How do I select between chip and coaxial terminations for different power levels and frequencies?
Chip vs. Coaxial Termination Selection
The choice between chip and coaxial terminations is driven by the physical integration level and the power handling requirement. In general: use chip terminations for on-board applications at low power, and coaxial terminations for system-level connections and higher power.
- 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
Frequently Asked Questions
Can I use a standard SMD resistor as an RF termination?
Yes, with limitations. A standard 49.9-ohm 0402 chip resistor (e.g., Vishay CRCW0402) provides adequate termination to approximately 6-10 GHz with VSWR < 1.5:1. Above 10 GHz: the parasitic inductance of the resistor body and solder pads degrades the match significantly. For frequencies above 10 GHz: use dedicated RF termination chips (e.g., Mini-Circuits ANNE-50+, Vishay FC series, or Susumu RR series) that are specifically designed with controlled parasitics for RF use. These cost $0.20-1.00 compared to $0.01-0.05 for standard resistors, but the improved RF performance justifies the cost.
What about high-power chip terminations?
For PCB-level terminations that need to handle 5-25W: use high-power surface-mount termination resistors from manufacturers such as: Anaren (now TTM) A25N50-1 series (25W, DC-6 GHz, flanged package), Florida RF Labs (high-power chip resistors to 100W), and API Technologies / Inmet (broadband chip terminations). These high-power chips use a BeO or AlN substrate for superior thermal conductivity and require a thermal via array under the PCB pad to conduct heat to the ground plane chassis.
When should I use a waveguide termination instead?
Use waveguide terminations when: the system uses waveguide interconnections (no coaxial connectors), the frequency is above the practical range of coaxial connectors (above 50-110 GHz depending on connector type), or the power level exceeds the coaxial termination's rating (waveguide terminations can handle kilowatts using water-cooled loads). Waveguide terminations use a resistive vane or wedge inside the waveguide that gradually absorbs the incident wave with minimal reflection. High-quality waveguide loads achieve VSWR < 1.05:1 across the waveguide band.