GB718439A - Improvements in and relating to generators of high frequency alternating electric currents - Google Patents

Abstract

718,439. Thermionic valve circuits. NATIONAL RESEARCH DEVELOPMENT CORPORATION Oct. 28, 1952 [Nov. 1, 1951], No. 25583/51. Classes 40 (5) and 40 (6). [Also in Groups XXXV and XXXVI] A load circuit, such as the treatment circuit of high frequency heating or diathermy apparatus, is automatically maintained in tune with the applied oscillations, by lightly modulating the oscillations at a lower frequency, deriving and selecting the separate upper and lower side frequencies from the load circuit, and utilizing the difference between their amp'itudes to provide a voltage for adjusting the load circuit towards resonance. Fig. 1 shows a crystal oscillator 1 associated with frequency-multiplying stages 2, 3 feeding energy at e.g. 27 me/s to a push-pull amplifier 5, 6 which is coupled by a co-axial line 14 to the load circuit comprising transformer 9, tuning reactances 12, 13 and treatment electrodes 10, 11. At the push-pull stage, modulating voltage of a lower frequency, e.g. 100 kc/s, is applied from an oscillatory source 8. A coupling 18 supplies modulated osci!lations from the load circuit to a mixer valve 23 which is also fed from a heterodyne source comprising crystal 19 and frequencymultiplying stages 20, 21. The anode circuit of the valve 23 includes tuned circuits 24, 25 for selecting respective'y the upper and lower side frequencies, as reduced by the heterodyne stage 19 . . . 23. The side frequencies are rectified at 26 and fed to D.C. amplifiers 27 . . . 30, the outputs of which are supplied to the divided field coil 31, 32 of a motor 33 Any error in the tuning of the load circuit unbalances the amplitudes of the side frequencies, and starts the motor in the direction to correct the error by adjusting the condensers 12; 13: When the tuning error is so large that no side frequencies appear in the load circuit, as may occur on starting up, the motor may be arranged to run in one direction con tinuously to bring the load circuit into approximate resonance, whereupon the AFC system takes charge. Fig 2 shows a suitable construction of the motor-adjusted tuning condensers. Verticallymovab'e condenser plates 12b, 13b are carried by an upright arm guided by a pin 41 and slot, and are mounted opposite fixed plates 12a, 13a. The motor 44 has a vertical shaft carrying worm gearing with a worm-wheel on the shaft of a cam 43. A roller 42 carried by the upright arm rests on the cam sur'ace, so that rotation of the cam raises or lowers the movable condenser plates 12b, 13b. The cam surface has a long easy rise joined by a step 43a which is sloped enough to allow reverse rotation.. The motor 44 may also drive a signal disc 49 through friction gear 47, 48 to give visual indication that the motor is operat- Meters 52, 58 are associated with the co-axial line 14, one being fed through a rectifier from the iunction of resistor 50 and condenser 51 connected across the line, and the other from the junction of resistor 56 and condenser 57. The meter 52 indicates the power fed from the amplifiers 5, 6 towards the load circuit, and the meter 58 the power reflected from the load circuit. The meter 58 indicates the degree of mistuning, and the difference between the two readings the power input to the load. The power dissipated m the final circuit is measured by a coil 55 coupled to the coil 9 and connected to a rectifier 53 and meter 54, the latter being calibrated to indicate the square of the current. To avoid overloading of the amplifier 5, 6 in the event of a sudden change in load, the anode circuit includes a relay 63a which when current is excessive opens a contact 63.1 thus causing relay 62 to release the switch 62.1, 62.2 to disconnect from the mains 61 the auto-transformer 7 supplying the HT rectifier 68, 72. At the same time a contact 63.2 is closed, thereby energizing a holding-coil 63b through a normally-closed contact 64. This contact is opened by returning the slider of the auto-transformer 7 to zero position, thereby releasing the coil 63b and returning its contacts 63.1, 63.2 to the positions shown, re-energizing 62 and re-connecting 7 across the mains. A thermal switch 66 delays the closure of the circuit of relay 62.