Patent GB-498811-A (GB)
Title: Improvements in or relating to devices for receiving high-frequency oscillations of variable frequency
IPC: H03J7/00
Filed: 1938-07-08
Abstract: 498,811. Wireless receiving systems. NAAMLOOZE VENNOOTSCHAP PHILIPS' GLOEILAMPENFABRIEKEN. July 8, 1938, No. 20327.. Convention date, July 10, 1937. [Class 40 (v)] In a wireless system in which the frequency of a received carrier wave is subject to variation, a tuning-control voltage corresponding at all times to the instantaneous detuning of the receiver is produced and influences the tuning of the receiver so that the detuning is always small. This enables a band-pass filter of width just sufficient to pass the sidebands to be used in an amplitude-modulation system even though frequency modulation also occurs, the tuning-control voltage controlling the parameters of the filter so that it always follows the variations of the carrier. The tuning-control voltage may be produced by the type of circuit, Fig. 5, used for ordinary automatic frequency control, the time constant however being made very small, e.g. one thousandth of a second, by suitable choice of elements R3 .. R6, C3 .. C5. An automaticvolume-control voltage may be derived at points 5, 6. Fig. 6 shows the application to a "straight" receiver, the output of the control circuit 62 after passing through a filter 63 controlling the tuning of the band-pass circuit 61 and also of the circuit 62 itself. In order to get a small enough time-constant for circuit 62, the width of the band-pass filter 61 may require to be greater than that required to pass the side-bands and in this case a separate amplifier 65 having only the side-band width may be used for the main signals. In a superheterodyne receiver, the tuning-control circuit acts on the local oscillator so that the intermediate frequency remains constant. To prevent interruption of the control due to an instantaneous interfering frequency or to a momentary cessation of the carrier, there may also be provided a slower control. responsive to the mean frequency of the carrier and in a double-heterodyne-stage receiver, one control may be applied at each stage. The amplitude of the oscillations supplied to the tuning- control circuit may be made constant by quickacting A.V.C. means and this may be combined with slower A.V.C. means to prevent fading. Fig. 12 shows a double - heterodyne system for ultra-shortwave reception, the instantaneous tuning- control being applied to each stage. The amplifier 21 supplying the first frequencycontrol circuit 22 has a very wide pass-band to give a small enough time-constant, the amplifier 29 for the first intermediate-frequency has a pass-band which is narrower than that of 21 but is still much wider than the side-band width, amplifier 31 feeding the second frequency-control circuit 32 is also comparatively large, but the amplifier 39 for the second intermediate-frequency is only of side-band width. The invention may be applied to systems in which the carrier is varied at a low-frequency for secrecy purposes, and also to systems employing frequency modulation in which case the tuning-control voltage itself gives demodulated signals without any further demodulator, or phase modulation, in which case the low-frequency signal is obtained from the tuning-control voltage via a network. whose amplitude-transmitting capacity is proportional to the reciprocal of the frequency.