How do I verify the phase stability of a coaxial cable assembly over temperature?
Cable Phase Stability Testing
Phase stability over temperature is one of the most critical cable specifications for phased-array radar, satellite communication ground stations, and precision test and measurement applications where multiple cables must maintain phase matching.
| Parameter | SOLT Cal | TRL Cal | eCal |
|---|---|---|---|
| Accuracy | Good | Excellent | Good-very good |
| Standards Needed | 4 (S,O,L,T) | 3 (T,R,L) | 1 (module) |
| Bandwidth | Broadband | Band-limited | Broadband |
| Setup Time | 5-10 min | 10-20 min | 1-2 min |
| Best For | Coaxial, general | On-wafer, waveguide | Production, speed |
Calibration Procedure
(1) VNA configuration: frequency range: at least the operating frequency of the application (typically 1-40 GHz). Number of points: 201-1601 (more points for better frequency resolution). IFBW: 100 Hz - 1 kHz (low IFBW for stable phase measurement). Source power: -10 to 0 dBm. (2) Thermal chamber: temperature range: at least -40°C to +85°C (MIL-STD) or -55°C to +125°C (extended). Temperature accuracy: ±1°C. The VNA must remain outside the thermal chamber (only the cable under test goes inside). Use feedthrough connectors or bulkhead adapters at the chamber wall for the VNA cables. (3) Reference cables: the cables from the VNA to the chamber feedthrough must be phase-stable (otherwise their phase change is included in the measurement). Use the shortest possible reference cables, or calibrate at the chamber feedthrough plane. (4) Soak time: at each temperature step, allow sufficient time for the cable to reach thermal equilibrium. Typical soak time: 15-30 minutes for coaxial cables (depends on cable mass and thermal conductivity). Verify equilibrium: the measured phase should not be drifting (less than 0.1° change per minute).
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Error Sources
PTFE (Teflon), the most common coaxial cable dielectric, has a crystalline phase transition at approximately 19°C (66°F). This transition causes: (1) A discontinuous change in dielectric constant (approximately -0.3% change in Dk). (2) A corresponding jump in phase velocity. (3) A sharp phase discontinuity of approximately 500-1500 ppm in a 1 m cable at the transition temperature. This effect is hysteretic: the phase vs temperature curve shows a different path for heating vs cooling, with the transition occurring at slightly different temperatures. Impact: for phased arrays operating near 19°C (room temperature in temperate climates), this transition causes a sudden phase jump that disrupts beam pointing. The effect is proportional to cable length and frequency. Mitigation: (1) Use expanded PTFE (ePTFE) dielectric (Gore, Micro-Coax): the molecular structure is modified to eliminate or reduce the phase transition. Phase stability: 50-150 ppm/°C with no transition. (2) Use solid PE or foam PE dielectric: no transition, but higher loss and lower temperature rating than PTFE. (3) Use air dielectric (semi-rigid with PTFE spacers): minimal dielectric effect, but the cable is rigid and expensive. (4) Temperature-compensated cables: designed with controlled thermal expansion to cancel the dielectric change.
Frequently Asked Questions
How do I specify phase stability for my application?
Determine the maximum allowable phase change from the system requirements: for a phased array with 0.25 lambda element spacing at 10 GHz: a 1° phase error causes approximately 0.01 lambda beam pointing error. For ±5° total phase budget over -40 to +85°C (125°C range): maximum phase change per cable = 5° / (125°C × f_GHz) = 5/(125 × 10) = 0.004°/°C/GHz. For a 1 m cable: this translates to approximately 120 ppm/°C. Specify cables with < 120 ppm/°C phase stability (select from the phase-stable cable category: Gore Phaseflex, Sucoflex 104, Times Phaseflex).
Does flexing the cable change the phase?
Yes. Bending a cable changes the phase due to: (1) Dielectric compression: bending compresses the dielectric on the inside of the bend and stretches it on the outside, changing the effective dielectric constant. (2) Physical length change: the outer conductor path length changes with bending radius. Phase change with flexure: standard cables: 5-20° per flex cycle at 18 GHz for a 90° bend. Phase-stable cables: 1-5° per flex cycle. Ultra-stable cables: < 1° per flex cycle. For rotating joints or continuously flexing applications: use flexible phase-stable cables designed for flex (Gore Phaseflex, Huber+Suhner Sucoflex). These cables are characterized for flex-induced phase change and can maintain < 2° per flex cycle over millions of cycles.
What about amplitude stability over temperature?
Cable insertion loss also changes with temperature: (1) The conductor resistance increases with temperature (copper: +0.39%/°C, silver: +0.38%/°C). This increases the conductor loss. (2) The dielectric loss tangent may increase or decrease with temperature (material-dependent). Typical amplitude change: 0.05-0.2 dB/m/GHz over a 100°C range for flexible cables. 0.01-0.05 dB/m/GHz for semi-rigid cables. For phase-critical applications: amplitude change is usually less important than phase change (the system can tolerate 0.5 dB amplitude variation more easily than 5° phase variation). But: for precision power measurement cables, specify amplitude stability requirements as well.