Patent GB-624741-A (GB)
Title: Improvements in or relating to frequency shift and like signalling systems
IPC: H04N5/40
Filed: 1947-01-28
Abstract: 624,741. Radio signalling. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Jan. 28, 1947, No. 2675. Convention date, Jan. 29, 1946. [Class 40 (v)] A radio telegraphy system employing a thermionic reactance for shifting the frequency of a variable-frequency oscillator embodies the following features: (1) the keying voltage is limited by a discharge tube; (2) the variablefrequency oscillator is synchronized with a crystal monitor when half the keying voltage is applied to the reactance; (3) variation of the multiplying factor of frequency-multipliers following the variable-frequency oscillator is co-ordinated with means for varying the keying-voltage applied to the reactance, with the object of maintaining a constant frequency deviation of the radiated wave; (4) variation of the keying voltage is linked with means for re-centring the frequency of the variablefrequency oscillator; (5) switches are provided to bring the crystal monitor into circuit for calibration and to utilize it for amplitudemodulation or on-off keying. The back-coupled oscillator 60 of medium frequency is associated with a reactance valve 40 for shifting its frequency under the control of a keying voltage applied to the grid. Output from the valve 60 is amplified in the valve 70, which is cathodecoupled to a transformer T1, the secondary of which is connected in push-pull to the grids of a balanced pair of mixing valves 82, 82<1>. Oscillations of higher frequency are applied to the valve grids in phase agreement through the transformer T2 from one or other of two crystal-controlled oscillators 120, 140, selection being effected by a switch S4 in their heater circuits. The anodes of the valves 82, 82<1> are connected in push-pull to a tuned output circuit 104 which selects either the upper or lower sidefrequencies and transmits them through amplifier 130, coaxial line J3, and amplifiers and frequency-multipliers 170, to the transmitting- aerial. Keyed tone currents from a source 10 are rectified at 16 and are applied across a limiting-tube 18 to a potentiometer 20. The keying-voltage is maintained by the limiter at a steady negative value for marks and at zero for spaces. The rate of build-up is reduced by a filter comprising condenser 23, inductance 24 and a condenser C3. The keying-voltage is applied through a contact on the three-position switch S3 to the control grid of the reactancevalve 40, thereby shifting the frequency of the oscillator valve 60 in the keying rhythm, and to an extent dependent on the adjustment of the potentiometer 20. The switch S3 in its upper position completes the circuits required for frequency-shift operation. In its midposition, the crystal monitor 205 is brought into circuit for adjusting the oscillator 60 to the correct mid-frequency, and in its lower position the valves 40, 60, 70 are dis-connected and the circuit is available for amplitudemodulated telephony or on-off telegraphy. When the frequency-multiplying factor at the transmitter stage 170 is doubled, a switch S2 is operated to put in circuit a resistance pad 28 designed to halve the keying voltage, thus maintaining the same frequency deviation in the radiated wave. When the switch S3 is in mid-position to bring the crystal monitor into operation, it also connects the jtmction of equal resistances R19, R20 to the reactance valve grid which thus receives a steady voltage equal to half the marking-voltage. The monitoring oscillations mix in the valves 82, 82<1> with the oscillations generated in the valve 60. The latter are adjusted by trimmer condenser C6 or C40 until zero beat is produced, being observed by a meter M1 connected by multicontact switches 250 in the cathode lead of the mixing valves 82, 821. The remaining contacts of the switch 250 are arranged to connect the meter M1 in turn into the anode, screen-grid and control grid circuits of the various valves, and to measure the keying-current flowing in the resistances R19, R20. When the keying- voltage is varied by adjusting the potentiometer 20, or by throwing the switch S2, the midfrequency of the oscillator 60 would be shifted away from the crystal monitor frequency. To prevent this the switch S2 is linked to a switch S102, Fig. 2, on a tapped battery or source 300, and the potentiometer arm of 20 is linked to a tapping on a resistance R100 connected in series with the battery across a resistance R103 in the grid lead of the reactance valve 40. The bias on this grid is thus adjusted 'so that the mid-frequency is unchanged. The switch S3 may also be linked to a switch S103 associated with a tapping on the resistance R103. Alternatively, the tapping 20 and the switch S2 may be linked to trimming condensers in the tuned circuit of the oscillator 60 (Fig. 3, not shown).