Competence from a Single Source.

Nowadays radio-frequency (RF) cables are highly specialized and can transmit signals up to 110GHz. Their electrical properties depend on various parameters like temperature, humidity, air pressure, mechanical robustness, shield properties and the number of bends. As these parameters cannot all be mapped into a single product, an initial proper selection based on a set of reliable measurements of an RF cable is crucial for the success of a project to avoid incalculable crucial follow-up costs. Most of the cables on the market are only inadequately measured for all relevant parameters, resulting in a high uncertainty of getting the right cable for your application. Therefore, el-spec GmbH and Deutsches Elektronen-Synchrotron DESY jointly offer a large set of measurements for the characterization of S-parameters, shielding, thermal, mechanical, cryogenic and magnetic properties. 

The electrical properties of high frequency cables depend on various parameters like temperature, humidity, air pressure, mechanical robustness, shield properties and the number of bends. As these parameters cannot all be mapped into a single product, a proper selection of an RF cable is crucial for the success of a project to avoid incalculable crucial follow-up costs. Unfortunately, most of the cables on the market are only inadequately measured and only good parameters are highlighted, resulting in a high uncertainty of getting the right cable for your application. Transferring the design process knowledge from the electro-magnetic compatibility community [1] to RF cables, as depicted in Figure 1, an unproper choice of RF cables during the design or testing phase let the costs growing exponentially in the production phase, e.g., by re-installation new cables or downtimes of the application.

[1] Page 5, Henry W. Ott, „Electromagnetic Compatibility Engineering“, WILEY, ISBN 978-0-470-18930-6

For this, the el-spec GmbH and DESY offer a range of measurements to test RF cables with respect to crucial parameters such as S-parameters, shielding, thermal, mechanical and cryogenic properties. The time-consuming and cost-intensive step of the testing phase and the risk of an unproper choice can be eliminated for the customer.

The S-parameter representation is the most common description for high frequency devices. The S-parameters give relevant information about a device. Nowadays S-parameters can easily be measured with network analyzers. A high frequency cable including connectors, with one input and one output is a 2-port device. With b=S ∙a  the definition of the s-parameters for a 2-port device is given by:

The knowledge and systematic characterization of a cable shield is fundamental for many applications, because the shield carries the return current of a signal and defines the signal transmission quality as well, which is often neglected. For higher frequencies the screening attenuation qualifies the cable shield in conjunction with the connector interface. Especially in devices and applications where high frequency interconnections are sensitive as a victim or culprit to radiation effects, the cable has to be specified to fulfill international standards of electromagnetic-compatibility. For lower frequencies the so-called transfer impedance of a shield defines the voltage drop caused from currents crossing the shield, which is important for interfaced electronic systems. The knowledge of the screen attenuation and transfer impedance of a specific cable contributes to a failsafe operation of electronics.

Measuring the Transfer Impedance and Screening Attenuation
The shield of a high-frequency cable carries the return current of a signal and defines the signal transmission quality as well, which is often neglected. Figure 3 (left) shows a typical structure of a high frequency cable with two layers of shields.  Figure 3 (right) defines the transfer impedance of a cable in the low frequency range, whereas the screening attenuation is defined by the ratio of the radiated P₂ to the input RF power P₁ for a certain cable length.

To test, improve and verify the quality of various cable shields and its connection to the assembled connector, Innovation & Technology Transfer (ITT), Machine-Beam-Control (MSK) and the cable specialist el-spec build up a new cable shield laboratory located in the Innovation Village at DESY. The setup is available for the accelerator & research community and industry in general. Figure 4 (left) shows a block diagram of the setup to measure the transfer impedance of a cable and the screening attenuation according to international standards. Here the cable under test is driven by a source generator of up to 8 GHz and terminated. The radio frequency (RF) energy radiated from the cable shield is collected and measured by a vector network analyzer. Figure 4 (right) shows a typical measurement of the frequency dependent transfer impedance and screen attenuation.

With this, the quality of the shield, the specific connector assembly or unwanted leakages during production and installation can be characterized and monitored. Figure 5 shows the cable shield setup in the laboratory located in the Innovation Village. As an example, Figure 6 shows a typical degradation of high-frequency cables by mechanical stress. The measured screen attenuation of a conformable cable, which is widely used in laboratories, is shown in dependence of its number of mechanical bends. The screen attenuation degrades from the measurement floor of about -125 dB/m by 15 dB for 10 bends, or 50 dB for 100 bends, respectively.  As shown in Figure 6, this is caused by microscopic cracks of the soldered shield braid (marked in red).

In many applications, where cables are localized in magnetic sensitive areas, the usage of non-magnetic cables and connectors are important. In medicine, nuclear spin tomographs operating at high magnetic fields of some Tesla require cables and connectors that are not magnetizable.  In the field of quantum computing, quantum sensors are highly sensitive to magnetic field requiring cables and connectors to their cryogenic units without any residual magnetic field. For this, el-spec GmbH offers measurements to determine the magnetization of connectors.

Measuring Magnetization of Connectors
To determine, if connectors are not magnetic, Figure 9 shows a setup to measure its magnetization. For this the sample is magnetized with static magnetic field and placed onto a differential measurement sensor. Fig. 9a shows an SMA to SMA adapter made of stainless steel with a high magnetization of about 7uT in comparison to a brass type with a low magnetization of about 0.1 uT.

Please contact our elspec team and we will get back to you with a solution.