GB1081130A - Improvements in modulation systems - Google Patents

Abstract

1,081,130. Modulating systems; automatic frequency control systems. GENERAL ELECTRIC CO. Dec. 20, 1965 [Jan. 4, 1965], No. 53865/65. Headings H3A and H3R. The output of a single sideband modulator 1 is amplitude limited and fed to a phase detector 20 controlling the frequency of an oscillator 14 feeding the transmitting aerial 16 through a class C power amplifier 15. The system has the bandwidth of a single sideband system, but since amplitude variations are suppressed and class C amplifiers are used the system has the efficiency of frequency modulation; the receiver may be a standard single sideband receiver. Single sideband modulator.- Signals from a microphone 3 are limited to a band from 300-3000 cycles by a filter 4 and fed through amplifier 5 to a balanced modulator 6 supplied with a carrier frequency fo from a crystal oscillator 7. The upper sideband is selected by a filter 8 and passed through amplifier 10 to the limiter 9 which is driven to full limiting. A part of the output of oscillator 7 is fed to limiter 9, and is swamped by the modulation signal when present; when there is no modulation the carrier is transmitted to suppress noise at the receiver. Phase-locked output loop.-The oscillator 14 has a nominal carrier frequency fc, and the output of amplifier 15 is fed to a balanced modulator 17 supplied with a carrier fr = fo - fc synthesized from a crystal oscillator 18. The output of modulator 17 is of the same frequency range as the output of modulator 1, and forms the other input of a phase detector 20. The error voltage from detector 20 is fed through a network 21 to control the frequency of oscillator 14. The network 21 is designed so that the A.F.C. loop responds at frequency rates between 300 and 3000 cycles but not at lower or higher rates; the loop thus acts as a filter and excludes undesired harmonics.