Directional Couplers Reflection Power Measurement Method
Understanding the Measurement Principle
As a key component in an RF system, the core characteristic of Directional Couplers is their ability to separate forward and reverse waves. When measuring reflected power, this characteristic is utilized: the signal enters from the input port, most of it is transmitted to the output port, and a small portion is coupled to the coupling port; theoretically, there is no signal leakage to the isolation port. In practical applications, the measurement framework must be built based on this principle to ensure the signal is transmitted along the expected path, laying the foundation for accurate measurement of reflected power.
- Separates forward & reverse waves for reflection analysis
- Input → output transmission with coupled sampling
- Isolation port ideally receives zero signal leakage
- Foundation for accurate reflected power measurement
Setting up a Standard Measurement System
When setting up the measurement system, all ports of the Directional Couplers must be correctly connected. Connect the input port to the signal source, the output port to the load under test, the coupling port to the power meter, and the isolation port to the matched load to avoid signal reflection interference. During the connection process, ensure good contact at all interfaces to reduce measurement errors caused by connection problems. Simultaneously, the signal source and power meter need to be warmed up and calibrated to ensure the equipment is in optimal working condition.
- Input → signal source | Output → DUT load
- Coupling port → power meter | Isolation → matched load
- Ensure tight connector contact & minimal interface loss
- Warm-up & calibrate source and power meter
Performing Reflected Power Measurement
After everything is ready, turn on the signal source to output a stable RF signal. At this time, the signal is transmitted to the load under test via the Directional Couplers, and part of the power is reflected back due to load impedance mismatch. The reflected signal is coupled to the coupling port through the Directional Couplers, and the reflected power value is read by the power meter. During the measurement process, the signal source parameters and the state of the load under test need to be adjusted multiple times, and multiple sets of data need to be recorded to improve the accuracy and reliability of the measurement results.
- Stable RF signal transmitted to load under test
- Reflected power generated by impedance mismatch
- Reflected signal coupled to power meter via coupler
- Multiple adjustments & data sets for reliability
Analysis & Correction of Measurement Errors
Because actual directional couplers have limited directionality, a small amount of forward signal leakage will reach the coupling port, affecting the accuracy of reflected power measurement. Therefore, after the measurement is completed, the measurement data needs to be corrected based on the directional parameters of the directional couplers. By comparing the ideal state with the actual measurement results, the error value is calculated, and the original data is adjusted to obtain reflected power data that is closer to the true value.
- Limited directivity causes forward signal leakage
- Correction based on coupler directivity parameters
- Compare ideal vs. actual to quantify error
- Adjust raw data for true reflected power value