US2679595A - High-frequency protective circuits - Google Patents

Description

May 25, 1954 P. L. SPENCER HIGH-FREQUENCY PR OTECTIVE CIRCUITS Filed Nov. 18, 1950 wdwh Patented May 25, 1954 UNITED STATES PATENT OFFICE HIGH-FREQUENCY PROTECTIVE CIRCUITS .Percy L. Spencer, West Newton, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application November 18, 1950, Serial No. 196,434

8 Claims. 1

This invention relates to electrical circuits, and more particularly to a system for automatic pro tection of a microwave oscillator producing continuous waves.

In electron tubes used to produce microwave energy such as magnetrons, for greatest elliciency the device should work into a load of op timum value. When this is the case, the voltage standing wave ratio in the transmission line connecting the tube and the load will be found to be unity. When this is not the case, the ratio will be at some higher value. Thus the voltage standing wave ratio may be taken as an indication of match or mismatch of the load and generator. As the standing wave is created by the presence of reflected energy'in the transmission line, a device, such as a directive coupler adapted to transmit energy in one direction only and terminated in a detector, can be used to detect the presence of standing waves in the transmission line.

The effect of reflected energy on the magnetron is to bombard the cathode with electrons and heat it. If this condition is not corrected prompt- 1y, serious damage may be done to the tube. Further increase in temperature of the cathode may be prevented if the filament voltage is promptly reduced.

It is the purpose of the present invention to provide means for sensing an increase in the refiected power and in response reduce the filament power. This is done by inserting a directive coupling in the transmission line and deriving from it a control voltage that in turn controls the current through a saturating winding on the filament transformer supplying filament power to the magnetron.

Such a control system automatically maintains the cathode of the tube at the optimum temperature in spite of considerable variation in load conditions. Thus the cathodes and other structures of the tube are not exposed to excessive heat for any appreciable length of time, thus prolonging the life of these relatively expensive tubes. Other automatic control devices for micro-wave tubes, such as that disclosed in Patent No. 2,498,719, depend upon critical spacing of one or more probes within the transmission line from generator to load. The present device is independent of the phase of the standing wave and so does not have to be positioned at any particular point in the transmission line.

These features make the device particularly useful in industrial application of microwave energy where the energy is used to heat various substances presenting various loads to the generator. It is also useful in unattended microwave communication links, such as television relays where the load conditions may change suddenly in the absence of a skilled operator.

Other and further advantages of this invention will be apparent as the description thereof progresses, reference being had to the accompanying drawing, wherein the single figure is a diagrammatic representation of a protective system according to this invention.

Now referring to the drawing, the numeral I0 refers generally to the magnetron generating microwave energy that is picked up by the an.- tenna probe I I and introduced into a wave guide I2. At a convenient distance a loop I3, of the type described by H. C. Early in the November, 1946, Proceedings of the Institute of Radio Engineers at page 883 and shown schematically in Fig. 1 of that article, and in greater detail in Figs. 6 and '7, is inserted into the wave guide I2 through a slot I4. One side of the loop may be connected through a coaxial cable I5 to an incident power indicator, not shown as it is no part of the present invention. The far side of the loop is connected through a length of coaxial cable I!) to a crystal rectifier [6 that is connected to ground through a resistor I1. The crystal I6 is connected in such polarity that, as the reflected power picked up by the loop I3 becomes greater, the junction point I8 becomes increasingly negative. The point I8 is connected to an amplifier 20 which may be of any conventional design adapted to amplify a slowly varying D. C. potential. The input should be of high impedance and the output of low impedance. This output is connected in series to a saturating winding 2| on the core 22 of a filament transformer 23. The primary 24 of this transformer is connected to a source of A. C. power 25. The secondary 26 is connected to the filament 2'! of the magnetron I0.

When the magnetron I0 is operating normally into a matched. load, the voltage standing wave ratio will be unity and there will be no reflected power to be picked up by the loop I3 and detected by crystal IS. The amplifier 20 passes a normal amount of current through the winding 2| leaving the core 22 unsaturated and permitted the magnetron filament 21 to receive its normal amount of energy.

When the load changes to create a mismatch and cause reflected energy to be picked up by the loop I3 and detected by the crystal I6, the flow of current through the saturating winding 2| increases to saturate the core 22 of the filament transformer 23. This reduces the amount of power transferred to the secondary 26 and delivered to the filament 21 of the magnetron, thus reducing the temperature of its cathode.

When the load conditions return to normal, the standing wave ratio in th transmission line i2 returns to unity and there is no reflected energy to be detected and no signal at the input of the amplifier 20. This reduces the flow of current through the saturating winding 2| on the core 22 of the transformer 23 permitting full heater energy to pass to the filament 21 of the magnetron in increasing its temperature, thus maintaining the cathode of the magnetron H3 at a constant temperature under a wide range of load conditions.

Any type of directional coupler could be used in place of the loop l3 shown. Also any con venient type of rectifier could be used in place of the crystal type I6 shown.

This invention is not limited to the particular details of construction and materials described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

l. A protective system comprising a source of radio frequency energy, a heater for said source, means for delivering energy to said heater, a transmission line for coupling said source to a load, means for detecting reflected energy in said transmission line, and means for varying the energy delivered to said heater in response to the output of the detecting means.

2. A protective system comprising a source of radio frequency energy, a heater for said source, means for delivering energy to said heater, comprising a transformer with a core and a saturating winding on said core, a transmission line for coupling said source to a load, means for detecting reflected energy in said transmission line, and means for varying the energy delivered to said heater in response to the output of the detecting means, comprising a saturable core reactance in said means for delivering energy to said heater.

3. A protective system comprising a source of radio frequency energy, a heater for said source,

means for delivering energy to. said heater, comprising a transformer with a core and a saturating winding on said core, a transmission line for coupling said source to a load, means for detecting reflected energy in said transmission line,

and means for varying the energy delivered to a said heater in response to the output of the detecting means, comprising means for coupling the output of said detector to the said saturating Winding.

4. A protective system comprising a source of radio frequency energy, a heater for said source, means for delivering energy to said heater, a transmission line for coupling said source to a load, means for detecting reflected energy in said transmission line, said means comprising a rectifier coupled to said transmission line through a directional coupling means, and means for varying the energy delivered to said heater in response to the output of the detecting means.

5. A protective system comprising a source of radio frequency energy, a heater for said source, means for delivering energy to said heater, comprising a transformer with a core and a saturating winding on said core, a transmission line for coupling said source to a load, means for detecting reflected energy in said transmission line, said means comprising a rectifier coupled to said transmission line through a directional coupling means, and means for varying the energy delivered to said heater in response to the output of the detecting means, comprising a saturable core reactance in said means for delivering energy to heater.

6. A protective system comprising a source of radio frequency energy, a heater for said source, means for delivering energy to said heater, comprising a transformer with a core and a saturating winding on said core, a transmission line for coupling said source to a load, means for detect- 25 ing reflected energy in said transmission line, said means comprising a rectifier coupled to said transmission line through a directional coupling means, and means for varying the energy delivered to said heater in response to the output of .0 the detecting means, comprising means for coupling the output of said detector to the said saturating Winding.

'7. A protective system comprising a source of radio frequency energy, an energy control for 35 said source, means for delivering energy to said energy control, a transmission line coupling said source to a load, an alternating current directional coupler connected to said transmission line for detecting substantially only reflected energy 4.0 in said transmission line, and means for varying said energy control in response to the output of said detecting means.

8. A protective system comprising a source of radio frequency energy, an energy control for said source comprising a saturable core reactance, means for delivering energy to said energy control, a transmission line coupling said source to a load, an alternating current directional coupler connected to said transmissionline for detecting substantially only reflected energy in said transmission line, and means for varying said energy control in response to the output of said detecting means.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,930,545 Wensley Oct. 17, 1933 2,149,080 Wolff Feb. 28, 1939 2,175,694 Jones Oct. 10, 1939 2,236,195 McKesson Mar. 25, 1941 2,262,044 Philpott Nov. 11, 1941 2,386,040 Edwards Oct. 2, 1945 2,408,091 Olesen Sept. 24, 1946

Images