US3096494A - Microwave amplitude modulator - Google Patents

Description

MODULATED OUTPUT VSL H. JACOBS ETAL Filed Dec. 30, 1960 MODULATED LIGHT SOURCE MAGNITUDE MICROWAVE AMPLITUDE MODULATOR L IN M LLIMETERS INPUT July 2, 1963 FIG UNMODULATE FIG. 2

3 4 2 a m /& 0a 3 a 2 mvslvroRs,

HAROLD JACOBS FRANK A. BRAND JAMES D. MEI/VOL 5 MICHAEL A. BENANTL Z4? $405M? ATTORNEY,

l 2 L IN MILLIMETERS United States Patent 3,096,494 MICROWAVE AMPLITUDE MODULATOR Harold Jacobs, West Long Branch, and Frank A. Brand, Elberon, NJ., James D. Meindl, East Pittsburgh, Pa.,

and Michael A. Benauti, Mamaroneck, N.Y., assiguors to the United States of America as represented by the Secretary of the Army Filed Dec. 30, 1960, Ser. No. 79,936 1 Claim. (Cl. 333-81) (Granted under Title 35, US. Code (1952), see. 266) Y he invention described herein may be manufactured and used by or tor the Government tor governmental purposes without the payment of anyroyalty thereon.

This invention relates to microwave amplitude modulator-s, and in particular to such apparatus utilizing semiconducting materials interposed in the path of the wave energy.

Hitherto, various arrangements have been suggested for operation of microwave devices as amplitude modulators with little or no change in phase. However, such arrangements have generally involved very complicated circuitry, are very costly, cumbersome, require high power capabilities and so are of limited applicability.

Accordingly, it is an object of this invention to provide improved microwave amplitude modulators with little or no phase As far as is known, there is no commercial product available which can satisfy the requirements of microwave amplitude modulators capable of maintaining very narrow band transmission, in order to communicate the maximum intelligence over a given portion of the electromagnetic spectrum, in a design which is as simple and compact as might be desired.

Accordingly, it is a further object of this invention to combine a waveguide and a semiconductor body into a simple compact unit which is susceptible of easy manufacture and which can be designed to fulfill all the requirements mentioned above.

For a more detailed description of the invention, together with other and further objects thereof reference is bad to the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a perspective view of a rectangular hollowpipe waveguide embodying the prinicples of the invention; and i FIGS. 2 and 3 are graphs illustrating the results of numerous experiments which serve as the basis of the present invention.

Before discussing specific embodiments of the invention, it will be helpful to develop some general principles.

It has been known for some time that when a semiconductor slab is inserted in a'rnicrowave field that a certain amount of absorption occurs. By varying the conductivity of the semiconductor, variations in absorption and hence transmission of power is attained. However, in all of these cases, the geometrical dimensions of the semiconductor slab has been ignored. The present invention is based on numerous theoretical and experimental considerations which show that, in order to obtain microwavemodulation with little or no phase shaft by conductivity changes in the semiconductor slab, the thickness dimension is critical. In other terms, it has been found when a semiconductor body is located across a waveguide with its specified thickness dimension being in the same direction as the propagated energy, that if the conductivity is modulated amplitude modulation is attained with little or no phase shift.

Referring to FIG. 1, there is shown a waveguide which may be of any desirable configuration, as for example a waveguide having a rectangular cross section.

Electromagnetic waves are propagated through the "ice waveguide 10* with linear polarization, its electric vector E as shown. This mode of waveguide propagation is denoted as TE mode, and this embodiment of the invention will be explained on such an assumption, although the invention is not altogether limited to this particular mode of transmission. The TE mode may be launched in the waveguide by any of several coupling arrangements well known in the The end 12 may also be coupled to a conventional receiver 13, in order to receive waves propagated through the waveguide.

A semiconductor body 16 is located within the waveguide 10 and disposed at some region between the input end 14 and the output end 12 of the waveguide, through which the electromagnetic wave is propagated. Semiconductor body 16 may be located midway between the H-plane or narrow walls of waveguide 10 and spaced therefrom, or may extend across the entire cross-section area of the waveguide. The thickness dimension L of semiconductor body 16, the critical dimension according to this invention, extends along a portion of the waveguide length and is parallel to the path of wave propagated energy.

The conductivity of semiconductor 16 is modulated in a conventional manner by variable light or by variable junction injection of excess minority carriers from a modulation source. Since the conductivity of semiconductor body 16 is proportional to the intensity of the injected carriers, modulation of the light or the junction will cause a modulation of the conductivity, and therefore an amplitude modulation of microwave energy traversing the waveguide 10. The carriers may be injected by incident light through apertures in the H-plane or narrow walls in the waveguide, or in the case of junction injection by horizontal wires contacting the semiconductor body 16 and going out through holes in the narrow walls in waveguide 10. Modulation by light is illustrated in FIG. 1. In the narrow wall 18, a small apelt-ure 20 is provided as shown, which permits light directed from an intensity modulated light source .22 to impinge upon the semiconductor body 16. The light from source 22 may be varied in any known manner.

in the case of modulation by light, absolute single crystals are not essential. Polycrystalline photosensitive semiconductor material, such as cadmium sulphide or lead sulphide can be utilized as long as the semiconductor materialh-as as long a liietime as possible. Experiments have been made using semiconductor material having a lifetime range from 1 microsecond up to 2000 microseconds.

Examples of semiconductor materials suitable for junction injection of the carriers are germanium, silicon and alloys or compounds made up firom the elements in the III and V group in the periodic table of elements. For purposes of the present example, a germanium body is selected, the intrinsic region preferably having a resistivity of at least 5 ohm-centimeters or higher. The germanium surface is processed for minimum surface recombination and the lifetime can be enhanced by the presence of irm purity levels, as by copper trapping.

The present invention is based on numerous experiments which show that there is provided a microwave amplitude modulator having little or no phase shift comprising a hollcwpipe waveguide section through which microwave energy can be propagated, a semiconductor body located within the waveguide in the path of said energy, the thickness of said semiconductor body being in the same direction as said propagated energy, said semiconductor body being characterized in that the thickness of said semiconductor body is calculated according to either one of the two following formulas:

and

where L=the thickness of the semiconductor body in millimeters, and V k=the wavelength of the propagated energy through the semiconductor body in millimeters.

In order to better explain the operation of the microwave amplitude modulator of this invention reference is made to the curves shown in FIGS. 2 and 3, wherein: 7 FIG. 2 is a representation of the magnitude of the ratio of electric field intensity, E transmitted through the germanium slab 16 to the electric field incident, E upon the front surface as a function of thickness, L, and conductivity 0', and

FIG. 3 shows the phase angle, 6, of the electric field intensity, E transmitted through the germanium slab, with respect to the electric field incident on the front surface, E as a function of thickness, L, and conductivity, 0'.

To show the changes of conductivity and thickness on the changes in transmission of the electric field through the germanium body 16, calculations were made in accordance with the following parameters:

(a) E /E the magnitude of the ratio of the trans mitted electric intensity to the electric intensity in air incident upon the surface;

. (b) 0, the phase angle in radians of E with respect to in;

(0) a, the conductivity at 2, 3, 4, 6 and reciprocal ohm-meters; and

(d) L, the thickness of the germanium slab 16 in millimeters.

The specific values shown in FIGS. 2 and 3 are for the propagation characteristics of electromagnetic waves being transmitted through the germanium body 16 at 10,000 megacycles per second. Using these data the following information is shown:

In examining FIG. 2, if it is assumed that the germanium slab 16- has a constant thickness L, it is seen that E /E will vary with conductivity. For instance, at 4 millimeters thickness, varying the'conductivity of slab 16 from 0 :2 to 0': 10, by some physical means such as light or uniform injection of excess minority carriers, will decrease E /E from about 56 percent to 10 percent.

In FIG. 3 is shown the phase shift due to conductivity modulation of the electric intensity. It is noted, that at 'a critical thickness 2.25 millimeters, a first node exists due to the internal reflections, and a second node of little phase shift starts at about 3.75 millimeters and extends to about 4.6 millimeters. In other words, if the conductivity is modulated with either of these specific thicknesses, amplitude modulation is attained with little or no change in phase.

Graphs were prepared similar to FIG. 2, formulated from calculations derived from the propagation characteristics of waves transmitted through the germanium body 16 at various frequencies. In all instances, it was observed if the thickness in millimeters of the germanium body 16 at the first and second nodes were divided by the wavelength in millimeters in the germanium body, the result was two respective constants .3 and .5. 1 This is exemplified by the following data:

. The invention is not restricted to the particular example described and illustrated. It is to be understood that the same constants .3 and .5, respectively, have been found toexist, in free space or in a Waveguidc,'for silicon and semiconductor alloys or compounds with resistivity greater than 5 ohm-centimeters and suflicient lifetime due either to intrinsic action or to traps. It is also interesting to note that these nodes in phase shift have been determined analytically and experimentally verified when the semiconductor slab is operated in the reflection mode. The constants, however, are different than those found for the transmission mode.

While there has been described what is at present considered a preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore aimed in the appended claims to cover all such changes and modifications :as fall within the true spirit and scope of the invention.

What is claimed is:

A microwave amplitude modulator having little or no phase shift of the reflected waves comprising a hollowpipe waveguide of rectangular cross-section and operating in the TE mode through which microwave energy can be propagated, 1a germanium body having a resistivity of at least 5 ohm-centimeters located within the waveguide in the path of said energy, the ratio of the dimension of said germanium body extending'in the direction of said propagated energy to the wavelength of the propagated energy through said germanium body being a con stant equal to .3, and said wavelength and said dimension being measured in the same dimensional units.

References Cited in the file of this patent UNITED STATES PATENTS 2,974,223 Langberg Mar. 7., 1961 2,977,551 Gibson et al. Mar. 28, 1961

Claims (1)

1. A MICROWAVE AMPLITUDE MODULATOR HAVNG LITTTLE OF NO PHASE SHIFT OF THE REFLECTED WAVES COMPRISING A HOLLOW PIPE WAVEGUIDE OF RECTANGULAR CROSS-SECTION AND OPERATING IN THE TE0,1 MODE THROUGH WHICH MICROWAVE ENERGY CAN BE PROPAGATED, A GERMANIUM BODY HAVING A RESISTIVITY

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