US2429823A - Crystal detector - Google Patents

Description

Oct.'28, 1947 c T. H. KHIQMANY El AL 2,429,823

CRYSTAL DETECTOR Filed Aug. 2, 1946 Ifiventors I Thomas H. Kinm'an, Bertram A.C.Tuc:ker",

by M

Their AtfcOl-"fiey Patented d. 28, 1947 CRYSTAL DETECTOR Thomas H. Kinman and Bertram A. 0. Tucker, Rugby, England, assignors to General Electric Company, a corporation of New York Application August 2, 1946, Serial No. 688,169 In Great Britain May 1, 1944 Section'l, Public Law 690, August 8, 1946. Patent expires May 1, 1964 3 Claims.

This invention relates to the construction of pre-adjusted crystal detectors of the silicontungsten type which have been found suitable for use in so called microwave receivers and are particularly applicable to wave-lengths within the band below 10 cms.

One object of the invention is to provide a mechanically stable assembly which can be handled without disturbing the catwhisker contact adjustment and which is not affected by variations in ambient temperatures.

A further object of the invention is to fix the crystal in a position where it can effectively be matched into a concentric line so that most of the signal power is absorbed by the detector and the standing waves set up in the line are reduced to a very small amplitude.

A crystal detector adapted for direct connection to and forming the termination of a concentric line according to the invention comprises a cylindrical metal head having an axial bore for receiving a crystal-mounting plunger adjustable therein, the crystal end of the head having a recess of such dimensions that, with the crystalmounting plunger in position, the crystal is mounted at a quarter wave length (or an odd multiple thereof) from the head, and a cupshaped insulating member secured to the head and supporting the cats whisker coaxially with the crystal-mounting plunger, the insulating member and cats whisker being so dimensioned as to form an effective half-wave length (or multiple thereof) connection to the central conductor of the concentric line.

For a better understanding of the invention reference should now be made to the drawing the single figure of which shows in cross-section the constructional features of a crystal detector assembly embodying the invention.

Referring to the drawing, this shows the crystal G and cat-whisker wire F which constitute a continuation of the inner conductor D of a concentric line, and which in conjunction with outer conductor C, comprising a metal tube attached to a metal head A, terminates a concentric line,

having substantially the same characteristic im-' pedance up to the point where the inner conduc- 2 be soldered firmly to an inner circular flange on the metal head A. The inner conductor is firmly attached and supported by a soldered joint through the hole, thus effecting a rigid assembly which has been found to be proof against mechanical shock or temperature changes after the contact adjustment. Contact adjustment iseffected by means of a slidable plunger H to which at one end the crystal wafer G is attached'by means of a plating and soldering process. After the contact adjustment has been made to obtain the desired detecting characteristic, the plunger H is fixed in position by means of a set screw J.

A flange B is provided on the head to facilitate the withdrawal of the crystal cartridge from its holder in the end of the line.

The impedance match is effected as follows:

The length of the recess K in the metal head A and the crystal-mounting plunger constitute a wave-length sleeve which ensures that the position of the crystal is definitely located in the line one quarter of a wave-length (or an odd multiple thereof) from a point of zero potential, or short circuit. This positioning of the crystal facilitates the matching of the latter through another eifective quarter wave section, comprising the cat-whisker wire which is dimensioned and so disposed to transform the relatively high crystal impedance into the line impedance in a manner well known in concentric line practice.

The apparent mismatch introduced by the presence of the insulating material, where the latter supports the inner conductor, is offset by making distance from the inner to outer faces of the ceramic supporting member on the axis an effective /2 wave-length long. This length, as is well known, corresponds to 7\//& where )\=wave length in air and e=dielectric constant of the material. Thus, the distance between the inner and outer surface constitutes M2 and the impedance will therefore be the same on each side of the insulation.

Crystal detectors so constructed have successfully been matched into concentric lines operating at wave lengths of the order of 1 cm. and the mismatch introduced; expressed as a standing wave voltage ratio set up in the line and measured by means of a travelling detector has been lessthan 1.2:1.

In the following claims Where we refer to an odd multiple or an integral multiple of a quarter or one-half wave-length, we intend to include the multiplier one.

While we have described only a preferred embodiment of our invention by way of illustration,

1. A high frequency crystal detector arranged to be directly connected to and to terminatea concentric transmission line comprising a cylindrical metal head having anannularrecess'at;one end thereof to form a central: mountmgpost said recess having a depth equal to an odd multiple of a quarter wave-lenth at saidirequency, a crystal.

element fixed to the end of said post, a cup-shaped insulating member fixed to said head and enclosing said crystal element, and a central con-ducting member projecting through said insulating cup and carrying a cats Whisker engaging said. crystal, said insulating member adjacent said central conducting member having a thickness effectively equal to an integral multiple of a half wavelength" at said frequency.

2. A high frequency crystal detector-"arranged tobe-directly connected to and toterminate a concentric transmission line comprisin a cylindrical metal head having an axial bore therein, a metal plunger fixed in said bore and carryin at one end a crystal element, the crystal endof said head being recessed to a depth equal to an odd multiple of a quarter wavelength at said frequency, a cup-shaped insulating member fixed to the recessed end of said head and having centrally mounted in the base thereof a conducting memberspacedfrom, said crystal, and acats whisker fixed to said central conducting member and engaging said crystal, the base of said insulating member having a thickness effectively equal to an integral multiple of a half wave-length at said frequency.

--Athigh 'frequency crystal detector arranged be directly-cennected to and to terminate a concentric transmission line comprising a cylindrical metal head having an axial bore therein, a metal. plunger adjustably mounted in said bore and carrying at one end a crystal wafer, the crysta-lL-end .of said head being recessed to a depth "equal to an odd multiple of a quarter Wave-length

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