Patent GB-563237-A (GB)
Title: Improvement in monitoring and measuring apparatus for frequency modulation signals
IPC: H03C3/00
Filed: 1943-03-01
Abstract: 563,237. Wireless systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. March 1, 1943, No. 3364. Convention date, Feb. 27, 1942. [Class 40 (v)] [Also in Group XXXVI] Apparatus for observing the swing in the frequency of frequency-modulated signals includes a source of such waves, a source of modulating signal waves, means for deriving from the frequency-modulated waves an electrical quantity which varies continuously as a function of the deviation of the frequency of the frequency-modulated waves from a mean frequency, and an indicator to which the electrical quantity and modulating signal waves are applied to indicate the deviation at all frequencies of the frequency-modulated waves, relatively to the amplitude of the modulating signal waves, of the frequency-modulated waves from the mean frequency. A source of frequency-modulated signals is connected to the input circuit 1 of a modulator 2, Fig. 1, a source of reference carrier frequency being connected to the input circuit 3. The output of the modulator 2 is connected through an amplifier 4 to the control electrode of a pentode 5 having high anode resistance. A high inductance 6 is included in the anode circuit of the valve 5 and the anode voltage is applied to the vertical deflecting plate terminals 7 of a cathode-ray oscillograph 11. Since the anode load of the valve 5 is substantially pure inductance, the voltage V developed across the anode circuit will be directly proportional to the frequency applied to the control electrode, so that the vertical deflection of the oscillograph is proportional to frequency. A source of signals corresponding to the modulating component of the frequency-modulated signal is connected to the terminals 8 of a variable phaseshifting circuit 9, the output of which is connected to the horizontal deflecting plate terminals 10 of the oscillograph 11, so that a timing voltage is provided which permits the instantaneous deviation in frequency of the frequency-modulated signal to be observed. In a modification, Fig. 2, the frequencymodulated signal to be observed is applied to the input terminals 20 of a modulator 21. A reference carrier frequency, preferably corresponding to the mean carrier frequency of the frequency-modulated signals is applied to the input terminals 30. The output of the modulator 21 is applied to the input terminals of two variable-gain amplifiers 23, 27, the outputs of which are connected to the control electrodes of pentodes 24, 28, which are of high anode resistance and have substantially pure inductive anode loads 25, 29. A peak voltmeter M is connected across the anode circuit of either of the valves' 24, 28 by a switch 22. Grid bias for the variable-gain amplifiers 23, 27 is obtained from a square wave limiter amplifier 33, which - is actuated through a balanced circuit 31 by signals corresponding to the modulating component of the frequency-modulated signals under observation. The output of the amplifier 33 is used to trigger the amplifier 23 on positive square-wave pulses and the amplifier 27 on negative square-wave pulses. The amplifier 23 then conducts during periods of positive deviation of frequency, the degree of deviation being indicated by the voltage developed across the inductive load 25. Similarly, the degree of negative deviation of frequency is indicated by the voltage across the load 29. Fig. 3 shows a circuit for indicating over - modulation. A source of frequency-modulated signals under observation is connected to terminals 40 and thence to the inputs of modulators 41, 43. Variable-frequency oscillators 42, 44 are connected to the inputs of the modulators 41, 43 and their frequencies are adjusted slightly above and slightly below the desired maximum and minimum limits of the frequency-modulated signal. The outputs of the modulators 41, 43 are connected through low-pass filters 45, 46 to indicators 47, 48. If the signal swings to frequencies producing beat frequencies which are passed by the filters 45, 46, one or other of the indicators 47, 48 is actuated. Specification 543,351 is referred to.