Telegraph receiving apparatus

1,123,416. Radio receiving systems; telegraphy. STANDARD TELEPHONES & CABLES Ltd. 19 Nov., 1965, No. 49261/65. Headings H4L and H4P. In an alternating-current telegraph receiver including two separate signal paths, the said paths are coupled to a common impedance by a hybrid circuit arrangement so arranged that substantially no power is transferred from either path to the other. As described, the output A of a first receiving channel of a radio diversity system is connected through a band pass filter 1 to a hybrid transformer 4, by which it is divided to feed a mixer 8 in a "mark" channel 33 and a similar mixer 9 in a "space" channel 35. Similarly, the output B of a second receiving channel is divided by a hybrid transformer 11 to feed a mixer 15 in a "mark" channel 34 and a similar mixer 16 in a "space" channel 36. A common local oscillator 17 is coupled by a hybrid transformer 19 to the mixers 8, 15 and a common local oscillator 18 is coupled by a hybrid transformer 24 to the mixers 9, 16. In the hybrid transformer 4, the value of the balancing resistor 7 is so chosen in relation to the input impedance associated with the primary winding 3 and the turns ratio between the secondary 5, 6 and the primary, that there is substantially no cross-coupling between the mixers 8 and 9. Similar considerations are applied to the other hybrid transformers 11, 19, 24. With a frequency shift keying system, the input carrier frequency may be 100 kc/s, with Œ0À1 kc/s shift, the oscillators 17, 18 being set, respectively, at 94À9 kc/s and 95À1 kc/s, so that the "mark" and "space" outputs are all at 5 kc/s. The arrangement may be extended for use in a frequency-shift diplex system, where four frequencies deviated from a carrier are used, or it may be adapted for on-off keying, when the oscillator 18 would be disabled. With a non-diversity system only one hybrid transformer, 4 or 11 would be used.