Directional Coupler with Combining Circuit for RF Power Measurement
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Solution Overview
Problem
Constructing low-loss directional couplers for RF circuits operating between 500 MHz and 3.8 GHz is challenging due to long wavelengths and transformer losses, making it difficult to implement effective power measurement and impedance mismatch detection.
Innovation Solution
A directional coupler circuit comprising current and voltage sensing circuits with a combining circuit, allowing for tunable phase and amplitude adjustments to measure incident or reflected power, and featuring a magnetic transformer and RC networks for adjustable transfer functions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If stripline directional couplers are constructed on integrated circuits, then power measurement capability is achieved, but the circuit becomes challenging to construct due to long wavelengths at 500 MHz to 3.8 GHz frequencies
Solution Approach 1:
The directional coupler is divided into separate functional modules: a first transmission line for forward power measurement, a second transmission line for reverse power measurement, and a bridge circuit for signal combination. This segmentation allows each module to be optimized independently and simplifies integration on PCBs for the specified frequency range.
Solution Approach 2:
A bridge circuit is introduced as an intermediary component to combine the output signals from the forward and reverse power measurement paths. This bridge circuit enables accurate directional power measurement by properly combining the coupled signals while maintaining impedance matching and minimizing interference.
2Adaptability or versatility
If magnetic-based directional couplers are used, then coupling functionality is achieved, but transformer losses and parasitics increase at 500 MHz to 3.8 GHz frequencies
Solution Approach 1:
The magnetic transformer component is extracted and replaced with electric-field-based coupling mechanisms using PCB transmission lines. This eliminates the transformer and its associated losses and parasitics while maintaining the directional coupling functionality through electromagnetic field interaction between adjacent transmission lines.
Solution Approach 2:
The magnetic coupling mechanism is replaced with electric field coupling between transmission lines. Instead of using magnetic transformers and inductive coupling, the invention uses capacitive and electromagnetic coupling between adjacent PCB traces, which has lower losses and fewer parasitic effects at the target frequency range.
3Adaptability or versatility
If quarter-wavelength stripline structures are used, then directional coupling is achieved, but the circuit length becomes too long for integrated circuit implementation
Solution Approach 1:
The circuit design uses transmission line segments with lengths optimized for the specific frequency range of 500 MHz to 3.8 GHz, rather than fixed quarter-wavelength structures. This allows the circuit to maintain directional coupling performance while adapting to the practical length constraints of PCB implementation at these frequencies.
Solution Approach 2:
The coupling mechanism transitions from requiring long quarter-wavelength paths to using adjacent transmission lines that couple through electromagnetic fields in the space between them. This dimensional approach allows coupling functionality to be achieved over much shorter physical lengths by utilizing the transverse electromagnetic field distribution between parallel conductors.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables accurate power measurement and impedance mismatch detection with high directivity, compensating for component variations and parasitic impedances, and achieving efficient RF signal monitoring in both forward and reverse directions.
Implementation Method 1
Low-loss magnetic-based directional couplers also are challenging to construct at this range of frequencies because of transformer losses and parasitics
Data Source
AI summary
A circuit includes a current sensing circuit comprising a current input terminal coupled to an input port, a current output terminal coupled to a transmitted port, and a current sensing output terminal configured to provide a current sensing signal proportional to a current flowing between the current input terminal and the current output terminal. The circuit further includes a voltage sensing circuit having a voltage input terminal coupled to the transmitted port and a voltage sensing output terminal configured to provide a voltage sensing signal proportional to a voltage at the transmitted port. A combining circuit has a first input coupled to the current sensing output terminal, a second input coupled to the voltage sensing output terminal, and a combined output node coupled to an output port.