GB628210A - Improvements in wireless receiving systems for carrier shift telegraphy - Google Patents

Abstract

628,210. Frequency modulation; automatic tuning control. HILL, R. F. Sept. 17, 1947, No. 25356. [Class 40 (v)] A radio receiver for frequency shift telegraph signals comprises a local oscillator and a first . mixing stage, and a further oscillator which supplies energy at the intermediate frequency to a second mixer stage which is also fed with the intermediate frequency from the first mixer stage, the resultant output from the second mixer stage being used to control the frequency of the local oscillator. In the figure, energy from aerial 1 is applied after amplification, to mixing stage 3, together with energy from a local oscillator 4. The output from mixer 3 is applied via intermediate frequency amplifier 5 and limiter stage 6, to a second mixing stage 7, together with energy at intermediate frequency. from another oscillator 8, which is crystal controlled. The output from mixer 7 is applied through a low-pass filter 10 to a thermionic reactance 11 to control the frequency of oscillator 4. The arrangement compensates for frequency drift and also varies the local oscillator during signals such that the " mark " and " space " intermediate frequencies are the same, thereby enabling a narrow pass-band intermediate frequency filter to be used. Automatic searching for signals is provided by means of a saw-tooth generator 12 which is coupled to reactance 11, and is controlled by a voltage from rectifying unit 13, such that oscillator 12 stops operating on receipt of an incoming signal. The output from filter 10 is applied directly, and also through a long time constant circuit 16, to a double triode valve 15, such that the slowly varying D.C. voltage from filter 10 is balanced in the combined output circuit of valve 15, whereas the fluctuation of voltage due to the keying signals produce an output which operates relay 18.