1. Introduction
Precision phase-stable cable assemblies consist of precision phase-stable RF coaxial cables and precision phase-stable RF coaxial connectors, assembled through precise installation processes and subjected to rigorous and responsible testing. These assemblies differ from standard RF cable assemblies and are distinct from common low-frequency cable harnesses.
Low-frequency cable assemblies only require low contact resistance and reliable mechanical contact. Standard RF cable assemblies, in addition to these requirements, must meet certain RF performance criteria. Precision phase-stable cable assemblies, however, must not only meet the requirements of standard RF cable assemblies but also ensure that these parameters maintain specific performance standards within the user-defined environmental conditions, such as variations in these parameters and their consistency.
The following sections elaborate on the definition, purpose, and scope of key technical requirements for precision phase-stable cable assemblies, as well as how users should specify their requirements and what manufacturers should pay attention to during production.
2. Definition and Purpose of Key Technical Requirements
2.1 Phase
Electrical Length: Defined as the electrical "length" of a cable assembly in terms of wavelength, electrical angle, or absolute phase.
Phase Matching: Includes absolute phase matching and relative phase matching.
Absolute Phase Matching: The difference in absolute phase between individual cable assemblies must fall within a specified range. Absolute phase matching ensures that the absolute phase of two or more assemblies falls within the user-defined tolerance range.
Example: “All cable assemblies must have an absolute phase of X ±3° at 2 GHz.” Every individual cable assembly must meet this requirement. Absolute phase matching is sometimes referred to as "infinite" matching.
Relative Phase Matching: In a batch of cable assemblies, one assembly is used as a reference, and the phase difference of the others relative to this reference must fall within a specified range. Unlike absolute phase matching, relative phase matching focuses only on consistency within a batch rather than on absolute values.
Example: “All cable assemblies must have a phase difference of X° at 2 GHz.” Each assembly within the batch must meet this requirement, but no phase matching is required between different batches. Relative phase matching is easier to achieve than absolute phase matching, but the tighter the tolerance, the higher the manufacturing cost.
Phase Temperature Stability: The extent to which phase changes over the entire operating temperature range, also referred to as phase response. Phase temperature variation in precision phase-stable cable assemblies is influenced by numerous factors, especially the dielectric material of the cable.
Phase Tracking: The ability of multiple identical cable assemblies to maintain consistent phase variations and accurately reproduce phase changes under varying temperatures.
In military applications, phase variations affect beamwidth, sidelobe suppression, and beam steering, which in turn impact system operational range, resistance to ground-based interference, rejection of spurious signals, and overall system accuracy. In digital systems, these errors increase signal distortion.
Most phase tracking failures result from poor materials, inadequate process control during cable and assembly manufacturing, or the use of mixed cable assemblies, such as cables and connectors from different manufacturers. These issues can be mitigated through strict control of cable and assembly production processes and comprehensive, responsible testing.
Mechanical Phase Stability: The extent to which phase changes when a precision phase-stable cable assembly is bent or flexed.
In phase-sensitive systems, maintaining phase stability during bending or flexing is crucial. Since installation, routine maintenance, and actual usage may involve bending of cable assemblies, phase changes during such movements must be accounted for in system phase design.
In some systems, predictability of phase variation is particularly important, such as in phased array systems, where multiple channels must maintain phase matching within a defined tolerance range.
Another often-overlooked factor is phase changes caused by system maintenance. If a cable is temporarily moved for access, returning it precisely to its original position is nearly impossible. Even if placed back in the same position, electrical length may change. Minimizing these changes is crucial to prevent system degradation. Unfortunately, phase variations due to bending or flexing are unavoidable. Since coaxial cables are cylindrical, the outer bend radius is always larger than the inner radius, leading to geometric and internal pressure changes caused by stretching and compression.
Coaxial cables consist of multiple cylindrical elements, including multi-core inner conductors. Differences in diameter, material, and structure among these elements cause varying changes in geometry, stress, and torque when bent, ultimately leading to phase variation.
2.2 Insertion Loss
Insertion loss refers to the attenuation of electrical signals due to the introduction of a cable assembly. It is as crucial as phase in precision phase-stable cable assemblies and involves factors such as absolute loss, absolute loss matching, relative loss matching, loss temperature variation, mechanical loss variation, and loss tracking. These terms are conceptually similar to those under “Phase” and will not be repeated here.
Insertion loss variation describes the change in insertion loss across different cables of the same model due to temperature, mechanical stress, and other factors. Loss tracking refers to the consistency of insertion loss variations under these conditions. Poor loss tracking affects system stability, including beam stability, sidelobe suppression stability, and beam steering control stability.
2.3 Voltage Standing Wave Ratio (VSWR)
The voltage standing wave ratio (VSWR) of a precision phase-stable cable assembly is calculated as:
S = S₁ × S₂ × S₃
where:
S₁ is the VSWR of the connector at one end of the cable assembly,
S₂ is the VSWR of the connector at the other end,
S₃ is the VSWR of the cable used in the assembly.
Amplitude Stability of VSWR: The ability of VSWR to maintain amplitude stability under variations in temperature and mechanical stress is referred to as VSWR amplitude stability. Typically, this must be within ±0.03.
3. Analysis and Discussion
Due to their unique operational environments and system applications, precision phase-stable cable assemblies must endure temperature fluctuations, vibration, shock, humidity, bending, flexing, and mechanical torque. Long-term exposure to such conditions can cause issues such as brittle solder joints due to metal fatigue and component failure. These assemblies must also minimize self-induced noise and noise effects under vibration. Most importantly, they must maintain electrical performance stability.
4. Conclusion
A thorough understanding of the technical requirements, purposes, definitions, and application backgrounds of precision phase-stable cable assemblies, as well as their impact on user systems, is fundamental to manufacturing high-quality cable assemblies. It is also crucial for users to make informed choices when selecting and utilizing these assemblies.
Users should define technical requirements based on system structure, performance needs, and operational demands to achieve optimal cost-performance balance. For phase and insertion loss matching, relative matching should be prioritized over absolute matching where possible. Maintaining and preserving absolute matching reference samples is extremely challenging.
