An unused duplexer port normally needs the termination specified by the measurement procedure. Correct termination helps establish the intended boundary conditions for an FDD test.
Why this matters in the industry
A three-port device is not fully defined by connecting only the two ports being measured. The remaining port can affect the result through coupling and mismatch.
The technical reasoning
A duplexer's third-port loading contributes to the measured network. The termination state should represent the intended test condition, and unwanted coupling can affect results at another port. Distinguish a network-analyzer measurement of the passive assembly from operation with connected transmit and receive electronics.
Characterizing the assembled network rather than one component
A multiport RF assembly includes transmission, reflection and coupling relationships between ports. A scalar loss measurement can answer some level questions, but it does not describe every interaction or phase response. Unused-port loading, fixtures and adapters contribute to the observed response. De-embedding attempts to remove a characterized fixture mathematically; it requires an appropriate model and stable connection conditions rather than a nominal dB subtraction.
How to structure the investigation
Confirm the required port impedance and termination quality across the test band. Connect the load at the specified reference plane and account for adapters. Use low-power network measurements where appropriate, keeping any active system bias requirements separate.
Define which network parameters matter to the experiment and establish reference planes for each port. Characterize the relevant routes with the actual unused-port states. For de-embedding, validate the fixture model with an independent check and retain its revision alongside analysis settings. Repeat affected measurements after interface repairs or changes that alter the assumed network.
Worked example or engineering scenario
Measuring a transfer path with the unused port open can give a different response from measuring it with a defined matched termination. The difference is part of the loaded network behavior.
Evidence to collect
| Record | Purpose |
|---|---|
| Follow the port map | Defines the tested state and scope of the comparison. |
| Check termination match | Makes the stimulus or route condition reproducible. |
| Include adapter effects | Supports interpretation of variation and possible confounding effects. |
| Document reference planes | Connects the observation with the stated engineering decision. |
Trade-offs and common interpretation errors
A nominal equal split or impedance does not establish perfect balance or zero reflection. De-embedding cannot reliably restore information lost through instability or an invalid model. State which parameters were measured, which were estimated and which interactions remain outside the method's scope.
What the result can support
State every port condition and reference plane when reporting duplexer measurements.
A high-power dummy load is not automatically a calibrated network-analyzer termination.
Further technical reading
Related industry knowledge
- Bias-Fed Cellular Test Paths: Where a DC Block Belongs
- Setting a Cellular Receiver Level: Source Display Versus DUT Input
Numerical scenarios are illustrative assumptions, not reported measurements of a supplied product or installation.