GB813264A - Improvements in and relating to modulating circuit arrangements particularly for voice frequency telegraph systems - Google Patents

Abstract

813,264. Keying carrier frequencies. ERICSSON TELEPHONES Ltd. Oct. 7, 1955, No. 28580/55. Class 40 (5). [Also in Group XL (b)] In a modulating circuit for a carrier transmission system in which a source of pulsating current is connected to the input of a ring modulator and the output of said modulator is divided and applied to separate parts of a primary winding of a transformer whose secondary winding is connected to the carrier transmission system, the connection in one way of the poles of a direct-current source to the modulator and transformer causes pulsating current to be suppressed from the transmission system, the connection of the poles of the D.C. source in the reverse way to the modulator and transformer permits pulsating current to pass to said transmission system, and pulsating current is suppressed automatically from the transmission system at all times that the said D.C. source is disconnected from said ring modulator and transformer. When the telegraph transmitter contact is on S the applied positive potential renders the elements MR9, MR10 and MRB2 of low impedance, whilst the elements MR 11, MR12 together with the bridge MRB1 are of high impedance. The voice-frequency current applied at OS1, OS2 to the primary winding of the transformer TA flows to the primary winding of the transformer TB from which it is impressed on the windings 1, 2 of its secondary winding.. Owing to the low impedance of the rectifier bridge MRB2 current flows through the winding 3 of the transformer TC and a current flow takes place in the opposite direction through resistor R and the winding 4 so that a very low output passes from the secondary winding of the transformer TC to the leads CL1, CL2 and the passage of tone current to the carrier equipment is virtually suppressed. When the contact of the D.C. transmitter is changed over to M, the rectifiers MR9, MR10 present a high impedance and MR11, MR12 have a low impedance. Also the bridge MRB1 is of low impedance, whilst the bridge MRB2 assumes a high impedance. The current through the primary winding 3 of the transformer TC is relatively small whilst the current passing through the primary winding 4 and the bridge MRB1 shunted by a relatively high resistance R1 is considerable so that tone current passes to the secondary winding of the transformer TC and over leads CL1, CL2 to the carrier equipment. When the D.C. keying arrangement is disconnected the elements MR9 ... MR12 have equal impedance and with the ring modulator shown no appreciable current flows through the primary winding of the transformer TB and current from the tone source is unable to pass to the leads CL1, CL2 and the carrier equipment.