GB690410A - Improvements in or relating to keying control circuits for radio transmitters - Google Patents

Abstract

690,410. Radio transmitting systems; keying carrier frequencies. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Jan. 31, 1952 [May 10, 1951], No. 11073/51. Class 40 (v). An on-off control circuit for a radio transmitter comprises a control valve with high negative feed-back and with the on-off control voltage applied to its grid in such manner that in the " off " condition the control valve is fully conductive and provides a low impedance or short-circuit across the output leads of the carrier source, and in the " on " condition the control valve is blocked and allows the carrier to pass to the aerial or amplifier. The control voltage is provided by a key for telegraphy, and by rectified speech signals for telephony. In telegraphy, the system permits " listening through " on a nearby receiver, and in telephony a quick change-over from reception to transmission on the same wavelength. In Fig. 1, carrier output from a crystal-controlled source 2 is fed from the anode of the valve across a resistor 13 to the grid of an amplifying-valve 9. The resistor 13 is in the cathode lead of the control valve, the anode and grid of which are held at zero carrier voltage by condensers 14, 15 coupling them to an earthed screening casing SB. When the valve 5 is blocked, carrier is fed through condensers 6, 7 to the amplifier 9. When the valve 5 is unblocked by positive voltage from a source 18, it acts as a lowimpedance shunt across the anode lead from the oscillator 2 and carrier transmission ceases. The source 18 may be controlled by a key for telegraphy, or by rectified speech currents for telephony, the absence of speech currents, by removing the carrier, leaving a transmit-receive set free for the reception of signals. In a modified circuit, Fig. 3, the oscillator anode is connected through condensers 20, 21 to the grid of the control valve 5, and through condenser 20 and resistor 29 to the anode of the valve 5 which is directly connected to the grid of the carrier amplifier valve 9. The cathode of the valve 5 is earthed for carrier frequency by a condenser 25 and is held positive by a D.C. source 24. In the sending condition, the valve 5 is blocked. When, however, speech ceases, the valve 5 is unblocked by positive voltage applied to its grid from the source 18, the valve forms a low impedance shunt across the oscillator output, and the passage of anode current from the battery 27 through the resistor 26 back-biases the grid of the carrier amplifier 9. The resistor 29 should have a value approximately equal to the reciprocal of the mutual conductance of the valve 5. It may, however, be omitted, Fig. 2 (not shown). A more detailed diagram, Fig. 4 (not shown), shows the valves 2, 5 in a single envelope, and indicates an arrangement of the cathode heater circuits for the three valves 2, 5, 9.