RF Test Fixture
Understanding RF Test Fixtures
RF test fixtures bridge the gap between coaxial test equipment and the real-world packaging of RF components. The fixture quality directly determines the accuracy of DUT characterization.
Test Fixture Types
- Coax launcher: SMA/K/V connector soldered to a PCB test board. Simple, universal. Usable to 40+ GHz with proper design.
- Probe station: Precision probes contact the DUT directly on-wafer or on-PCB. Highest accuracy. GSG (Ground-Signal-Ground) probes to 110+ GHz.
- Split-block: Machined metal fixture that clamps around the DUT. Used for waveguide MMIC and chip testing.
Fixture Design Rules
- Minimize transition length between connector and DUT.
- Maintain controlled impedance through the entire transition.
- Include calibration structures (thru, open, short) on the fixture board.
Frequently Asked Questions
What is an RF test fixture?
A fixture that connects coaxial test equipment to non-coaxial DUTs. Types: coax launchers (PCB), probe stations (on-wafer), split-block (waveguide). Fixture quality determines measurement accuracy.
How accurate is a test fixture?
A well-designed fixture with proper de-embedding: residual error < 0.1 dB in insertion loss, 0.1 degree in phase. Without de-embedding: fixture adds 0.5-2 dB loss and significant phase error depending on frequency.
When should I use probes instead of fixtures?
Above 20 GHz where fixture parasitics become large, for on-wafer measurements, and when highest accuracy is needed. Probes provide the shortest, most controlled transition to the DUT.