Field of the invention

The invention relates to broad band coaxial load for RF signals. The load may be used as a calibration device for RF measurement equipment like a network analyzer.

Description of the related art

Network analyzers normally are calibrated by calibration standards. The precision of a calibration is largely dependent on the precision of the calibration standards used. Therefore, calibration standards are required, which provide specific characteristics over a broad bandwidth. Most calibration methods use through connection, open, short and a load. A calibration load must have a constant impedance over a specified frequency range resulting in a high return loss.

A coaxial load is disclosed in DE 2042821. Here, a rod of conductive material is held within a tapered outer conductor. A calibration load disclosed in

US 9,423,481 B2 is based on the same basic concept. Here, in addition, the inner conductor is suspended flexible to compensate for mechanical tolerances of an external connector mated with the calibration load. Generally, a flexing of the center conductor out of its precise center position is an asymmetry which affects impedance matching and therefore return loss specifically at higher frequencies.

Summary of the invention

The problem to be solved by the invention is to provide a broad band coaxial load which may be used for frequencies up to 150 GHz and more. The load should be easy and cost efficient to manufacture. It should have a high return loss over a wide bandwidth together with excellent long term characteristics. Solutions of the problem are described in the independent claims. The dependent claims relate to further improvements of the invention.

A broad band coaxial load is based on a conductive rod which is held coaxially within an outer conductor transition element, which is mentioned herein as a coaxial support. The coaxial support may have a body with a round or circular hole having at least three different sections.

The body itself may include a polymer or plastic material with a metallic surface. Such a metallic surface may be made by sputtering, anodizing or any other suitable process. The metallic surface should be at least in the region of the hole, such that the inner side of the hole has a conductive surface. It is not necessary to provide a complete metallic surface at the whole body.

The conductive body may be manufactured by an additive manufacturing method. Such additive manufacturing methods may include stereolithography or Photopolymerization, e.g. two-photon-lithography, two-photon-polymerization, multi-photon-polymerization. This allows an extreme miniaturization, because it is very difficult to machine a precise contour of the hole. Further, all components like springs which will be explained later in detail may be manufactured from one piece which results in increased stability and durability. Finally, this manufacturing method allows an extreme flexibility and scalability, such that the body may be adapted to a large scale of conductive rods and frequency ranges.

The coaxial support comprises at least three sections which at least differ in the shape of the hole.

A first section, which is starting from a first side of a coaxial support includes an exponential funnel. This funnel has a first larger diameter which may be adapted to a coaxial system which may be connected to a first side of the coaxial support and will be discussed later. The funnel decreases to a second diameter which is within the body and which may match to the diameter of the conductive rod. Basically, the funnel may also have a linear or straight slope between the first diameter and the second diameter, but a higher return loss of a load may be achieved by using an exponential slope.

The funnel may include at least one of a linear funnel, a curved funnel, circular funnel, a hyperbolic funnel and an exponential funnel. Basically, the funnel may be rotational symmetric to its center axis. It may have in a sectional view a linear, a curved, a circular, a hyperbolic an exponential shape.

After the funnel, a second section follows, which is a contacting section and which may be configured to contact the conductive rod. The second section may include a plurality of radially oriented contact springs. These contact springs generate a radial force to the conductive rod in order to electrically contact the conductive rod and to provide a comparatively short current path to the interior of the hole. The spring section may have a diameter which may be matched to the diameter of the conductive rod. Besides providing an electrical contact, the spring section provides at least some mechanical friction to the conductive rod and therefore may hold the conductive rod in its position within the body.

A third section following the second section comprises a cylindrical hole which has a diameter matching to the diameter of the conductive rod. This third section provides a precise guidance of the conductive rod, such that the conductive rod is stabilized in an axial orientation of the body. The rod may only be moved in an axial direction through the body. As the rod may comprise a comparatively stiff material, e.g. glass or metal as will be explained later in detail, the rod keeps a comparatively fixed position within the funnel of a first section due to the guidance of the third section. This is important as radial deviations in the position of the rod against the exponential funnel would change the impedance of the coaxial load and therefore reduce the return losses of the coaxial load. The hole through all sections may be a through hole.

The conductive rod must be adapted to the characteristic impedance of the coaxial system of the broad band coaxial load. It may comprise in an axial direction between a first end and an opposing second end multiple sections with different properties. There may be a first high conductive zone at the first end. This high conductive zone may be used to contact a coaxial conductor system like a center conductor. Next to this high conductive zone is at least a first resistive zone which may have a constant or a varying resistance over its length. After this first resistive zone, there may be a second resistive zone which may have a different resistance. Finally, there may be a further low impedance conductive zone, which may be used to be contacted with the contact springs of the coaxial support. The conductive and/or resistive zones are connected to neighboring zones.

The conductive rod may include a dielectric rod, e.g. a glass rod or glass fiber which may receive a conductive surface. Specifically from telecommunication applications, glass fibers can be manufactured with a very high precision. This is important, because variations in thickness or diameter of the conductive rod vary the impedance and affect the return loss characteristics. Such a glass fiber, herein also named as a glass rod, may receive a conductive surface by sputtering or any other suitable method like anodizing. It may also be possible to combine different technologies like anodizing and sputtering. Further, thin metal rings may be used to provide the low impedance sections.

When assembled, the conductive rod may protrude from the second side of the coaxial support. This may allow gripping the end of the conductive rod and to move it in an axial direction of the hole for precise adjustment. In a further embodiment, the end of the conductive rod may be glued to the second side of the coaxial support. This may allow a good fixation providing a long-lasting precise characteristic. Due to the force applied by the second section, this may not be necessary, but it may be used to improve quality.

In an embodiment, the coaxial support together with the conductive rod may be held by a shell. There may further be a coaxial outer conductor holding a coaxial center conductor, which both may be made of metal or of a material having at least a conductive surface. The coaxial center conductor may be connected to the conductive rod and the coaxial outer conductor may be in electrical and mechanical contact to the first side of the coaxial support. Further, the first diameter of the exponential funnel may have the same diameter as the inner diameter of the outer conductor.

There may be provided a further housing to increase stability and to improve the handling characteristics, as for high frequencies, the coaxial support is comparatively small and can barely be handled with bare hands. The housing may enclose a hollow space at the second side of the coaxial support. This hollow space prevents any object touching the conductive rod at the second side of the coaxial support and therefore changing adjustment.

As the contact springs are monolithic with the coaxial support, if they have a comparatively high stability and a common surface, which allows a conductive surface coating without interruptions. The contact springs together with the contact protrusions are arranged radially and they may be at the same axial distance to provide the same electrical length from the beginning of the conductive rod to each of the contact points. There are at least three contact springs which may be evenly angularly spaced. In an embodiment, six contact springs are provided even a higher number of contact springs depending on the possible degree of miniaturization may be used. In an embodiment, the conductive rod has a low impedance surface over its length or comprises a metallic material. Such an embodiment would be a short- circuit which may also be used for calibration.

In an embodiment, the conductive rod may be printed of an absorbing material, e.g. carbonyl iron.

Description of Drawings

In the following the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment with reference to the drawings.

Figure 1 is a sectional view of an embodiment.

Figure 2 shows a coaxial support in detail.

Figure 3 is a cross-sectional view through the spring section.

Figure 4 is a total view of the coaxial support.

Figure 5 shows a detail of the spring section.

Figure 6 is a sectional view of the coaxial support together with a conductive rod.

Figure 7 shows details of the conductive rod.

Figure 8 is a sectional view of a contact spring.

Figure 9 is a sectional view of a second embodiment of a coaxial support.

Figure 10 is a view from the second side of the second embodiment. Figure 11 is a sectional view of a third embodiment of a coaxial support.

Figure 12 is a sectional view of a modified embodiment.

Figure 13 is a front view of body parts.

Figure 14 is a front view of a modified embodiment.

In Figure 1, a sectional view of an embodiment is shown. A coaxial support 200 holds a conductive rod 300 coaxially and centered to its center axis 105. The coaxial support 200 may be held in between a shell 152 and an outer conductor 154. The outer conductor 154 may further hold a center conductor 144 by means of spacers 145. The center conductor 144 is in electrical contact with the conductive rod 300. A load housing 150 may be provided to protect the interior components and to increase the size of the device to improve handling by bare hands. Further, the load housing 150 may enclose a hollow space 159 behind the coaxial support 200.

In Figure 2, a coaxial support 200 is shown in more detail. The coaxial support 200 includes a body 270. The body may include a polymer material which may have been manufactured by an additive manufacturing process. Such additive manufacturing processes result in a large flexibility to modify the size and structure adapting to specific requirements. The body 270 includes a circular or round hole with at least three sections. The hole may have a conductive inner surface which may be obtained by sputtering or anodizing or any other suitable method. The first section 210 of the hole includes an exponential funnel 214. This funnel has a first diameter 211 at a first side 201 of the body 270. The diameter of the funnel decreases towards inside of the body to a second diameter 212. The second diameter 212 may be adapted to the diameter of the conductive rod whereas the first diameter 211 may be adapted to an input coaxial system. The second section 220 may include a spring which will be explained in more detail later. This second section may have an inner space 222 with contact springs 221 which may further be adapted to the diameter of the conductive rod.

The second section is followed by a third section 230 which has a straight bore 231with a diameter adapted to the diameter of the conductive rod, such that the conductive rod may slide through this section along the center axis 280.

In Figure 3, a cross-sectional view through the spring section 220 is shown. At the center are individual contact springs 221.

In Figure 4, a total view of the coaxial support is shown.

In Figure 5, a detail of the spring section is shown. The springs 221 may include elongated pieces of material which may flex when receiving a radial force, e.g. from the conductive rod. The springs 221 may also include contact protrusions 223 which improve contact and generate a higher point pressure on the surface of the conductive rod.

Figure 6 shows a sectional view of the coaxial support together with a conductive rod, here a possible alignment between the conductive rod and the coaxial support is shown. The second high conductive zone 340 at the second end may be aligned with the contact springs 221 of the second section 220. The first resistive zone 320 and the second resistive zone 330 may be within the exponential funnel and the first load impedance conductive zone 310 may be outside of the exponential funnel and outside of the coaxial support for contacting an external device or a center conductor 144.

Figure 7 shows details of the conductive rod. The conductive rod may include at least two conductive sections at its ends and resistive sections in between, whereas neighboring sections are connected to each other. At a first end 301, there may be a first conductive section having a low impedance 310. This may be followed by a first resistive zone 320 which may have a constant resistance over length or a variable resistance over length. This resistance may be adjusted by laser cutting or by adjusting the layer thickness which may e.g. be generated by a sputtering process. Following this zone, there may be a second resistive zone 330 which may have different resistance. This is followed by a second high conductive zone 340 close to the second end 302 of the conductive rod. This second conductive zone may be in contact with the contact springs 221 of the second section of the coaxial support 200.

Figure 8 shows a sectional view of a contact spring. The contact spring 221 has a contact protrusion 223 which presses against the conductive rod. When the rod is inserted, the contact protrusion is displaced outwards in the direction of arrow 224 as shown by dashed lines.

Figure 9 is a sectional view of a second embodiment of a coaxial support 400. A body 470 may include at least one slot 430 in the region of the contact protrusions and/or at the second side 202. The at least one slot 430 may basically extend in a radial direction from the outer surface of the coaxial support. The embodiment may work best if there are 2, 3, 4, 5 or 6 slots, which may be equally spaced angularly. The embodiment shown in the figure has 4 slots 430. This embodiment may work like a collet. By radially compressing the body by applying a radial force 440, the diameter of the second section 220 may be reduced such to assert or increase a force to the conductive rod. Such radial force may be applied by pressing the body into a conical section of an outer conductor 154 or shell 152. Instead, a nut with a conical thread either in the nut or on the body may be provided.

There may be a metal insert 420 which may include sheet metal or a metal film.

The metal insert may also be a metal tube. The metal insert may at least cover the surface of the first section 210 of body 470 including exponential funnel 214 and optionally the second section 220 and/or third section 230. Such a metal insert may provide a smooth inner surface oriented towards the conductive rod, further providing an optimized broad band return loss. Due to the metal insert, deviations in shape of the body like burrs may have no effect.

Figure 10 is a view from the second side 202 of the second embodiment of a coaxial support.

Figure 11 is a sectional view of a third embodiment of a coaxial support. The body 570 has a plurality of slots 530 similar to the previous embodiment. Here the body 570 may include a metal.

Fig. 12 shows a modified embodiment based on Fig. 3. Here, the coaxial support 205 is split into at least two body parts, a first body part 271 and a second body part 272. The parts may be split in a plane through center axis 280. This allows a simplified manufacturing process, specifically, if manufactured by a 3D printing process. There may be protrusions 273 and recesses 274 matching together to connect the first body part 271 with the second body part 272. The body parts may be identical, such that each body part includes protrusions 273 and recesses 274.

Figure 13 is a front view of the body parts. It shows, that the second body part 272 is the same as the first body part 271, but rotated about 180° around the center axis.

Figure 14 is a front view of a modified embodiment. Here, the first body part 271 with the second body part 272 are connected together to form a single body. There may be at least one or a combination of a snap connection, a press fit, a glued connection or a welded connection. List of reference numerals

100 broad band coaxial load

105 center axis

113 first end

114 second end

144 center conductor

145 spacer

150 load housing

152 shell

154 outer conductor

155 inner diameter of outer conductor

159 hollow space

200 coaxial support

201 first side

202 second side

205 first part

210 first section

211 first diameter of exponential funnel at outer end

212 second diameter of exponential funnel at inner end

214 exponential funnel

220 second section

221 contact springs

222 inner space

223 contact protrusion

224 displacement of contact protrusion

230 third section

231 diameter of third section

234 cylindrical hole

270 body first body part second body part protrusion recess center axis conductive rod first end second end high conductive zone at first end first resistive zone second resistive zone high conductive zone at second end second embodiment of coaxial support metal insert slot radial force body third embodiment of coaxial support slot body