US2844725A

US2844725A - Pulsed oscillator system

Info

Publication number: US2844725A
Application number: US586965A
Authority: US
Inventors: Thomas J Ryan
Original assignee: Philco Ford Corp
Current assignee: Space Systems Loral LLC
Priority date: 1956-05-24
Filing date: 1956-05-24
Publication date: 1958-07-22
Anticipated expiration: 1975-07-22

Description

July 22, 1958 T. J. RYAN 844,725

' PULsED oscILLAToR SYSTEM FiledMay 24, 19,56

United States Patent C) PULSED oscrLLAToR SYSTEM Thomas J. Ryan, Langhorne, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsyl- .valia Application May 24, 1956, Serial No. 586,965

8 Claims. (Cl. Z50-36) The present invention relates to signal generating systems and more particularly to systems for generating short bursts or pulses of radio frequency energy.

Pulsed magnetrons have been used for a number of years to generate short pulses of microwave frequency energy. Pulsed magnetrons are normally operated in a relatively strong magnetic ield, for example of the order of 5,000 to 7,000 gauss. The amplitude of the pulse applied between the anode and the cathode to cause the pulsed magnetron to oscillate may be of the order of 5,000 to 20,000 volts. Special oxide coated cathodes are employed in order to supply the high peak cathode current during the generation of the pulse. The peak output power of a pulsed magnetron is of the order of several kilowatts. Pulses having a duration of a fraction of microsecond may be generated with tubes of the type just described. Pulsed magnetron tubes are large and expensive. The modulator circuits required to drive these tubes are also large and expensive. Continuous wave or C. W.. magnetrons have been employed for a number of years to generate continuous oscillations at microwave frequencies. C. W. magnetrons normally operate with a relatively weak magnetic eld, for example of the order of 1,000 gauss, and with a fixed anode potential of the order of 500 volts. The power output of a typical C. W. magnetron is of the order of one watt.

The anode structure and other physical characteristics of C. W. magnetrons are generally similar to the `anode structure and corresponding physical characteristics of pulsed magnetrons. This fact, coupled with thel fact that the low power C. W. magnetrons and power supplies of the order of 500 volts are smaller and cheaper than high power pulse tubes and power supplies of the order of several kilovolts, has led to attempts to `construct low power pulse systems employing C. W. magnetrons as the microwave pulse generator. C. W. magnetrons have the further advantage over pulsed magnetrons that they may be operated at muchhigher duty cycles. Pulsed magnetrons usually are limited to duty cycles of the order of .001 while C. W. magnetrons may be operated with duty cycles `approaching 1.0. However, in many instances it has been found that C. W. magnetrons will not oscillate in response to short duration pulses having amplitudes equal to or even slightly in excess of the normal operating potential of the tube. The inability of the C. W. magnetron to respond to pulses having a duration of a few microseconds or less is believed to be due to the fact that the normal oscillation build up time in a C W. magnetron is generally rather long, for example of the order of a millisecond to several hundred milliseconds in some commercially available C. W. magnetrons. This build up time should be compared to the oscillation build up time of a pulsed magnetron which may be of the order of a few millimicroseconds. The oscillation build up time is believed to be related to the noise generating properties of the magnetron, and the fast'build up time of pulsed magnetrons is assumed to result from the generationv of' random noise in response to the leading edge of the modulating pulse. This random noise contains some components at the proper frequency to pre-excite the cavities of the pulsed magnetron. For reasons which are not entirely clear, C. W. magnetrons cannot be caused to exhibit a corresponding fast build up time without going to high magnetic ields and high peak anode Voltages. This cannot be done without redesigning the tube to withstand the high peak voltages, that is without rebuilding the C. W. magnetron into a pulsed magnetron. It is not known whether this limitation applies to all C. W. magnetrons which are commercially available. but it is believed that it exists in most such tubes.

I have discovered a novel circuit which overcomes the above-described limitation of C. W. magnetrons and which permits C. W. magnetrons to be operated as pulsed magnetrons at relatively low anode potentials and ield strengths. The C. W. magnetrons may be operated over a wide range of duty cycles, for example from very low duty cycles such as .001 to very high duty cycles approaching 1.0.

Therefore it is an object of the present invention to provide a novel circuit for pulsing C. W. magnetron tubes.

A further object of the present invention is to provide a circuit in which the amplitude of the modulating pulse supplied to a C. W. magnetron may be less than the normal anode potential of the C. W. magnetron.

Still another object of the present invention is to provide a simple economical circuit in which the oscillation build up time of a C. W. magnetron is greatly reduced.

A further object of the present invention -is to provide a simple economical circuit for causing the oscillation build up time of a C. W. magnetron to approach that of a pulsed magnetron.

These and other objects of the invention are realized by providing a circuit which impresses a D. C. voltage, hereinafter referred to as the oscillation enhancement voltage, on the C. W. magnetron anode. This enhancement voltage is made less than the voltage which will cause the C. W. magnetron to oscillate.. A modulating pulse equal to or slightly greater than the diierence between the normal anode potential of the C. W. magnetron and the enhancement voltage is superimposed on the enhancement voltage at such times as the C. W. magnetron is to generate microwave oscillations.

For a better understanding of the present invention together with other yand further objects thereof reference should now be made to the following detailed description which is to be read in conjunction with the accompanying drawing in which:

Fig. l is a schematic diagram of a preferred form of pulse modulator circuit;

Fig. 2 is a plot showing a typical anode current vs. anode voltage characteristic for a C. W. magnetron; and

Fig. 3 is .a plot magnetron anode voltage as a function of time for the circuit of Fig. 1.

In Fig. l the cathode of magnetron 12 is connected to ground. The anode of magnetron 12 is connected toa source of anode supply potential, represented by the symbol B+, through the secondary winding 14 of a pulse transformer 16. In the circuit of Fig. l it is preferable to operate the anode above ground potential in order to minimize the number of power supplies required. In many C. W. magnetrons the shell is not connected to the anode so that it is possible to operatel the anode above ground potential without placing the shell above ground potential. If the particular C. W. magnetron employed has an exposed anode or a connection between the anode and the shell and it is undesirable to operate the anode or the shell above ground potential, it will usually be possible to follow the practice employed .in pulsed magnetrons and operate the anode at ground potential and apply negative potentials tothe cathode.= The circuit modifications required will be obvious to anyone skilled in the art so no further explanation of this modication will be given herein.

The circuit shown for pulsing magnetron 12 comprises a pentode amplifier tube 18 which has the primary winding 20 of transformer 16 as an anode load impedance. As shown in Fig. l the primary winding 20 is also returned to the supply represented by the symbol .B+. The remaining connections to tube 18 are conventional. The suppressor grid is connected to the cathode. The resistor capacitor network 24 is provided for maintaining the screen grid at a fixed positive potential. A resistor capacitor coupling network 26 provides means for supplying positive pulses to the control grid of tube 18. The resistor in coupling network 26 is returned to a negative bias source represented by the minus sign in Fig. l. The radio frequency signal generated by the circuit of Fig. l appears at output connection 28 which is coupled to the anode cavity of C. W. magnetron 12 in an appropriate manner.

The operation of the system of Fig. 1 will be explained with 'reference to Figs. 2 and 3. As shown in Fig. 2, the relationship between the anode current and anode voltage of magnetron 12 is represented by the curve 32. Radio frequency oscillations are generated by magnetron 12 only if the anode potential is above the knee 34 in curve 32. It is generally assumed that, for anode potentials below the knee 34, the magnetron will operate in a so-called noise mode. That is, it is generally assumed that a magnetron operated below the knee 34 will generate random noise signals which will appear at the output circuit even though the magnetron will not generate sustained oscillations at the low anode potentials.

The presence of these noise signals is usually highly objectionable, particularly in pulsed type systems, since the noise generated in the magnetron may obscure received signals of small amplitude. However, I have discovered that, contrary to the usual assumption, many C. W. magnetrons do not generate noise signals even when the anode potential is only slightly below the knee 34 of curve 32. I have also discovered that C. W. magnetrons which normally have a relatively long oscillation build up time, if pulsed from zero potential to a potential above= the knee 34 of curve 32 have a relatively short oscillation build up time, that is, a build up time of the order ot20 millimicroseconds if pulsed from a potential just below the knee 34 of curve 32 to a potential above the knee of this curve. Again this is true even though the C. W. magnetron may `not generate any appreciable noise signals for anode potentials below the knee 34 of curve 32. Therefore, the anode supply potential for magnetron 12 is selected to have a value slightly below the knee 34 of curve 32 as shown by the point on the voltage axis labeled B+.

Tube 18 of Fig. 1 is normally held below anode current cutol by the negative bias supply associated with coupling network 26. However, positive pulses supplied by way of network 26, at the times that pulses of radio frequency energy are to be generated by C. W. magnetron 12, cause tube 18 to conduct heavily. The conduction through primary winding 20 induces a signal in secondary winding 14. The polarities of

windings

14 and 20 are so arranged that the potential appearing across winding 14 is in a direction to add to the anode supply potential. Therefore the operating point of magnetron 12 is moved to a point above knee 34 for the duration of the pulse supplied by transformer 16.

Fig. 3 is a plot of the anode potential of magnetron 1 2 as a function of time. The dashed line 40 represents the potential of the anode supply source represented by the symbol B+. The broken line 42 represents the minimum anode supply potential necessary to generate sustained oscillations. Curve 44 is a plot of the actual potential at the anode of magnetron 12 if pulses of time duration D are supplied to the control grid of tube 18 at times spaced apart by intervals T. It should be noted that the pulses suppliedby winding 14 cause the anode need be set on the maximum 'amplitude of the anode' potential.

It has been observed that certain C. W. magnetrons exhibit a relatively sharp cathode saturation effect. This characteristic causes the amplitude of the oscillatory signal to be independent of anode potential once the anode potential exceeds the value which will cause saturation. The separation between the anode potential which will permit oscillation and the anode potential which will result in cathode saturation may be as small as l0 volts for some tubes. This effect causes the oscillatory pulses to have a constant amplitude throughout each pulse even though the pulse supplied by transformer 16 is not a square-topped pulse.

It should be noted that the pulse supplied by winding 14 is only a fraction of the total anode supply potential. In actual practice the pulse amplitude may be less than one-fifth of the potential represented by line 42. This fact makes it possible to employ a relatively low power tube in the pulse generating circuit. Ten watts of microwave power have been obtained using a subminiature tube for tube 18.

The oscillation present in curve 44 following each of the pulses is the well known ringing effect caused by stray capacitance associated with transformer 16. It has been found that this ringing does not result in the generation of noise signals in the circuit of the present invention provided maximum amplitudes of the positive peaks do not approach the level represented by line 42. This ringing may be controlled by controlling the damping of transformer 16 or by other means well known to persons familiar with pulsed magnetron circuits.

The power required from the pulsing circuit may be further reduced if the period of the oscillation or ringing in curve 44 is adjusted so that the pulse supplied by winding 14 occurs in time coincidence with a positive peak of the oscillation in anode potential. This is easily accomplished at relatively high repetition rates by designing the pulse transformer to have the appropriate inductance and capacitance.

Y The presence of the steady D. C. potential on the anode of magnetron 12 in the interval between pulses results in back-heating of the cathode of magnetron 12. Therefore the filament power supplied to the magnetron may be reduced as soon as anode potential is supplied to the tube. Insome types of C. W. magnetrons the filament power may be entirely removed once the cathode has reached its proper operating temperature. As the above description indicates, the circuit of Fig. l differs from conventional pulse magnetron circuits in the presence of the D. C. supply connected to the anode of magnetron 12. Also, as pointed out above, the advantages of this novel circuit have been made apparent by applicants discovery that the oscillation starting time of a C. W. magnetron may be materially reduced if a steady D. C. potential, herein designated as the oscillation enhancement voltage, is supplied to the magnetron, and also the discovery that the application of the oscillation enhancement voltage would not cause the generation of noise in the output of the C. W. magnetron.

. A circuit having the following parameters has been tested and found to operate in a highly satisfactory manner. However, it is to be understood that these parameters are given only by way of example and the invention is not to be limited thereby.

While the invention has been described with reference to a single embodiment thereof, it will be apparent that various modications and other embodiments thereof will occur to those skilled in the art within the scope of the invention. Accordingly I desire the scope of my invention to be limited only by the appended claims.

What is claimed is:

l. A radio frequency pulse generator circuit comprising a magnetron having an anode and a cathode, said magnetron having the characteristic that it generates substantially no radio frequency energy at anode potentials below a first value and generates sustained oscillations at anode potentials within a selected range of potentials, the upper and lower limits of.said selected range being greater than said first value, a source of anode supply potential, a pulse transformer having a primary winding and a secondary winding, the secondary winding of said pulse transformer being connected in series circuit with said source of anode supply potential, said series circuit being connected between said anode and said cathode of said magnetron, the potential supplied by said source of anode supply potential being less than said first value, and means for supplying a signal to theprimary of said pulse transformer, thereby to produce a voltage pulse across said secondary winding, the sum of the potential supplied by said source of anode supply potential and the potential appearing across said secondary winding of said pulse transformer being equal to a potential in said selected range.

2. A radio frequency pulse generator circuit comprising a magnetron having an anode and a cathode, said magnetron having the characteristic that it generates substantially no radio frequency energy at anode potentials below a first value and generates sustained oscillations at anode potentials above a second value which is greater than said first value, a source of anodesupply potential, a pulse transformer having a primary winding and a secondary winding, the secondary winding of said pulse transformer being connected in series circuit with said source of anode supply potential, said series circuit being connected between said anode and said cathode of said magnetron, the potential supplied by said source of anode supply potential being less than said first value, and means for supplying a signal to the primary of said pulse transformer, thereby to produce a voltage pulse across said secondary winding, the sum of the potential supplied by said source of anode supply potential and the potential appearing across said secondary winding of said pulse transformer being greater than said second value.

3. A radio frequency pulse generator circuit comprising a magnetron having an anode and a cathode, said magnetron having the characteristic that it generates substantially no radio frequency energy at anode potentials below a first value and generates sustained oscillations at a second value of anode potential which is greater than said first value, a source of anode supply potential, a pulse transformer having a primary winding and a secondary winding, the secondary winding of said pulse transformer being connected in series circuit with said source of anode supply potential, said series circuit being connected between said anode and said cathode of said magnetron, the potential supplied by said source of anode supply potential being less than said first value, and means for supplying a signal Yto the primary of said pulse transl former thereby to produce a voltage pulse across said secondary winding, the sum of the potential supplied by said source of anode supply potential and the potential appearing across said secondary winding of said pulse transformer being equal to said second value.

4. A radio frequency pulse generator circuit comprising a magnetron havingan anode and a cathode,'said magnetron having the characteristic that it generates substantially no radio frequency energy at anode potentials below a first value and generates sustained oscillations at anode potentials above a second value which is greater than said first value, a first source of anode supplypotential, a pulse transformer having a primary and a secondary winding, one terminal of said primary winding and one terminal of said secondary winding being connected to a first terminal of said source of anode supply potential, a second terminal of said secondary winding being connected to the anode of said magnetron, a sec- Ond terminal of said source of anode supply potential being connected to the cathode of said magnetron, an electron tube having at least an anode, a cathode and a -control grid, the anode of said electron tube being connected to a second terminal of said primary winding, the cathode of said electron tube being connected to said second terminal of said source of anode supply potential and means associated with said control grid for periodically causing said vacuum tube to be rendered conductive, the potential supplied by said source of anode supply potential being less than said first value, the sum of the potential supplied by said source of anode supply potential and the potential appearing across said secondary winding as a lresult of said conduction through said electron tube being greater than said second value.

5. A radio frequency pulse generator circuit comprising a magnetron having an anode and a cathode, said magnetron having the characteristic that the anode current increases relatively slowly for unit increases in anode potential below a first value of anode potential and increases relatively rapidly for unit increases of anode potential above said first value, said magnetron being further characterized vby the fact that said magnetron generates substantially no radio frequency energy at anode potentials below a rst value and generates sustained oscillations at a second value of anode potential which is greater than said first value, a source of anode supply potential, the potential supplied by said source being less than said first value by a small amount, a pulse transformer having a primary winding, one terminal of said primary winding and one terminal of said secondary winding being connected to the positive terminal of said source of anode supply potential, means connecting the cathode of said magnetron to the negative terminal of said source of anode potential, a second terminal of said secondary winding being connected to the anode of said magnetron, an electron tube having at least an anode, a cathode and a control grid, the anode-cathode circuit of said electron tube being connected between a second terminal of said primary winding and said second terminal of said source of anode supply potential and means associated with said control grid of said electron tube for rendering said electron tube periodically conductive, the sum of the potential supplied by said source and the pulse potential supplied by said secondary winding of said pulse transformer during the intervals that said electron tube is conducting being equal to said second value.

6. A radio frequency pulse generator circuit comprising a magnetron having an anode and a cathode, said magnetron having the characteristic that oscillations are generated only at values of anode potential above a first value, a source of anode supply potential, the potential supplied by said source being less than said rst value by a small amount, a source of substantially unidirectional 7 pulse signals connected in series with said source ofY anode supply potential, the polarity of said pulse signals being such as to, increase directly the potential appearing across the terminals of said series combination upon the occurrence of each of said pulse signals, said series vcombination being connected between said anode and said cathode of said magnetron, the sum of the potential supplied by said source of anode supply potential and the peak potential of said pulse signals being at least equal to said second value, whereby said magnetron is caused to generate oscillations in response to the application of each of said pulse signals, said oscillations having a relatively rapid build-up rate.

7. A radio frequency pulse generator circuit comprisinga magnetron having an anode and a cathode, said magnetron having the characteristic that the anode current increases relatively slowly for unit increases in anode potential below a first value of anode potential and increases relatively rapidly for unit increases of anode potential above said lirst value, said magnetron being further charaeterized by the fact that said magnetron generates substantially no radio frequency energy at anode potentials below a iirst value and generates sustained oscillations at a second value of anode potential vwhich is greater than said first value, a source of anode supply potential, the potential supplied by said source being less than said first value by a small amount, a source of substantially unidirectional pulse signals connected in series with said source of anode supply potential, the polarity of said pulse signals being such as to increase directly the potential appearing across the terminals of said series combination upon the occurrence of each of said pulse signals, said series combination being connected between said anode and said cathode of said magnetron, the sum of the potential supplied by said source of anode supply potential and the peak potential of said pulse signals being at least equal to said second value, whereby said magne- Y g tron is caused to generate oscillations in response to `the application of each of said pulse signals, said oscillations having a relatively rapid build-up rate.

8. A radio frequency pulse generator circuit comprising a magnetron having an anode and a cathode, said magnetron having the characteristic that the anode curt rent increases relatively slowly for unit increases' in an- `ode potential between a rst value of anode potential and a second value of anode potential and increases relatively rapidly for unit increases of anode potential above said second value, said magnetron being further characterized bythe fact that it generates substantially no radio frequency energy at anode potentials below said second value and generates sustained oscillations at a third value of anode potential which is greater than said second value, a source of anode supply potential, the potential supplied by said source being less than said second value by a small amount, a source of substantially unidirectional pulse signals connected in series with said source of anode supply potential, the polarity of said pulse signals being such as to increase directly the potential appearing across the terminals of said series combination upon the occurrence of each of said pulse signals, said series combination being connected between said anode and said cathode of said magnetron, the sum of the potential supplied by said source of anode supply potential and the peak potential of said pulse signals being at least equal to said third value, whereby said magnetron is "caused to generate oscillations in response to the application of each of said pulses, said oscillations having a relatively rapid build-up rate.

References Cited in the file of this patent UNITED STATES PATENTS 2,507,351 Sherbatskoy May 9, 1950 2,572,707 Elie OCt. 23, 1951