US2961531A - Microwave crystal mixer - Google Patents

Description

NOV. 22, 1960 s, ow 2,961,531

MICROWAVE CRYSTAL MIXER Filed July 6, 1956 2 Sheets-Sheet 1 INVENTOR, I STANLEY E. HOWE ATTORNEYS Nov. 22, 1960 s. E. HOWE MICROWAVE CRYSTAL MIXER 2 Sheets-Sheet 2.

Filed July 6, 1956 Fig. '3

INVENTOR. STANLEY E. HOWE ATTORNEYS MICROWAVE CRYSTAL MIXER Stanley E. Howe, Wakefield, Mass, assignor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Filed July 6, 1956, Ser. No. 596,202

10 Claims. (Cl. 250-20) This invention relates in general to microwave apparatus, and in particular to improved holders for crystal mixers and detectors.

In microwave devices, the use of Wave guide to conduct energy is most common. In conjunction with the wave guides it has also become conventional to employ crystal diodes rather than vacuum tubes as detectors and as mixers. The primary reason for the use of crystals rather than vacuum tubes is the fact that the s:'gnal-tonoise ratio of the crystal is more favorable than that of the vacuum tube. Other factors favoring the use of crystals are their small physical size and their simple design which facilitate their use with wave guide or coaxial cable.

When crystals are being used simply for detection, the radio frequency signal is passed down a wave guide to the crystal detector where demodulation takes place and an output signal is then derived. In mixer applications, a radio frequency signal and a local oscillator signal are simultaneously passed down a wave guide to the crystal where the two signals are mixed together. The output or intermediate frequency signal resulting is then usually carried by coaxial cable to a receiver.

Early in the practice of the art, it was discovered that a considerable loss of radio frequency energy from the incoming signal took place at the intermediate frequency output connection. To avoid this loss, a device known as the radio frequency choke was designed. The radio frequency choke consists of a cavity formed about the point of connection of the output line for the intermediate frequency signal. The cavity so formed is of a depth comparable to a half of the wave length of the radio frequency energy. The closed end of the cavity so provided is reflected back to the mouth of the cavity surrounding the intermediate frequency output resulting in a low impedance appearing at the mouth of the cavity adjacent the output connection. frequency chokes as devices for preventing loss of radio frequency energy is that they are inherently effective only over a relatively narrow band of frequencies.

More. recently, it has become desirable in detecting and in mixing applications to employ crystals which are operative efficiently over a broad band of frequencies. Although crystals have been developed which are effective in detecting and mixing such signals, conventional choke structures have been ineffective in avoiding losses exceptin the band of frequencies for which they are designed. This problem has not gone unrecognized and some effort has been devoted to designing detecting and mixing apparatus capable of operation over a broad band of frequencies; One of the devices developed for mixer applications includes a set of ascending brass steps mounted in the wave guide down which the'radio frequency and local oscillator energy are beng fed. The stepsare of-varying length and extend from apoint well within the wave guide to the body'of the mixer assembly. In conjunction with the steps, there is normally used a probe which constitutes an extension of the center con- I United States Patent The only shortcoming of radio e tained from this mixer were reasonably good over a broad band of frequencies but the extreme difficulty of fabrica tion and the high cost of the mixer apparatus have prevented its Widespread adoption. Broad band detection apparatus has been equally complex and expensive. It is with the improvement of wave guide to coaxial cable transitions for detectors and mixers, particularly where energy of a broad band of frequencies is being handled, that this invention is concerned.

Therefore, it is an object of the present invention to provide detecting and mixing apparatus capable of efiiciently handling energy .of a broad band of frequencies.

It is a further object of the present inventon to provide detectors and mixers wherein little radio frequency energy leakage is encountered at the output connections.

It is another object of the present invention to provide detectors and mixers for use in wave guides without causing signficant mismatch of impedances.

It is still another object of the present invention to provide suitable detector and mixer apparatus for broad band crystals.

In general, the present invention consists in a crystal holder in which a coaxial type crystal is held. The crystal holder is mounted on the boad side of a wave guide and has a central conductor which extends into the wave guide. Plug-in connections for the crystal are provided by resilient fingers on the barrel of the holder for engaging the external shell and by resilient fingers, at the end of the probe for engaging the internal element of the crystal. A fine wire extends from the tip of the probe within the wave guide to a sde wall of the guide or to a central conductor of a coaxial output line mounted on the side wall of the guide. In the embodiment wherein the fine wire extends through the side wall of the wave guide to the central conductor of a coaxial output line, the out put is taken from the coaxial line and a DC. return is provided from the crystal to the Wave guide by direct metallic contact. In the structure wherein the fine wire extends only to the side wall of the wave guide, the intermediate frequency output is taken from the top of ,a tripolar crystal, and a DC. return is provided by the wire; For a better understanding of the present invention to gether with other objects, features and advantages, ref erence should be made to the following detailed de scrip tion of a preferred embodiment thereof which should be read in conjunction with the accompanying drawings in wh ch:

Fig. 1 is a front elevation, form of the mixer apparatus,

Fig. 1A is a sectional view of a crystal of the type used with the mixer apparatus illustrated in Fig. 1,

Fig. 2 is a side elevation of the mixer apparatus illustrated in Fig. 1,

Fig. 3 is a front elevation in section of an alternative form of mixer apparatus utilizing no coaxial output line connected to the wave guide,

Fig. 3A is a sectional view of a tripolar crystal foruse in the mixer of Fig. 3, and

Fig. 4 is a side elevation partly in section of the embodiment of the mixer apparatus illustrated inFig. 3

In general, mixer structures are more complex than detector structures and for that reason the embodiments of the invention shown are mixers. The application of the principles of the invention to detectors-is of course, obivous from the disclosure relating to mixers. In the partly in section, of one mixer structure of Figs. 1 and 2, there is shown a wave guide 12 through which radio frequency and local oscillator energy may flow. A flange 13 is provided at the input end of wave guide 12 to permit the attachment of other sections of guide from which energy may come. Connected to the narrow side of wave guide 12 'is a coaxial output line 14. An adaptor 16 serves as the outer conductor of the coaxial line and terminates in an end surface which is flush with the inside of the wall of wave guide 12. The center conductor 17 of the coaxial line also terminates flush with the inside of the wall of wave guide 12. A similarly terminated insulating sleeve 18 is disposed between the center conductor 17 and adaptor 16.

An opening is provided in the broad wall of wave guide 12 and that opening receives a-plug 19 which is generally annular in shape. Plug 19 is brazed or soldered in position and terminates in a surface which is flush with the internal surface of the broad wall of wave guide 12. Adjacent the upper end of plug 19 is a threaded section 21 to accommodate a cap (not shown) which may be used to retain a crystal detector in the plug. At the extreme upper end of plug 19 there is formed a receptacle portion 22 which may be integral with plug 19 or otherwise brazed or soldered in place. Resilient fingers as at 23 are formed in the receptacle portion 22 to provide good physical and electrical contact with the device to be inserted therein.

Parenthetically, it should be noted that the crystal which plug 19 is designed to receive is shown in Fig. 1A, and is described in detail below. The crystal has been shown independently of the mixer apparatus in order that detail of the mixer and the crystal not be confused.

A probe 24 lies along the axis of annular plug 19 and has an area of reduced diameter where it is in contact with an insulating bead 25. Shoulders are formed on the probe in this manner at the flat surfaces of bead 25 which assures positive support to the probe. The upper end of the probe 24 is hollowed out and there are formed a series of resilient fingers 26 for telescopically receiving the internal element of the crystal cartridge. The lower end of probe 24 penetrates to a point well within wave guide 12. Probe 24 is preferably spaced from the end 27 of the wave guide by a distance approximately equal to a quarter of a wave length of the energy being transferred in the guide. For broad-band use the actual distance chosen is a compromise, however.

At the lower end of probe '24, a fine wire 28 is attached and extends toward the narrow wall of the wave guide 12 at which point it is connected to the inner conductor 17 of coaxial output line 14. The diameter of the wire 28 need not be held to a particular size. For reasons which will be apparent from the explanation which follows as to the operation of the device of this invention, the wire must be of less diameter and preferably of substantially lesser diameter than the probe 24. From the standpoint of the operation of the device the only limitations on the minimum diameter of the wire are its adequacy with respect to its current carrying capacity and its physical strength. Since the currents involved are normally very small, the latter limitation'is the only one which is of any practical concern.

In Fig. 1A the crystal used with the mixer disclosed in Fig. 1 is shown in some detail. The crystal includes an external sleeve 31 which is preferably silver plated or otherwise treated to form a good conductive external surface. Sleeve 31 is of a diameter sufficiently large that tight engagement is afforded by resilient fingers 23 When the crystal is placed therein. A pin 32 is centrally disposed within the sleeve 31 of the crystal and extends along its axis. An insulating member 33 between pin 32 and sleeve 31 isolates the one from the other electrically and provides physical support to the pin. The outside diameter of pin 32 is sufiiciently lareg to assure tight engagement of the pins in resilient fingers 26 of the probe 24. A conventional resilient cat whisker 34 is mounted at the other end of pin 32 and is in contact with a die 35 of silicon, germanium, or other suitable semi-conductive material. Die 35 is soldered or otherwise conductively attached to a conductive plug 36 which is pressfitted into sleeve 31.

The structure as illustrated in Figs. 3 and 4 is similar in many respects to that illustrated in Figs. 1 and 2. Certain modifications have been made in order that the socalled tri-polar crystal such as that illustrated in Fig. 3A may be used in conjunction with the mixer apparatus. Energy from a radio frequency source and from a local oscillator passes through wave guide 42. A flange 43 is provided at the input end of wave guide 42 to permit the connection of other wave guide sections (not shown) from which input energy may be derived. A plug 49 similar in all respects to plug 19 as shown in Figs. 1 and 2 is disposed in an opening in the broad side of wave guide 42. Plug 49 is generally annular in shape and contains a probe 54 which is held in position centrally of the open ing in plug 49 by an insulating head 55. The diameter of probe 54 is reduced for a distance along its length and the intersections of the portion of reduced diameter with the other portions of probe 54 form shoulders which engage the fiat end surfaces of insulating head 55. In this fashion, adequate support is provided to retain probe 54 centrally in the opening in plug 49. The upper end of probe 54 is hollowed out to form an axial opening. Slotting of the hollowed out portion of the probe provides resilient fingers 56. A threaded section 51 is provided adjacent the top of plug 49 and extending upwardly above threaded section 51 is a receptacle portion 52 having resilient fingers 53 formed in the end thereof. Receptacle portion 52 may be formed integrally with plug 49 or may be brazed or soldered in position.

At the lower end of probe 54, a fine wire 58 is soldered or brazed and extends to the side wall of wave guide 42 where it is also soldered or brazed in place. The tip of probe 54 is located at a distance from the guide end 57 which is approximately equal to one-quarter of the wave length of the radio frequency energy being transferred in the guide 42.

In Fig. 3A there is illustrated a type of tri-polar crystal which is used in conjunction with the apparatus shown in Figs. 3 and 4. The crystal detector shown in Fig. 3A is in many respects similar to that shown in Fig. 1A. It includes a conductive external sleeve 61 of sufficiently large diameter to be tightly engaged in good physical and electrical contact by resilient fingers 53. A pin 62 is centrally disposed along the axis of sleeve 61. Pin 62 is isolated electrically from sleeve 61 and held in fixed physical relationship thereto by an insulating bead .63. The outer end of pin 62 is of a sufliciently large outside diameter to fit tightly within the resilient fingers 56 of the hollowed out portion of probe 54. At the inner end of pin '52 there is mounted a resilient cat whisker 64 which bears upon a die 65 of silicon, germanium, or other semi-conductive material. Die 65 is soldered or otherwise conductively attached to a conductive plug 66. Plug 66 in this instance, however, is insulated from electrical contact with sleeve 61 by a thin insulating member 67. Plug 66 has an extension which permits external electrical contact to be made thereto although an opening to receive a plug from a coaxial line may replace the extension. The outer conductor of the coaxial line may be threaded to the portion 51 of plug 49 and the inner conductor of the coaxial line connects directly to the extension or opening of plug 66.

Sleeve 67 may be composed of an epoxy resin, a polyester resin, a copolymer comprising polymerized dichlorostyrene such as identified by the trademark Styron, or a tetrafiuoroethylene resin such as that designated by the trademark Teflon. In some instances it has proven desirable to enclose the insulating sleeve in a conductive jacket in the form of a thin metallic sleeve which facilitates insertion of the insulated plug into the external sleeve during the fabrication of the device.

The insulation of the plug from the shell results in a DC. isolation of the crystal and its supporting plug from the shell or outer conductor 61. In addition, the effect of the insulating sleeve 67 is to constitute a condenser of small capacity which may be utilized to by-pass certain frequencies while having no appreciable effect on rela tively lower frequencies. When, as here, the crystal is employed as a mixer or a video detector, the insulating sleeve, the shell and the plug constitute a low capacity condenser of coaxial configuration, the capacity of which may be made such as to bypass effectively the radio frequency signal while causing no substantial attenuation of the desired intermediate frequency or video output signal resulting from the mixer action.

The device illustrated in Figs. 1 and 2 operates in a manner which is quite similar to that of conventional mixers. The radio frequency and local oscillator signals which are being conducted in wave guide 12 are picked up by the probe 24 and are rectified with mixing action taking place in the crystal. The intermediate frequency signal resulting from the mixing action is conducted from the probe 24 to the coaxial line center conductor 17 by Wi1'6 28. No radio frequency choke is needed at this coaxial output. wire is of such small size that it does not cut a sufiicient number of lines of the electric field to pick up any appreciable radio frequency energy. The further fact that radio frequency energy is not transmitted from the probe to the intermediate frequency output by the Wire is believed to be due to the fact that a great mis-match of impedances at radio frequency'exists between the probe and the wire and also between the Wire and the coaxial output line center conductor. The intermediate frequency signal is not affected because the length of wire is much too short at intermediate frequencies commonly used to be a significant portion of a wave length. In one'specific instance a deviceas shown in Figs. 1 and 2 was employed in rectangular wave guide suitable for operation in the frequency range from 15,000 megacycles per' second (mc.p.s.) to 22,000 mc.p.s. The wire used was 0.0035 inch in diameter and approxir'riately inch in length. The probe was 0.063 inch in diameter. When operated at a radio frequency 'of 18,000'mc.p.s. with a local oscillator frequency of 18,030 mc.p.s., resulting in an intermediate frequency of 30 mc.p.s., the efiiciency of conversion of the signal from the radio frequency to the intermediate frequency was found to be comparable with that obtained using a conventional narrow band holder and a choke suitable for operation at 18,000 mc.p.s.

Operation of the device illustrated in Figs. 3 and 4 is similar to that of the device illustrated in Figs. 1 and 2 but with some significant differences. As has been pointed out, the device in Figs. 3 and 54 does not have a coaxial output line connected to the wall of the wave guide. As is clearly shown in Fig. 3 the wire 58 merely extends from the tip of the probe 54 to the narrow wall of Wave guide 42. The output signal is most conveniently taken from a coaxial line having its outer conductor threaded on portion 51 and its inner conductor connected to the plug 66. This is made possible because of the selective by-pass effect previously mentioned as being obtained from the sleeve 67 between the cartridge 61 and the plug 66. Energy proceeding down wave guide 42 is picked up by probe 54 and rectified by the crystal which has been inserted in plug 49. Mixing of the radio frequency and local oscillator energy takes place in the crystal and the intermediate frequency derived is conducted out through the coaxial line formed by the upper end of plug 66 and receptacle portion 52. In this instance, the wire 58 provides a suitable direct current return for the crystal and the low capacity condenser formed The reason is believed to be that theby the sleeve 67 effectively short-circuits energy at the radiojor local oscillator frequency. In this embodiment of the invention 'as in the embodiment previously dis cussed, the wire 58 does not cut enough of the lines of the electric field to pick up any appreciable radio frequency energy, nor does it transmit radio frequency energy from the probe to the wave guide wall.

In detector applications, either similar apparatus may be used, or somewhat simplified structures are also practical. Only a radio frequency signal is then present in the wave guide, and the video or other output is taken directly from one of the elements of the crystal, "either by way of the probe and fine wire as in the first embodiment discussed above or by Way of the back plug of the crystal as in the second embodiment above. The advantages of broad band operation and elimination of radio frequency chokes are of course retained in the detection apparatus.

Although preferred embodiments of the present invention have been shown and described, these "should not be construed as limiting the invention to the precise structures disclosed. It is believed that numerous modifications which are within the purview of the present invention will suggest themselves to those skilled in the art. The invention should be limited only by'the spirit and scope of the appended claims.

What is claimed is: V

1. Crystal holder apparatus comprising 'a sectionof wave guide having a pair of opposed narrow walls and a pair of opposed broad walls for conducting an ultra high frequency signal, a crystal rectifier for providing an output signal which is a function of said ultra high frequency signal, a pair of output terminals, and a circuit in series across said terminals, including said'crystal rectifier, a probe within said Waveguide for picking up said ultra high frequency signal, a fine wire 'of thickness less than that of said probe within said waveguide disposed perpendicular to the narrow walls of the wave guide, and said wave guide.

2. Mixer app aratus comprising a section of wave guide for conducting energy of at least two ultra high frequencies, a crystal mixer having at least two terminals for providing a signal of intermediate frequency obtained by the beating of said ultra high frequencies, an output line for said signalof intermediate frequency, a probe for picking up energy within the wave guide extending into saidwave guide and connected to one terminal of said crystal mixer, a fine wire of thickness less than that of said probe disposed within said wave guide and con- I nected to said probe and extending in a direction perpendicular to said probe, said crystal mixer, said fine wire, said probe and said wave guide being connected in series circuit relationship across said output line.

3. Mixer apparatus comprising a section of wave guide for conducting energy of at least two ultra high frequencies, a probe extending into said wave guide, a crystal mixer connected between said wave guide and said probe, a two-conductor output line for extracting signals of intermediate frequency provided by said crystal from the beating of said energy of ultra high frequencies, and a fine wire of thickness less than that of said probe disposed within said wave guide and connecting said probe to one of said two conductors of said output line.

4. Mixer apparatus comprising a section of rectangular wave guide, a coaxial line connected to a narrow wall of said wave guide, and means for providing a signal of intermediate frequency to said coaxial line in response to signals of at least two ultra high frequencies being conducted in said wave guide, said means including a probe extending into said wave guide through a broad wall thereof, a crystal connected between said broad wall and said probe, and a fine Wire directly connecting the tip of said probe within said wave guide to the inner conductor of said coaxial line.

5. Mixer apparatus comprising a section of wave guide for conducting energy of at least two ultra high frequencies, a crystal mixer for providing a signal of intermediate frequency obtained by the beating of said ultra high frequencies, an output line directly connected to said crystal mixer, a probe connected to said crystal mixer and extending into said wave guide, and a fine wire disposed within said wave guide and connected between said probe and said wave guide.

6. Mixing apparatus for broad hand signals comprising a section of wave guide for conducting energy of at least two ultra high frequencies, a crystal mixer for providing a signal of intermediate frequency obtained by the beating of said ultra high frequencies, said crystal mixer a rectifying path formed therethrough, an output line having one terminal thereof connected to one end of said rectifying path, a probe extending into said wave guide and connected to the other end of said rectifying path, and a fine wire connected between the end of said probe within said wave guide to the internal wall of said wave guide, said wave guide being capacitively connected to said rectifying path of said crystal mixer.

7. Mixing apparatus for broad band signals comprising a section of wave guide for conducting energy of at least two ultra high frequencies, a probe extending into said wave guide through a broad wall thereof, a fine wire connected between the tip of said probe and the central area of a narrow wall of said wave guide, a coaxial output section mounted on said wave guide, said coaxial output section mounted on said wave guide, said coaxial output section including a tri-polar crystal having one element thereof directly connected to said probe, 21 second element thereof forming the center conductor of said output section, and a third element thereof directly connected to said wave guide.

8. Mixing apparatus for broad band signals comprising a section of wave guide for conducting energy of at least two ultra high frequencies, an annular plug section mounted on a broad wall of said wave guide, a probe centrally disposed in said annular plug section and penetrating said wave guide, a wire of diameter much smaller than that of said probe connected between the tip of said probe and a narrow Wall of said wave guide, and a tri-polar crystal mounted in said annular plug, one element of said crystal being directly connected to said probe, a second element of said crystal being directly connected to said annular plug, and a third element of said crystal being capacitively connected to said annular plug.

9. Crystal holding apparatus for broad band signals comprising a section of rectangular wave guide having a pair of opposed narrow walls and a pair of opposed broad walls, an annular plug mounted on a broad wall of said wave guide, an opening being formed in said wave guide in register with the opening of said annular plug, a probe centrally disposed in said annular plug and penetrating said wave guide, a crystal rectifier, resilient fingers being formed at the outer end of said probe for engaging one element of said crystal rectifier, resilient fingers also being formed at the outer end of said plug for engaging a second element of said crystal rectifier, a coaxial output line having inner and outer conductors, said outer conductor terminating in and connected to said wave guide and said inner conductor passing through a narrow wall of said wave guide, and a fine wire disposed within said wave guide parallel to said broad wall thereof, one end of said fine wire being connected to the tip of said probe within said wave guide and the other end of said wire being connected to said inner conductor.

10. Mixer apparatus comprising a section of wave guide having a pair of opposed narrow Walls and a pair of opposed broad walls for conducting energy of at least two ultra high frequencies, a crystal mixer for providing a signal of intermediate frequency obtained by the beating of said ultra high frequencies, a pair of output terminals, and a circuit in series across said terminals, said circuit including said crystal mixer, a probe for picking up energy within said wave guide, a fine wire of thickness less than that of said probe within said wave guide and disposed perpendicular to the narrow walls of the wave guide, and said wave guide.

References Cited in the file of this patent UNITED STATES PATENTS 2,502,456 Hanson et al. Apr. 4, 1950 2,605,399 Pound July 29, 1952 2,642,494 Zaslavsky et al. June 16, 1953 2,731,561 James et a1. Jan. 17, 1956 FOREIGN PATENTS 1,102,769 France May 11, 1955

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