Choosing and using cables for RF calibration

Coaxial cables are common throughout RF and microwave calibration, representing significant investment as precision cables can be very expensive. Choosing an appropriate cable type is often critical to successfully making accurate, repeatable measurements. Characteristics of the cable, the connectors, and the attachment of the connectors to the cable all contribute. The key characteristics are attenuation, phase shift (delay) and match, and their stability with time, temperature and flexing/movement of the cable and connectors. Maintaining the cable and connectors in good condition is essential to minimizing errors and uncertainties.

For less demanding applications such distributing reference frequencies, around the laboratory or between individual instruments within a system, typical general purpose RG58 cables using BNC connectors will suffice. However these types of cables are not appropriate for metrology applications where signal level or phase accuracy and stability or impedance match is critical. Generally the BNC connector is not appropriate for calibration applications, but there are some higher quality BNC connectors available which are typically used with higher grade cables in oscilloscope calibration. The majority of oscilloscopes appearing in the calibration workload have BNC connectors, so use of a BNC connector is unavoidable.

At RF and microwave frequencies, the cables and connectors are transmission lines. Using cable and connector types appropriate for the frequency range is essential in order to avoid excessive attenuation and other unwanted effects as the transmission line approaches its cut-off frequency. Coaxial connectors are described by the transmission line diameter (internal diameter of the outer connector). At higher frequencies the smaller transmission line dimensions dictated by shorter wavelengths place greater demands on mechanical tolerances. Cables and connectors become smaller in diameter, more fragile, and require greater care in their use, handling and storage. The precision N-type (7 mm) connectors common in many metrology applications are useable up to 18 GHz. Other types, including PC3.5 mm, 2.92 mm (K), 2.4 mm, 1.8 mm (V), 1.0 mm, and so forth, are designed for higher frequencies (110 GHz for the 1.0 mm). Do not exceed the manufacturer's frequency range specificstions for the cables and connectors. Some of these connector types are non-destructively inter-connectable, but doing so introduces excessive attenuation and mismatch.

Unsurprisingly, the improved performance of metrology grade cables is accompanied by higher costs, typically an order of magnitude more expensive than general purpose cables, with the higher precision cables being even more expensive. These flexible and semi-flexible cables are of the “level stable” type, where attenuation characteristics are not significantly affected by variations in temperature and flexing. Good practice is to observe a minimum bend radius of around 100 mm. Kinked cables will have unpredictable performance and should be discarded to prevent inadvertent use.

Phase stable cable types, as their name implies, also maintain phase (delay) characteristics with time, temperature and flexing. Cable of this type is commonly used as Vector Network Analyzer (VNA) test port cables where good flexibility and immunity to bending and flexing are required. 

The manner in which the connector and cable are joined – crimped or clamped - is also important, both electrically and mechanically. Mechanical arrangements differ with connector design, with potential discontinuity of the transmission outer conductor through the termination resulting in variations in transmission line characteristic impedance and therefore contributes to match (mis-match) performance. In a crimped connector the cable outer conductor is secured by compression between a metal sleeve and the connector body. In a clamped connector there is a nut and ferrule securing the cable outer conductor to the connector body. Crimping has the potential to add further transmission line discontinuities if the pressure applied to form the crimp distorts the cable or connector components. Clamping has the potential for a smoother transition of the transmission line outer conductor, and therefore better match. However, there is opportunity for loosening of the clamping nut with cable movement, etc, degrading the connection impacting attenuation and match performance, potentially in an intermittent fashion. Crimped and clamped terminations have different attributes and users should choose according to their needs.

It is good practice to consider cables much like any other calibrated item within the laboratory, including them within routine maintenance and calibration schedules, and to serialize or asset tag cables as a means of identifying individual items. Many higher grade cables are supplied with measured data for attenuation and match and users may make their own measurements, for example using VNAs. Regularly inspect cables and connectors for damage and any other degradation that might affect performance, monitoring characteristics, changes and where appropriate account for the characteristics during use.

If you are interested in learning more about choosing cables and connectors for RF calibration, as well as other common sources of measurement errors in RF calibration, watch this on-demand web seminar: How to identify and avoid common errors in RF calibration. 

You may also be interested in this application note: "RF calibration best practice guide: coaxial connectors."