GB698018A - Improvements in or relating to frequency modulated communication systems - Google Patents

Abstract

698,018. Automatic frequency control systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Oct. 9, 1951, No. 23513/51. Class 38 (iv). [Also in Group XL (c)] A self-excited high-frequency re-transmitter 14 of a radio relay station is constrained to follow the frequency modulation of the wave received at the station, by heterodyning both waves to the same mean intermediate frequency and utilizing the phase variation between these intermediate frequencies to regulate the instantaneous frequency of the self-excited source 14. As shown, the F.M. wave received on the aerial 10a is mixed at 10 with a heterodyne source 11, and the resulting IF is amplified at 12 and applied as one input to a phase-comparing balanced bridge 13. The other input to the bridge is an I.F. derived by mixing the output of the self-excited magnetron 14 with a second heterodyne source 17 in the mixer 16. The bridge output is applied to a frequency control element 15 associated with the magnetron 14; this element may comprise a thermionic reactance valve, or, as shown, a splitanode magnetron 40 operating as a variable capacity coupled into the cavity of the magnetron oscillator 14 through the loop 38. The change in the mean frequency of the relayed F.M. wave is determined by the difference between the frequencies of the heterodyne sources 11, 17. These may be independent crystal-controlled sources, or their frequencies may be derived by frequency conversion or multiplication from a single source. If the output carrier from the source 14 is of the same frequency as the input carrier at 10a, the same source 11 serves for heterodyning both.