US2817820A - Frequency-modulated communication systems - Google Patents

Description

Dec. 24, 1957 R. E. HC JUSE 2,817,820

FREQUENCY-MODULATED COMMUNICATION SYSTEMS Filed Dec. 31, 1952 2 Sheets-Sheet 1 a I MIL-v ul: I

- /NVEN To/2 ROBE/2T 5. House 1 'eygg ATTo/NEv Dec. 24, 1957 v R. E. HOUSE FREQUENCY-MODULATED COMMUNICATION SYSTEMS Filed Dec. 31, 1952 2 Sheets-Sheet 2 ANODE CURRENT /N MILLMMPERES 2% w. mu m N Na Q EH m WE OT T A i w PV. 5

2,8 1 7,820 Patented Dec. 24, 1957 United States Patent Ofiice FREQUENCY-MODULATED COMMUNICATION SYSTEMS Robert E. House, Quincy, Mass, assignor to Raytheon Manufacturing Company, Newton, Mass, a corporatioh of Delaware Application December 31, 1952, Serial No. 328,927

2 Claims. (Cl. 332-) This invention relates to frequency-modulated communication systems, and more particularly to those utilizing an electrically-tunable magnetron.

Electrically-tuned magnetrons are well known in the art. In such magnetrons, the output frequencyis adjusted by injecting electrons into the resonator Cavities of the magnetron. However, when these devices are used in circuits, they produce undesired interfering modulation components on the output carrier in addition to the desired intelligence modulation components applied to the modulating circuit of the magnetron.

It has been found that the undesirable interference modulation components result in part from operating instability in the magnetron anode circuit which allows the magnetron a choice of operation at a plurality of dif ferent points in the magnetron anode-voltage versus anode-current curve. However, at a number of these points, the magnetron may operate in a regien of discontinuity .in which the oscillations shift rapidly between two pi modes in a manner to generate undesired spurious oscillations. In accordance with this invention, however, it has been found that, if the magnetron is not given such a choice but is restricted in its possible operations to points on the anode-voltage versus anode-current characteristics at which such discontinuities do not occur, the above undesirable result is eliminated. The desired manner of thus restricting the operation of the magnetron is to supply it from a source of constant current. Such regions of instability exist whether or not the magnetron is of the frequency modulated type or is tunable. Thus the use of such a constant current source isuseful in stabilizing and improving the conditions of operation substant-ially in all types of magnetrons in which such regions of discontinuity exist. p

This invention discloses a specific constant current circuit for electrically-tunable magnetrons. Specifically, the circuit comprises a grid-control electron discharge device in series with the magnetron anode supply'and the mag ne-tron with the control grid of this tubebeing returned to its cathode through a cathode load resistor through which flows the anode current of both the magnetron and the said device. The heater supply for the cathode of the magnetron is made in a manner to introduce as little capacity into the circuit. as possible. The heater supply for the magnetron. is returned. tothe cathode of the series tube through a capacitor, resulting in a reduction of the effective stray capacitances by a factor substantially equal to l-I-the gain of the electron discharge device. The reduction of the stray capacitances allows the anode supply circuit toact as a substantially constant current generator over a relatively wide range of modulation frequencies, for example, up to ten megacycles. The constant current effect can also be obtained by introducing an inductance between the series tube and the magnetron cathode.

This invention also includes the use of a heater for the main cathode of the magnetron which is not directly vention Will become apparent as the description thereof progresses, reference being had to the accompanying drawings, wherein:

Fig. 1 illustrates a circuit embodying the invention;

Fig. 2 illustrates a partially broken away view of an gectrzically-tunable magnetron of the type indicated in Fig. 3 illustrates the graph of the operating characteristics of the magnetron illustrated in Fig. 2; and

Fig. 4 illustrates a second circuit embodying the invention.

In Fig. 1, reference numeral 10 designates generally a magnetron comprising a main anode structure 11 connected by an output coupling 12 through a coaxial line 13 to an antenna 14. The magnetron 10 has a main cathode 15 which is heated by a heater 16 supplied from the secondary 17 of a transformer 13 having a minimum of capacity between its windings. The primary 20 of this transformer is connected to a suitatble source of low frequency alternating current. A suitable magnetic field 1S lmpressed across the magnetron 10 by a permanent magnet 21. The value of this field is selected so that, when a suitable potential is applied between the anode 11 and the cathode 15, the magnetron 10 will oscillate at a frequency determined by the resonant frequency of the anode cavities.

The desired anode potential is applied to the magnetron 10 by connecting the cathode is to the plate 22 of a beam power amplifier tube 23, such as a type 4D32. The cathode 24 of the tube 23 is connected to a source 25 of negative potential through a resistor 26. The cathode 24 is also coupled to one end of the secondary 17 of the transformer 18 through a capacitor 27 and to the screen grid 28 of the tube 23 through a capacitor 30. The capacitor 27 serves to couple the stray capacities of the system represented by the capacitor 29 back to the cathode 24. The screen grid 23 of the tube 23 is connected through a current-limiting resistor 31 through voltage regulator tubes 32 and 33 connected in series to the negative terminal of the source 25 of negative potential. The junction 34 between current-limiting resistor 31 and the. voltage regulator tube 32 is connected to the positive terminal of the source 25 through a voltage dropping resistor 35. The polarity of the voltage regulator tubes 32 and 33 are so arranged that the junction 34. is maintained positive with respect to the negative terminal of the voltage source 25 by the combined voltage regulating drop of the voltage regulator tubes 32 and 33 with the remainder of the negative high voltage supply appearing across resistor 35. Voltage regulators 32 and 33 are shunted by resistors 36 .and 37 connected in series. The junction of these two resistors is connected to the junction between voltage regulators 32 and 33, thereby insuring substantially stable operation of this voltage re ulating system. negative terminal of the source 25 through a potentiometer 40 having an adjustable tap 4-1 which is connected to the control grid 4-2 of the tube 23 and to the negative terminal of the source 25 through a bypass condenser 43. The positioning of tap 41 adjusts the current through tube 23 and hence the point of operation of magnetron 10 on its characteristic curves.

In order to vary the oscillating frequency of the magnetron 10, there is provided an auxiliary source for di.

lated from the main cathode 15 and having asseparate The junction 34' is also connected to the s3 heater 45 which is connected to a suitable low frequency heater current supply. The auxiliary cathode 44 is connected to the anode 11 through a modulator 46.

Fig. 2 shows the detailed construction of a representa two form of electronically-tunable magnetron that can be used as the magnetron in the circuit shown in Fig. 1. The anode structure 11 comprises a plurality of anode members 50 which extend inwardly from a metallic anode cylinder 5.1 comprising a portion of the envelope of the magnetron. The inner ends of the anode member 50 define a cylindrical space in which is positioned the main cathode is of the magnetron. The main cathode is supported at its lower end by a lead-in support member 52 which extends upwardly through an opening in a lower end plate 53 which is sealed to the anode cylinder 51 and through a lower magnetic pole piece 54 positioned in the aperture in the plate 53. The support member 52 is hermeticallysealed to the pole piece" 54 by an insulated seal 55, partially shown in Fig. 2.

i The escape of energy generated in the magnetron, throug the aperture adjacent to the cathode lead-in support 52, is prevented by a choke member surrounding the lead-in member 52 with the inside pole piece 54 spaced therefrom. A heater structure 16 is positioned within the cathode 15 and is insulatedly supported at the upper end by an insulating member 57. The heater 16 is covered with insulating material, such as alundum, to prevent shorting of the heater 16 to the cathode 15, as well as to prevent shorting between turns of the heater. The ends of the heater at the lower end of the cathode 15 are connected to separate lead-in members 58 which extend downwardly through a cathode support 52 spaced therefrom by an insulator 60 and extend out beyond the end of the cathode support 52 through insulating seals (not shown). An output coupling loop 12 is connected to the anode structure 11. and fits an output coaxial line 13. The anode members are alternately connected on their upper edges adjacent to their inner ends by a pair of conductive straps 61, according to a well-known practice.

Means are also provided for mechanically tuning the magnetron in order to adjust the center frequency of the device. The tuning means comprises a cylindrical member 62 adapted to be inserted between the straps 61, thereby varying the capacitance at the inner ends of the anode members 50, and as a result varying the resonant frequency of the magnetron anode cavities. A flexible her metic seal is provided between the tuning cylinder 62 and the anode by means of a diaphragm 63 sealed at its inner edge to the cylinder 62 and at its outer edge to an upper metallic cover plate 64 which in turn is sealed to the anode cylinder 51. The cylinder 62 is attached to a cylinder support portion 65 which extends up through and slidably engages an upper magnetic pole piece 66 which is positioned in an aperture in the upper end plate 64. A threaded stub portion 6'7 extends outwardly beyond the upper end of the pole piece 66 and engages a mechanical means (not shown) for adjusting the position of the tuning cylinder 62. The permanent magnet 21 engages the pole pieces 66 and 54, thereby creating the desired magnetic field in the interaction space between the. main cathode 15 and the tip of the anode members 50.

The electronic tube cathode comprises a flat disk 68 surrounding the main cathode 15 and spaced therefrom below the anode members 51. The upper surface of the disk 68 is coated with an electron-emissive material. Positioned below the disk 68 is the heater coil 42 for the cathode 41, said heater coil being insulatedly supported in an annular recess 70 stuck downward into a plate 71 positioned below the disk 70 and rigidly attached thereto as by welding. The cathode 4-1 is supported by means of straps 72 con nected between plate 71 and support pins 73 which ex tend downwardly to apertures 74 in the lower end plate 53 outside the magnetic pole piece 54. The support pins '73 are set in insulating members 75 positioned in metallic cups '76 which are, in turn, sealed to recesses in the lower end plate 53 surrounding the aperture 74. The lead-outs A. for the heater 42 and the cathode 41 are provided by substituting suitable insulating seals provided with lead-in members for the support pins 73 and ceramic member 75 in the metal cups 76. These details, however, are not shown in the drawings.

A graph illustrating the operating characteristics of the magnetron, illustrated in Fig. 2, is shown in Fig. 3. Along the axis of ordinates is plotted the anode voltage applied between the anode 11 and main cathode 15 in volts, and along the axis of abscissae is the anode current in milliamperes. Curve 77 represents a tuning current from cathode 41 of substantially milliamperes, and moves gradually from an anode voltage on the order of 1,995 volts at an anode current of 12 /2 milliamperes, represented by the point 78, to a maximum anode voltage of approximately 2,005 volts in the anode current in the region of fifty milliamperes, represented by the point 80, and then decreases to an anode voltage slightly less than 2,000 at an anode current of 175 milliamperes, as shown by the point 81. The magnetron is capable of operating smooth- 1y from point to point along this curve. Thus there are no regions of instability which may be termed regions of discontinuity in which spurious oscillations may arise. Therefore, operation at substantially any point along this curve is acceptable.

This same condition of satisfactory operation is represented by the curves 82 and 86.

The curve 82 is for a tuner current of 100 milliamperes and moves gradually from anode voltage of around 1,990 volts at 12 /2 milliamperes of anode current, as represented by the point 83, to a maximum of slightly more than 2,000 volts at an anode current in the region of fifty milliamperes, as represented by the point 84, and then decreases gradually to an anode voltage of 1,980 volts at an anode current of 175 milliamperes, as represented by the point 85.

Curve 86 corresponds to a tuner current of seventy-five milliamperes and rises gradually from an anode voltage of slightly greater than 1,985 volts at an anode current of 12 /2 milliamperes, as represented by point 87, to a maximum of approximately 1,990 volts at an anode current of fifty milliamperes, represented by point 88, after which it gradually decreases to a minimum voltage of approximately 1,965 volts at an anode current of 165 milliamperes, as represented by the point 90 on the curve, after which the voltage again rises gradually with increased anode current.

However, the above conditions of stability are not true for other curves shown in Fig. 3. Thus curve 91, which corresponds to a tuner current of fifty milliamperes, substantially coincides with curve 86 from points 87 to 103. Beginning substantially at this point of deviation, the curve enters a region of discontinuity represented by the dotted portion of the curve. As pointed out above, the operation of the magnetron in this region is unsatisfactory because of a shift of the oscillations into a split pi mode operation in which the oscillations shift rapidly between two pi modes resulting in a spurious oscillation which seriously interferes with the desired performance. This region of discontinuity ceases at substantially point 91 and continues smoothly to a minimum voltage of approximately 1,960 volts at an anode current of milliamperes, as indicated by point 92 on the curve, after which the anode voltage gradually rises with increasing anode current.

Curve 93, which corresponds to a tuner current of twenty-five milliamperes, exhibits a similar type of dis continuity between points 94 and 95. It will be noted that curve 93 reaches a maximum anode voltage at an anode current of about twenty-five milliamperes and thereafter drops into the region of discontinuity described above.

Curve 96, which corresponds to zero current, starts at approximately 1,960 volts with an anode current of twelve and one-half milliamperes as represented by point p 97. Very soon after it begins to rise it reaches the point 98, where it enters a region of instability which persists until it has passed its maximum anode voltage and has dropped, at point 100, to a voltage at which the anode current is approximately fifty milliamperes. Here, likewise, this region of discontinuity is represented by a dotted portion of the curve.

From the curves of Fig. 3, it will be seen that, if a load line of the system were permitted to vary in its angular relationship with respect to the curves of Fig. 3, the danger would exist that that load line would intersect the curves of Fig. 3 at some portion of its operation in a region of discontinuity. Fig. 3 also shows that, if that load line were maintained fixed, a portion of the characteristic of the tube could be selected in which the load line would not intersect the curves in any such regions of discontinuity. A preferable way in which this load line is fixed is to provide that the anode current of the magnetron shall remain substantially constant throughout the various operating conditions of the magnetron. Thus, in accordance with the invention, the magnetron is supplied from a substantially constant cur rent source. If, for example, the constant current supply were such as to produce a fixed current of the order of 150 milliamperes, an idealized load line represented by the heavy line 102 in Fig. 3 would result. Such a load line is one which has infinite slope and thus represents substantially a maximum impedance in the supply circuit. However, in actual practice, this idealized conditionis not obtained so that the load line has less than infinite slope. However, it is relatively easy to maintain that load line in a position where it does not intersect any of the magnetron operating curves in a region of discontinuity throughout the useful operating range of the magnetron.

It has been found that good results are obtained with the circuit of this invention when the anode supply circuit and the modulating circuit are adjusted to produce an anode current on the order of 150 milliamperes, and a tuner by-pass current on the order of 120 milliamperes. This operating point is represented on the graph of Fig. 3 by the point 104. If the tuner current is now modulated about this point 104, the magnetron operates in a highly satisfactory manner.

The circuit of Fig. 1 may be modified as shown in Fig. 4 by elimination of the capacitor 27 and by the insertion of an inductance 110. Otherwise the circuit is the same as that shown in Fig. 1. This modified circuit does not give as good results as that of Fig. 1, due to the stay capacities in the circuit that the capacitor 27 of the circuit of Fig. 1 serves to reduce.

The magnetron used with the circuit of this invention need not necessarily be of the type shown in Fig. 2. Any electrically-tuned magnetron may be used. The modulating circuit may produce video or other modulation, such as narrow band frequencies. The output of the magnetron need not necessarily be fed directly to the antenna, as shown, but may be fed to other suitable amplifying devices or to any other desired load.

This invention is not limited to the particular details of construction, materials and processes 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:

1. An electron discharge system comprising a magnetron having a cathode and an anode, and a power supply for said magnetron, said power supply comprising a grid controlled electron discharge device having a cathode, an anode and a grid, with the cathode and anode of said controlled tube connected in series with the cathode and anode of the magnetron through a resistor and an inductance with the grid of said controlled tube returned to its cathode through said resistor to provide negative feedback and insure constant current.

2. An electron discharge system comprising a magnetron having a cathode and an anode, means for frequency modulating said magnetron, and a power supply for said magnetron, said power supply comprising a grid con trolled electron discharge device having a cathode, an anode and a grid, with the cathode and anode of said controlled tube connected in series with the cathode and anode of the magnetron through a resistor and an inductance with the grid of said controlled tube returned to its cathode through said resistor to provide negative feedback and insure constant current.

References Cited in the file of this patent UNITED STATES PATENTS 2,122,495 Scott July 5, 1938 2,135,199 Ponte et a1. Nov. 1, 1938 2,149,080 Wolfi Feb. 28, 1939 2,495,776 Royden Jan. 31, 1950 2,591,932 Hansell Apr. 8, 1952 2,658,117 Sunstein et al. Nov. 3, 1953

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