Cable Assemblies can be matched in Phase, Delay, and Amplitude. The most common matching required is Phase matching. The match can be specified in electrical degrees at a specified frequency or in time delay. Of interest are 3 groups of candidates for phase matched cable assemblies:
Normally two specifications are used for phase matched sets of cables assemblies:
A) Matching to a Standard:The phase standard is usually a piece of hardware, a “Gold Standard”, it also could be an unchanging software standard; i.e. a specified electrical length at a certain frequency.
B) Matching as a Set:Cable assemblies matched as a set means that the assemblies of the same set are matched to each other. The cables in one set may not match those of another set.
1) Frequency of Operation:The higher the operating frequency and the closer the required phase match, the more difficult the matching process and the cost will be.
2) Length of Cable Assemblies:Short Cable Assemblies are usually easier to match and to test than longer assemblies. With longer assemblies there is already the problem that they need to be coiled for testing and this results in phase changes already, a wider match window is required.
3) Connectors of the Assemblies in one set:There is no problem to use different connector styles in a set of matched cable assemblies; it may add additional cost though, as the matching process may get more complicated because of different connector lengths and dielectrics.
4) Installation Process:Especially for matched sets with long cable assemblies phase changes can be expected during installation. Phase adjustable connectors or adapters can be used for phase correction after installation.
5) Variation of Velocity of Propagation:Manufacturing without tolerance is not possible, unfortunately. For a cable the velocity of propagation is usually specified at +/- 2%, or maybe at +/- 1%, resulting in different electrical lengths of cable assemblies with identical physical lengths. This will be noticeable especially with long cable assemblies or when using cable manufactured from different lots.
6) Temperature:Change in temperature will result in change of electrical length of the cable assembly, caused by the dielectric of the cable. Cables using solid extruded PTFE dielectric are generally strong mechanically but higher for insertion loss and show worst phase changes over temperature when compared to cables using air content in their dielectric. These latter cables are weaker mechanically but lower in insertion loss and have better phase versus temperature characteristics.
7) Testing Phase Match:Usually Vector Network Analyzers will be used in a temperaturecontrolled room. But it has to be taken into consideration that test results taken even with the best equipment are subjected to tolerances.
8) Phase Tracking:Phase tracking is usually caused by three parameters:
Preconditioning:The cables of a phase-matched set need to be thermally stress relieved before phase matching. At Spectrum Elektrotechnik GmbH the cable is preconditioned by exposing it several times to temperatures of -54°C to +125°C, in some cases even between -71°C to +200°C. This will assure good phase tracking.
Temperature:The phase change with temperature may not be that critical if the complete set is exposed to the changing temperature, as the phase will shift equally in all of the assemblies of the set, assuming that assemblies are not formed in a bundle where the inner assemblies will see the temperature change much later than the outer assemblies. It will be most critical when assemblies of the same set, matched at ambient, are subjected to different temperatures in the system.
Bending:As outlined earlier for sets with long cable assemblies phase changes can be expected already after static installation. For dynamic installation phase tracking will depend on the bend radii, the number of cycles, and the similarity of the flexure cycles of the assemblies in the set.