US2977551A - Microwave modulator - Google Patents

Description

March 28, 1961 A. F. GIBSON ET AL 2,977,551

MICROWAVE MODULATOR Filed March 11, 1958 v I a COOLING RADIATOR MODULATING SIGNAL SOURCE PIPE CONNECTION t ENHANCED TO RICONDUCTIVITY 9 TRIC v RAD|AT|OR LAYER BULK HEAD R IO 7 3 DIELECTRIC BULK HEAD PIPE coo CONNECTION F s TO RADIATOR mvnmmons: ALAN FRANK (mason JAMES WILLIAM GRANVILLE y United States Patent Claims priority, application Great Britain Mar. 18, 1957 14. Claims. (0. 332 62) from low values but, at high values of electric field, tends to a limiting, or saturation, value. Thus the microwave absorption, which :is proportionalto the 'rate at which the drift velocity increases with increasing electric field,

changes froma high value to a very low one as the electric field is increasedto ahigh value. Similar effects are displayed by silicon.

If, however, an attempt ismade to make use of this property to provide a practical microwave modulator by positioning a body of a semiconductor material in a microwave radiation field so that the direction of'the E-vector of the field coincides with the direction of an electric field applied to the body as a modulating signal, it is found that an inconveniently large potential is required to establish the electric field and also, even when the electric field is applied as 'a pulse signal, overheating of the body du'e'to the current it carries can easily occur.

It is an object of the invention therefore to provide an improved construction for a microwave modulator of the type making use 'of'the variation of absorption of a semiconductor material as the electric field applied to it is varied. H v According to the invention a microwave modulator comprises a waveguide for propagating microwave radiation having its E-vector transversely of the waveguide, thewaveguide having aflsection of reduced dimensionin which the reduction is in the direction of the E-vector,

a body of semiconductor material of oneconductivity ty iextending in the direction of the E-vector across the section and secured by an ohmicconnectionto one wall of the waveguide being electrically unconnected to the opposing wall, and a connection lead making. a sec- I 0nd ohmic connection to the part of the body adjacent to the opposing wall so that an electric field applied via the waveguide and the connection lead is in the direction of the E-vector, whereby in operation variation of the electric field elfects a corresponding variation in microwave radiation propagated along the waveguide.

, Conveniently the waveguide. is of rectangular cross section and the semiconductor body extends into a slot 7, cut'in the opposing wall of the waveguide and is insulated from the sides of the slot. 7

Advantageously the second. ohmic contact is made noninjecting by providing a zone'of enhanced conductivity I'materialat the contact; this entails an NN} junction fo'r'an N-type germanium body for example.

A. microwave modulator according to the invention will now be described by way of example reference being made to the accompanying drawings in which the figure shows a cut-away perspective view of a microwave modulator.

Patented Mar. 28, 1961 A slab 3 of N-type germanium of 5 ohm-cm. resistivity is soldered to the bottom wall 4 ofthe waveguide 1 and extends across the reduced section 2. The slab 3 extends into and is insulated from a slot 5 'in the upper wall 6 of the reduced section2., Typically the slab 3 'is l0-l5 mils in thickness and about 6 mm. in extent in the direction A of propagation along the guide 1. The effective height of the slab 3 from the bottom wall 4 is substantially that of the dimension of the reduced section 2. The top layer 7 of the slab 3 is made as a region of enhanced conductivity (N-]) and to this region a soldered connection 8 connects a connection lead 9. A space 10 exists between the edges of the slot 5 and the slab 3 and this is filled with an insulating material (not shown); a solid dielectric such as mica has the advantage of assisting to locate the slab 3 Within the slot 5 and enabling the slotto be made reasonably narrow.

'In operation microwave radiation of 8 mm. wavelength is propagated along the waveguide 1 in the direction A the reduced section causing no undue increase in attenuation. The waveguide 1 is taken to be connectedto earth as a datum potential and this is shown symbolically at the right-hand of the waveguide 1;.tlie connection lead 9 is ,connected to an earthed modulating signal source 11 tion. The'microwave radiation passing in the direction A from the reduced section 2 is thereby modulated in accordance with the modulating signal.

Typically a modulating signal of amplitude 200 volts reduces the attenuation of the microwave radiation in the 'guide 1 substantially to zero. This relatively low value of voltage is achieved because the eifective part of the slab 3 along which the electric field is established is made 1 small (20 mils)." The slab 3,. by virtue of its soldered connection to the bottom wall'4 of the waveguide 1, is

effectively cooled when the modulating signal is applied,

slab .3 does not makehelectrical contact with the sides of I the slot 5. v

large extent by'the use of conventional matching tech- .lReflections from the slab 3 do not appear to" bese'rious" but the efiect of such reflections can be reduced to a nique's; for instance, the provision of suitable adjustable stubs in front of and behind the slab 3 in the direction of propagation'A. a

If his desired to reduce radiation through the space j 10 of the slot 5 suitable choke structures can be added.

,It is possible that the power handling capacity of a modulator can be improved by the use of cooling fins v attached to the bottom wall 4 of the waveguide 1 or by the use of a liquid dielectric in the guide 1 around the slab 3. A silicone oil or carbon tetrachloride may prove suitable asaliquid dielectric and cooling would thenbe' efiected by providing a small natural circulation cooling radiator or suitable dielectric circulating means.

The reduced section 2 of the guide round the slab 3 would be sealed by dielectric bulk heads and the liquid dielectric would be carried into and out of the inside of the section by connections made to small holes in its lower and upper walls.

A microwave modulator as described above has a short response time, ideally of the order of seconds, and can therefore be used as a short-pulse or wide-band modulator of microwave radiation.

We claim:

l. A microwave modulator comprising a waveguide for propagating microwave radiation having its E-vector transversely of the waveguide, the waveguide having a section of reduced dimension in which the reduction is in the direction of the E-vector, a body of semiconductor material of one conductivity type extending in the direction of the E-vector across the section and secured by an Ohmic connection to one wall of the waveguide being electrically unconnected to the opposing wall, and a connection lead making a second ohmic connection to the part of the body adjacent to the opposing wall so that an electric field applied via the waveguide and the connection lead is in the direction of the E-vector, whereby in operation variation of the electric field effects a corresponding variation in microwave radiation propagated along the waveguide.

2. A microwave modulator as claimed in claim 1, wherein the wall of the waveguide adjacent the second ohmic connection defines a slot into which the semiconductor body extends remaining electrically out of contact therewith.

3. A microwave modulator as claimed in claim 2,

wherein the semiconductor body comprises a slab definand that part of the waveguide defining the sides of the h slot.

7. A microwave modulator as claimed in claim 4, comprising a cooling radiator, and connections therefrom to the opposing walls of the waveguide transverse to the direction of the E-vector for conveying liquid dielectric between the waveguide and the radiator.

8. A microwave modulator as claimed in claim 3, comprising cooling means making thermal connection with the outside of the waveguide corresponding to where the semiconductor body is secured on the inside.

9. A microwave modulator as claimed in claim 1 wherein the waveguide is connected as a point at datum potential, in combinationwith a modulating signal source connected between the datum potential point and the connection lead to the semiconductor body.

10. A microwave modulator as claimed in claim 1 wherein a region of the semiconductor body to which the second ohmic connection is made comprises a layer of semiconductor material of enhanced conductivity;

11. A microwave modulator as claimed in claim 4 wherein insulation material is inserted between the body and that part of the waveguide defining the sides of the slot.

12. A microwave modulator as claimed in claim 4 comprising cooling means making thermal connection with the outside of the waveguide corresponding to where the semiconductor body is secured on the inside.

13. A microwave modulator as claimed in claim 3 wherein the waveguide is connected as a point at datum potential, in combination with a modulating signal source connected between the datum potential point and the connection lead to the semiconductor body.

14. A microwave modulator as claimed in claim 3 wherein a region of the semiconductor body to which the second ohmic connection is made comprises a layer of semiconductor material of enhanced conductivity.

References Cited in the file of this patent UNITED STATES PATENTS Australia Sept. 20,1954

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