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A traceable type K connector for 43.5GHz measurement
2021-09-14
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Author:Frank

Compared with its predecessor, 4G, the 5G mobile communication standard is a big leap. The application focus is no longer limited to the sub-6GHz communication field, but more different application frequency bands are accommodated in the communication chain. Most of the used frequency bands have standard test equipment, such as the 10GHz, 20GHz and 40GHz frequency bands. The 5G millimeter wave spectrum from 37GHz to 43.5GHz has created new demand for measurement equipment. Test and measurement (T&M) equipment manufacturers have released 43.5GHz equipment to meet the needs of wider frequency coverage. However, how good the measurement results are, you need to have the components that transmit signals to and from the test equipment, and how good the connectors of these components are. To accurately measure 43.5GHz, you need to use the 2.92 mm (K-type) connector that can work up to 40GHz, while providing mode-free performance that can work up to 43.5GHz and a clear traceability path.

Why choose 43.5GHz?

Although many initial 5G deployments use the sub-6GHz frequency band, millimeter waves (ie, 24GHz and above) have the advantage of greater bandwidth. Many countries are allocating spectrum in the 37-43.5GHz range to 5G millimeter wave communications (see Figure 1). In June 2018, the US Federal Communications Commission (FCC) proposed the use of the 42~42.5GHz frequency band for broadband or fixed wireless services, while Brazil and Mexico also made similar proposals to use the 37-43.5GHz frequency band for mobile broadband services. Japan and the European Union have also proposed to apply similar mobile broadband services in the 40.5-43.5GHz frequency band. China may be the biggest promoter of millimeter wave applications using frequencies up to 43.5 GHz. The Ministry of Industry and Information Technology of China has been at the forefront of 5G R&D and testing. In addition to planning for 5G spectrum, China has also conducted research and development trials, and began to verify such pcb products at the end of 2018.


Figure 1: 5G millimeter wave spectrum planning and application in various countries1

In the past few years, this frequency expansion has been quietly adopted by many test and measurement companies, adding this frequency option to existing and new products. One of the many aspects that provide 43.5GHz measurements is the connector interface, which is the interface between the user equipment and the test equipment. Currently, there are two ways to allow users to use 43.5 GHz:

Use 2.4mm connectors on the test equipment-this method has a dual advantage. First, the connector meets the working performance of 50GHz, and secondly, traceability is established. However, a problem faced by this method is that the user must replace all cables, adapters, calibration tools, and other components with 2.4mm connector interfaces. The cost of this is very expensive, because 2.4mm connectors are usually more expensive than 2.92mm connectors. Another problem is that many DUTs use 2.92 mm (K) connectors, which means that users must add additional adapters to connect the 2.4 mm connector on the test equipment to the 2.92 mm DUT.器连接。 Connected. Although most manufacturers that use 2.4mm connectors provide 2.92 mm adapters, unless the adapter indicates that it can be used up to 43.5 GHz at the 2.92mm end, the performance of 43.5 GHz is limited by the excess output generated on the connector. Over-moding is limited and cannot be guaranteed. This will continue to be discussed below.

Use 2.92 mm connectors on the test equipment-the second method is to use 2.92 mm connectors on the equipment, but it should be noted that from 40GHz to 43.5GHz, this kind of connector cannot be traced back, and its nominal performance It is "measured". The disadvantage of this method is that the connector may not be tested individually and can only be used as part of the DUT for "whole" measurement.

Overmold

pcb board

The two most important indicators of the electrical performance of a connector are its frequency scalability and whether it meets the performance required for the 43.5GHz frequency. In order to obtain the best performance, some modes should be prevented from spreading in the connector. For 2.92mm(K) connectors, theoretically only transverse electromagnetic (TEM) waves can propagate up to about 46GHz. In fact, the cut-off frequency will be lower: the dielectric support beads need to take into account the mechanical stability of the connector, and because the wavelength in the medium is shorter than that in the air, the electromagnetic waves of the other modes can also propagate below 46GHz. This is why K-type connectors are usually rated to work at a maximum of 40 GHz.

Above the cut-off frequency, an additional mode, TE11 mode, will also propagate. It is not lateral and propagates at a higher frequency like other modes of waves2. This is a problem, because the energy of the input signal can be converted between different modes, and this conversion is caused by tiny defects on the surface of the support bead (as shown in Figure 2). The overmolding phenomenon in the connector can be revealed during the measurement process. It is clearly visible during the transmission measurement of the connector, as shown in Figure 3, that the large attenuation peak appears in the narrow band. Once the frequency resonance is missed-the energy coupling between modes is not efficient-the energy will be reflected back to the original transmission path.





By reducing the perimeter of the dielectric support bead, optimizing the impedance of the support bead, and reducing the tolerance to reduce the chance of energy coupling into the transmission mode, the occurrence of overmode can be prevented. Assuming that a manufacturer overcomes all obstacles and designs a 2.9mm connector that will not produce over-mode at 43.5GHz, can this provide sufficient confidence in the measurement? The answer will vary from application to application, depending on the rigor of the test specification. This information will be explained in the data sheet.

Why is traceability so important?

A term used in the electrical specifications of measuring instruments in the frequency range of 40 to 43.5 GHz is "measured index". A measured index or characteristic index is a measurement result that can provide a set of data, and these data can be quantified with a certain degree of confidence and used to characterize all equipment. Although this method of measuring electrical indexes is not uncommon and is becoming more and more common, the difference between measuring indexes below 40 GHz and measuring indexes above 40 GHz is traceability. Below 40 GHz, the uncertainty budget is clearly defined through a complete traceability approach; measurement results between 40 and 43.5 GHz usually do not have the same confidence. For manufacturers, uncertainty may be important, because the measurement results of the product will determine whether it can pass the requirements of the test specification.

Although traceability is a way to establish a reliable uncertainty budget, it is more important: the quality associated with recognized national metrology institutions such as the National Institute of Standards and Technology (NIST) or the Swiss Federal Institute of Metrology (METAS) Assurance system. Not all connectors are traceable, such as SMA connectors. Although the connector is widely used, it is generally considered to be untraceable due to the irregularity and poor repeatability of its dielectric materials. This is the reason why SMA connectors cannot provide accurate measurement results.

Fortunately, the basic characteristics of the K-type connector ensure its traceability, and after careful design, the frequency range of reasonable and recordable uncertainty can be increased to 43.5GHz. The most basic aspect of connector traceability is impedance, which depends on the size evaluation and control of the airline used to measure the connector. Dimensional measurement uses traceable tools such as laser rangefinders, coordinate measuring devices, and air gages. Once these measurements are complete, the next step is to transfer the air line performance to a single connector through calibration tools and other components (as shown in Figure 4). The IEEE P287 coaxial connector standard lists the connectors used for evaluation
Traceable K-connector
In order to design a traceable 43.5 GHz connector, Anritsu has designed a new connector function called Extended-K™ (Extended-K™). The extended K-type components with 2.92mm connectors will not be overmolded and provide traceable indicators of 43.5GHz, thus avoiding the expensive investment of migrating the measurement system to 2.4mm connectors. Anritsu provides a complete 43.5 GHz K-connector measurement system, including test port cables, 2.4 mm adapters, portable TOSL calibration tools (both male and female) and Anritsu’s ShockLine™ vector network analyzer (with extended K Type function). Anritsu’s adapters are also traceable, allowing users to quantify their uncertainty budget.