. ELECTRO-MECHANICAL RESEARCH, Inc. Nov. 9, 1950 [Nov. 22, 1949], No. 27438/50. Class 37. [Also in Group XL (c)] Demodulating apparatus for FM signals includes a trigger circuit to which input pulses of varying frequency are fed, the two outputs of which consist respectively of (a) pulses of constant width separated by variable time intervals, and (b) pulses of variable width separated by constant time intervals, each output being fed to a separate smoothing circuit, and the difference between the smoothed voltages being a measure of the frequency variation of the initial input pulses. The discriminator is particularly designed for use in multi-channel telemetering systems, using frequency-modulated sub-carrier waves. In Fig. 1, a frequency-modulated sub-carrier wave is selected from the radio receiver 10 by a filter 11 and after amplification at 12 is converted to a rectangular wave by a limiter 13. This wave is converted to a series of sharp pulses by a pulse generator 14 the frequency of which follows that of the received sub-carrier, and the pulses are applied to a trigger circuit discriminator 15 giving a rectangular wave output of the form shown in Fig. 3A or Fig. 3B. The outputs are smoothed and their difference is passed through a low-pass filter 16 and amplifier 17 to an indicator. The duration of one portion t1 of the wave is constant and is determined by the constants of the trigger circuit 15, whilst the duration t2 of the other portion is such that t1+ t2 is equal to 1/# where # is the momentary frequency of the sub-carrier. In Fig. 2 the pulse generator 14 comprises a differentiating network 20, 21 and the pulses are applied to the negatively-biased grid of a blocked valve 18. The negative pulses are of no effect, but the positive pulses open the valve and provide sharp negative pulses at its anode, which are applied to the trigger circuit 15 comprising a reciprocally-coupled pair of valves 27, 35. The anode of the valve 27 is coupled to the positive grid of the valve 35 by a timeconstant network including a condenser 33 and resistor 41, whilst the anode of the valve 35 is directly connected by a resistor 42 to the negative grid of the valve 27. The trigger circuit normally assumes the condition in which saturation current passes in the valve 35, and the valve 27 is cut off. When a negative pulse from the valve 18 is applied to the anode of valve 27 and through the condenser 33 to the grid of valve 35, the latter valve is immediately blocked, its anode voltage rises and the valve 27 immediately passes its saturation current. This condition persists for a constant period t1 until the condenser 33 has discharged sufficiently to re-open the valve 35, and thus cut-off the valve 27. This normal or rest condition continues for a period t2 until the next pulse arrives from the circuit 14. The pulse voltages at the respective anodes of the valves 27, 35 are shown in Figs. 3A and 3B, and these voltages are smoothed by networks 45... 48 to provide voltages E2, E3, Fig. 3C, one of which rises, and the other falls, in linear relation with the changes of frequency f of the incoming sub-carrier wave. The difference between E2 and E3 provides the de-modulated signal, and has a zero value when t1 is equal to t2. In a more detailed circuit, Fig. 4 (not shown), the anodes of the trigger valves are connected to the grids of a pair of cathode-follower stages, the cathode outputs of which are fed through smoothing-networks. These networks, together with those elements of the trigger circuits which determine the period t1, may be built as interchangeable, plug-in units designed for widely varying mean values of the sub-carrier frequency. In Fig. 5 the output terminals 105, 113 of the first cathode-follower stages are connected to a pair of screen-grid valves 107, 115 also connected as cathode followers passing the de-modulated signal through the cathode leads 124, 125 to attenuator 126 and galvanometer or recorder 127. Excess current to the devices 126, 127 is avoided by oppositely connecting the grid-cathode circuits of two valves 133, 149 across the leads 124, 125 and connecting the anodes of these valves in parallel to the control grid 138 of a further valve 132. Normally the valves 133, 149 are cut off, but if the current in leads 124, 125 becomes excessive one of the valves becomes conductive and its falling anode voltage is communicated to the grid of the valve 132. The impedance of this valve rises, and as the screen grid current of the cathode-follower valves 107, 115 passes through the valve 132, the screen voltage falls and limits the output. A dummy load 131 may be connected in place of the devices 126, 127 by operating switches 129, 130, and a thermal relay 165 protects these devices until a predetermined time has elapsed after power is applied..