US3091741A - Attenuators - Google Patents

Description

May 28, 1963 c. EISAMAN ET AL 3,091,741

ATTENUATORS 3 Sheets-Sheet l Filed April 18, 1957 new o o O 3 2 COIL CURRENT- MA.

INVENTOR. LEO C. EISAMAN BY SEYMOUR SCHIFF ATTORNEY May 28, 1963 I.. c. EIsAMAN ETAL 3,091,741

ATTENUATORS Filed April 18, 1957 5 Sheets-Sheet 2 OSC FREQUENCY ,4

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DEVICE (CAVITY) /f/ ADJUSTABLE VOLTAGE 25- mi DEVICE /26 /Q24 *27 REGULATED VOLTAGE SOURCE y \I3 FREO. ADJUSTING DEVICE /'0 /30 /40 OUTPUT R.I'. POWER ATTENUATOR ATTENUATOR o O o l GO/ GD/ c RELAY Isa Ieb o I7 VOLTAGE ATTENUATION DB m A m I'G 3 E5 55 FREQUENCY- KMC.

May 28, 1963 1 c. EISAMAN ET AL 3,091,741

ATTENUATORS Filed April 18, 1957 5 sheets-sheet s United States Patent O 3,091,741 ATTENUATORS Leo Coburn Eisaman and Seymour Schilf, Rochester, NX., assignors to General Dynamics Corporation, Rochester, NSY., a corporation of Delaware Filed Apr. 18, 1957, Ser. No. 653,678 1 Claim. (Cl. 331-183) This invention relates to attenuators and is particularly directed to attenuators for microwave energy, with special attention to means for changing attenuation rapidly from one level to another.

So called ferrite devices, using the Faraday principle of rotation of electric lines of force in a paramagnetic body, have been used in wave guides for various purposes. A typical device comprises a disk or rod of a magnetized metal compound, such as nickel-zinc ferrite, placed in a wave guide with a magnetizing coil around and coaxial with the rod. The plane of polarization of the electric field in the guide is rotated through an angle of up to 90, the amount of rotating being a function of and dependent upon the magnitude of the coaxial magnetic field. Fortunately, this rotation is effected with substantially no insertion loss. A flat metal vane or blade mounted downstream from the ferrite plug will absorb little of the microwave energy when the plane of polarization is norrnal to the vane but `will attenuate a maximum amount when parallel to the plane of polarization.

The vane in such an assembly has been mechanically rotated to control attenuation, but the manufacture of suitable rotating joints in the Wave guide is costly and tbe finished product has many operating disadvantages.

An object of this invention is to provide an improved microwave attenuator, a more specic object being the elimination of the mechanical rotating wave guide joint. In some microwave equipments, the attenuation must be done in steps from one accurately predetermined level to another. In such equipments, it is sometimes desired to make the step changes suddenly to produce a substantially square-cornered change in the microwave energy. Obviously, the mechanical inertia of any moving parts mitigates against fast attenuation changes. Accordingly, a further object of this invention is to provide improved means for changing the attenuation in a microwave system with no time delays.

Further, in the ferrite devices of the type contemplated here, it has been found that attenuation is a function of frequency of the microwave source and that the device is quite sensitive to changes in frequency. Accordingly, the step-levels of attenuation may be unpredictable with small drifts in frequency of the microwave source. Recalibration of the attenuator for such frequency drift is impractical. Hence, a still further object of this invention is to provide improved means for making the attenuator insensitive to frequency.

Still further, it has been found that the attenuation for any particular level of magnetizing current may be of two different values, depending on the direction of change of magnetizing current as the current arrives at the new level. |For example, if the magnetizing current is changed from -5 milliamperes to +5 milliamperes the attenuation might be l0 db, whereas if the current is changed from milliamperes to +5 milliamperes the attenuation could be about 13 db. The explanation of these inconsistent results is probably found in the hysteresis loops of the magnetic materials involved. Accordingly, a still further object of this invention is to providerneans for obviating the hysteresis effects of the control current-attenuation characteristics of ferrite-type attenuators.

The objects of this invention are attained by coupling one or more ferritetype attenuators in a microwave transmission line, such as a wave guide, each of the attenu- 3,091,741 Patented May 28, 1963 ators having a magnetizing coil and being responsive to the changing magnetizing force of said coil to change the magnitude of attenuation of the microwave energy; a switch being connected between each coil and said source for selectively adding predetermined amounts or steps of magnetizing current, and hence causing steps of attenuation in the high frequency system. The voltage source for the magnetizing current is regulated and is adjustable in response to frequency changes in the microwave energy.

`Other objects and features of this invention will become apparent to those skilled in the art by referring to the specific embodiments described in the following specification and shown in the accompanying drawings in which:

FIGURE la is a longitudinal cross-section of one ferritetype attenuator device which can be used in the system of this invention,

FIGURE 1b is an end View of the device of FIGURE la,

FIGURE 2 is a graph of the coil current-attenuation characteristic of the device of FIGURE 1,

FIGURE 3 is a circuit diagram of one embodiment of the attenuator system of this invention,

FIGURE 4 is a graph of the frequency-attenuation characteristic of the system of FIGURE 3, and

FIGURE 5 is a circuit diagram of a voltage regulator of this invention.

vThe wave guide 1a of FIGURE 1 is assumed to be connected between a source of microwave energy on the one hand and a load or utilization means on the other so that microwave energy ilows from left to right. The wave guide is preferably round in cross-section, is of good electrical conductive metal and is internally polished to reduce losses. Concentrically within the guide is the rod 2 of compressed particles of magnetic metal, such as nickelzinc ferrite. The axial dimensions of the plug should be of the order of a Wavelength at the operating frequency. A low loss dielectric material, such as Teflon at 3, supports the ferrite rod. Surrounding the wave guide and coaxial therewith is the direct current magnetizing coil 5 with leads 6 which are connected to a direct current source for establishing an axial magnetic field. Because of the Faraday effect rotation, the plane of polarization of electric fields approaching the rod 2 is rotated through a finite angle at the other end and the energy continues down the wave guide with but little attenuation. The amount of rotation depends upon the magnitude of the magnetic field and, hence, may be controlled by the direct current applied to leads 6. Downstream from the rod is mounted the vane 7 of poor electrically conductive or lossy metal, mounted centrally in the guide on one diameter thereof. When the plane of polarization of the microwave energy is perpendicular to the vane 7, absorption of energy in the vane is small. As the component of energy parallel to the plane increases, attenuation increases.

lFIGURE 2 shows the change 4in attenuation of such a device as the magnetizing coil current changes. It is to be noted that the attenuation is different for an increasing coil current than for a decreasing coil current, and that the characteristic curves are analogous tov a magnetic hysteresis loop. 'Ibis feature will be referred to in greater detail hereinafter. -It will be noted further that the curvature of the characteristic Varies considerably and that minimum attenuation (nearly zero) in one particular attenuator occurs when the coil current is about 30 ma. in one direction, which is referenced as negative in this example. Attenuations of 5 or more db lis obtained when the coil current is reversed and returned to zero.

In FIGURE 3 the attenuator 10, of the type of FIG- URE 1, is shown with a microwave source 1 at one end and a microwave output on the other. The two leads 6 of the magnetizing coil are connected through switches 11 and 12, to the regulated direct current voltage source 13, through the adjustable voltage regulator 14. The voltage output of regulator 14, in FIGURE 3, is stable at any level, but is adjustable to different levels. Adjustment is interlocked or ganged with the frequency determining device at the microwave source 1 so that voltage applied to the attenuator coils is a function of microwave frequency. The interlock will be referred to more in detail below.

In the embodiment shown in FIGURE 3, it is contemplated that two predetermined values of coil current be selectively applied to the attenuator 10 to provide two accurately predetermined levels of attenuation. Nhen an attenuator is used having the characteristic of FIGURE 2, a reversible coil current is indicated. Conveniently, the switches 11 and l2 .are connected to reverse the coil current and are operated by the relay winding 15, which isv in turn energized through switch 16 and the relay voltage source at 17. The magnetizing circuit shown has one side grounded, as at 18 and 19. The amplitude of the magnetizing current is adjustable separately for either polarity by potentiometers 2) and 2l, each adjustment being made to establish the two desired levels of attenuation in attenuator 1t). Obviously, the switching and potentiometer arrangement shown can be easily modified to provide .any two attenuation levels. For example, instead of switching from minus to plus polarity or vice versa, switching may be made between two plus levels, between two minus levels, or between zero and plus or minus.

Switch 16 for operating the relay may be operated manually or automatically, as by cams. Further, it is contemplated that the relays shown, including switches 11 and l2, may be replaced with any desired system of switching, including cam operated microswitches That is, it is within the scope of this invention to program the attenuator 10 to operate between its two levels of attenuation according to any predetermined plan, such as .a punched-tape program.

By connecting two, three, or more attenuators in the transmission system, the diversity of the program may be increased. For example, attenuators 3@ and 4G may be connected electrically in series with the high frequency circuit of attenuator l0. By connecting their coil leads 6a and 6b through switches 11a, 12a, llb, .and 12b, the three attenuators may be programmed to attenuate the wave guide energy to any of a large number of levels. Relays 15a and 15b are actuated by switches lea and leb, respectively, to control attenuators 30 and 4o. If, for example, attenuators 1t?, 30 and 40 are, respectively, adjusted to each attenuate zero to 10, zero to 20, and zero to 30, any attenuation up to 60 db may be obtained, in 10 db steps by manipulating switches 16, 16a, .and lob.

As mentioned above in connection with FIGURE 2, two inconsistent values of attenuation can be obtained from a single setting of attenuator coil current. According to an important feature of this invention, it is contemplated that the controls for the coil current be so constructed and arranged that the level of coil current for a desired level of attenuation will always be approached from one direction. `In the embodiment shown, in which only two values of coil current are employed, either one of the values may be the reference for the other. By a certain rotational placement of the vane 7 with respect to the normal polarization plane, one value of coil current, for example, may be highly negative to reduce attenuation to near zero, and the other value may be at some less negative or positive potential corresponding to a predetermined attenuation value. Let it be assumed that the second selected coil current is to be arrived at only by increasing the coil current in a positive direction. That is, potentiometer 20 could normally be held in circuit when relay coil 15 is de-energized, .and potentiometer 20 could be adjusted for the necessary high negative Coil current and its corresponding minimum attenuation. Upon energization of the relay, the current direction is reversed in leads 6 and potentiometer 2l is brought into the circuit which may now be adjusted for desired ,attenuation, say l0 db. Hence, upon the opening of control switch 16, the relay relaxes, the armature drops back, and the attenuator is automatically returned to nearly Zero insertion loss. It follows that each time switch 16 is closed the coil current through potentiometer 2l `always increases in a positive direction to the preset coil current and attenuation level.

Although a two-position reversing switch is shown for each attenuator, it is contemplated that three or more potentials could be applied to the attenuator coil. Such a modification would merely require multiplication of the switching circuits Ll ,and 12 and the provision of the necessary additional potentiometers corresponding to 20 and 21 to provide the additional attenuation levels.

It is further contemplated that the attenuators in the high frequency transmission system may be placed in various parallel .and series-parallel combinations, as well as the simple series arrangement shown.

The attenuator of the fcrritetype shown here is quite frequency sensitive. Curve A in FIGURE 4, for example, shows that attenuation of the commercially available attenuator of the type known .as the Gyraline, No. R- 1350, varies between 18 yand 4 db over a frequency range of 8.9 kilomegacycles to 9.6 kilomegacycles per second. According to a still further feature of this invention, means is provided for rendering the insertion loss substantially independent of frequency over the operating range. The characteristic of the attenuator, when operated according to this invention, is shown by curve B.

It is contemplated that the oscillator-source 1 be adjustable over a relatively wide range of frequencies, such as 8 to 10 kilomegacycles. From curve A of FIGURE 4 it is apparent that considerable coil voltage adjustments must be made as the frequency of the source changes. If the oscillator is or the microwave type which can be tuned by the resonant cavity 22, the knob 23 can be employed for changing the size of the cavity and for changiner the frequency of oscillation. According to an important feature of this invention, the frequency adjusting device 23 is mechanically interlocked, through linkage 24, with the adjustable voltage device 14 so that the regulated voltage on the leads 19-25 becomes a function of the frequency of the system.

One voltage adjusting device 14 is shown, by way of example, in detail in FIGURE 5. To the leads 26 and 2,7, from the regulated source 1.3, is preferably applied two regulated voltages, on either side of ground. Across the leads 26 and 27 is connected the potentiometer comprising resistors 28, 2.9, and the anode-cathode space of double triode 31. As the grid voltage changes the cathode end of resistor 29 can be smoothly varied from positive to negative values including ground potential. The grids .are connected in parallel to the output of the second twin cathode-follower 32, one grid 33 of which is connected to the adjustable biasing potentiometer 34 in the cathode circuit as shown. The adjustable contact of the potentiometer is mechanically linked to the frequency adjusting device 23 through linkage 24. It is relatively simple to select the gain of the cathode followers and calibrate the output D.C. voltage at 19-25 in terms of shaft position at 23 and, hence, in terms of microwave frequency. Other circuits could, of course, be designed for adjusting attenuator controlling current in terms 0f oscillator frequency.

Many modifications may be made in the relay circuits of FIGURE 3, or the Voltage regulator circuits of FIG- URE 5, or the mechanical details of the attenuator of FIGURE 1 without departing from the scope of this invention as defined in the following claim.

We claim:

The combination comprising a microwave source with a frequency adjusting device, an attenuator for high frequency connected in the output of said source, said attenuator being responsive to .a variable direct current to change the level of lattenuation; a direct current voltage source connected to said attenuator, means for adjusting the voltage of said direct current voltage source; and common control means for changing said frequency adjusting device and said direct current voltage source, the incremental change in direct current per unit of frequency change being so selected as to produce uniform attenuation over a predetermined range of frequencies.

2,355,338 Stewart Aug. 8, 1944 6 Haeff et a1 Mar. 18, Zalesk Mar. 9, Norton Aug. 2, Hogan May 29, Reggia July 2, Lee Feb, 25, Zaleski Oct. 21, Fox Feb. 24,

OTHER REFERENCES Fox et al.: Behavior and Applications of Ferrites in the Microwave Region, the Bell System Technical Journal, vol. 34, Number l, January 1955, pages 1-104 (pages 2633 especially relied upon).

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