Patent GB-590476-A (GB)
Title: Improvements in or relating to apparatus for generating frequency modulated oscillations
IPC: H04B7/02
Filed: 1944-08-28
Abstract: 590,476. Wireless signalling. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 28, 1944, No. 16436. Convention date, April 18, 1941. [Class 40 (v)] [Also in Groups XXXV and XXXVI] The outputs from two sources of oscillations of different frequency and/or amplitude are applied simultaneously to an amplitude limiter, the output from the latter being a further oscillation modulated in frequency in dependence upon the frequency and amplitude differences in the two input sources. If the two amplitudes are substantially equal, the output frequency is approximately the mean of the input frequencies except for very great momentary frequency shifts at the instants of opposing phase of the two input currents. If the relative amplitudes of the input currents is reversed, the direction of the momentary frequency shift is reversed as this direction is always away from the frequency of the weaker input current towards and through the frequency of the stronger input current. The frequency shift is a maximum when the amplitudes are equal. Measurement and adjustment of transmitter frequency.-To facilitate the adjustment of the frequency of a transmitter 10, Fig. 5, to a standard frequency, a portion of its output from the variable coupling 14 and the output of a standard frequency oscillator 12 are fed to an amplitude limiter 16 whose output is fed to the frequency modulation detector 20. The amplitude peaks thus produced, which correspond to the frequency deviations in the output of 16 are displayed on the screen of a cathoderay oscillograph 26. The transmitter output is arranged to be of slightly smaller amplitude than. that from the standard source and the polarity of the peaks then indicates whether the transmitter frequency is above or below that of the standard oscillator, and the frequency difference is equal to the number of cycles visible on the screen times the oscillograph sweep frequency. The output from the frequency modulation detector may operate a relay, Fig. 6 (not shown), controlling a reversible motor which restores the transmitter tuning upon any deviation from the frequency of the standard source. The output of the latter is controlled in amplitude, by a voltage derived from the transmitter output, so as' to be of substantially a fixed ratio relative to the transmitter amplitude. Automatic gain control.-The invention may be used as an amplitude comparison device. Thus, the output from a variable gain amplifier 31, Fig. 7, feeding carrier telegraph signals to a radio transmitter 33<1>, and which is of constant frequency, is applied, together with the output from a standard oscillator O of constant amplitude and fixed, but different frequency, to the amplitude limiter 35. The output from the latter feeds a frequency modulation detector 37 and peak current rectifier 39 associated with a reverse current relay controlling an A.G.C. motor in the amplifier 31, When the amplifier output is of amplitude equal to that of the oscillator O, the detector output is zero and the relay returns to neutral, stopping the motor. Diversity reception.-In another embodiment of the invention, making use of amplitude comparison, two phase modulation superheterodyne receivers fed from spaced aerials produce signals at different but fixed intermediate frequencies, portions of which are fed over circuit 50, Fig. 8, to an amplitude limiter 54 followed by a frequency modulation detector 56. The polarity of the amplitude pulses from the latter depends on the relative amplitudes of the inputs and is used to determine which of the phase modulation detectors 48, 48<1> of the two receivers shall be operative. The electronic switches performing this function are so arranged that only signals from the receiver giving the greater amplitude at any given time are present in the common output circuit 64. Amplitude-sensitive receiver with noise suppression.-A superheterodyne receiver for on-off telegraph signals, but which can be adapted for amplitude or frequency modulated telephony and signalling, which is responsive to signals of a predetermined narrow amplitude range and thus quenches signals accompanied by noise, comprises a local oscillator 86, Fig. 9, having a fixed frequency different from the I.F. signals from the frequency changer 81, 82 and of constant amplitude. The I.F. signals and the output of the oscillator 86 are applied to the amplitude limiter 88 followed by the frequency modulation detector 90 and the amplitude pulses thus produced provide a signal of a second intermediate frequency which disappear if the amplitudes of the first I.F. signals and the output of oscillator 86 are substantially different. The pulses are rectified in 92 to provide the final receiver output signal. Suppressed carrier restoration.-In a receiver for double side band, suppressed carrier signals, the I.F. output from frequency changer 102, 104, Fig. 11, comprising the two side bands is applied to the amplitude limiter 106 followed by frequency modulation detector 108 from whose output, pulses appear which occur at the times when the phase of the combined double side band currents suddenly reverses. These pulses are used to effect reversals in the polarity of the D.C. output of circuit 110 which controls electronic switches in signal amplifiers 114, 116 which are connected to a common output circuit 120 with reversed polarities. The result is that the current in the common output 120 is equivalent in frequency and phase to the suppressed carrier, the phase reversals having been eliminated. The reconstituted carrier is passed through frequency selective circuits and an amplitude limiter 124, to the main detector 128 of the receiver, to which the side bands from 104 are also supplied. Specifications 343,680 and 489,094 are referred to.