Low Insertion Loss RF Power Splitter/Combiner Technology Analysis

　　I. Basic Concepts and Functional Differentiation

　　1. Definition

　　RF power splitters and combiners are core passive devices in RF systems. They have symmetrical structures but opposite functions:

　　* **Splitter:** Distributes a single input RF signal evenly or proportionally to multiple output ports, such as 2-channel, 4-channel, or 8-channel splitters.

　　* **Combiner:** Combines multiple input RF signals into a single output, ensuring no interference between the signals and low signal loss after combination.

　　**Core Common Requirement:** Low insertion loss (IL), meaning minimal power attenuation after signal passing through the device, directly impacting the energy efficiency and signal coverage of the RF system.

　　2. Functional Relevance

　　Ideally, the same device can be used bidirectionally (a splitter in reverse becomes a combiner). However, in practical applications, the adaptability of parameters such as isolation and power capacity must be considered. Designs are typically optimized for either splitting or combining scenarios.

　　II. Key Performance Parameters (Focusing on Low Insertion Loss Related Indicators)

　　1. Insertion Loss (IL)

　　Definition: The ratio of output port power to input port power (expressed in dB, calculated as: IL = -10lg(P_{out}/P_{in})). A lower value is better.

　　Low Loss Standards:

　　Low Frequency Band (<1GHz): The theoretical loss of an ideal 2-way splitter is approximately 3dB (power evenly distributed), and the actual device IL should be ≤3.2dB.

　　High Frequency Band (2-6GHz, such as 5G applications): IL for a 2-way splitter should be ≤3.5dB, and for a 4-way splitter ≤6.5dB (theoretical loss 6dB).

　　Loss Sources: Conductor loss (resistance of metal materials), dielectric loss (tangent of substrate dielectric loss angle), radiation loss (high-frequency signal leakage), port reflection loss.

　　2. Other Related Key Parameters

　　Isolation: Refers to the degree of signal crosstalk between multiple ports. Low isolation will cause crosstalk power to consume effective signal power, indirectly increasing the "equivalent loss." Therefore, isolation ≥ 20dB is typically required.

　　VSWR (Standing Wave Ratio): This measures the degree of impedance matching at the port. Ideally, VSWR = 1. Poor impedance matching will cause signal reflection, and the reflected power cannot be effectively transmitted, effectively increasing loss. In practical applications, VSWR ≤ 1.5 is required.

　　Power Rating: This indicates the maximum input power a device can withstand. Overloading the input power will cause the device to overheat and its parameters to degrade, thus increasing loss. Therefore, devices with appropriate power ratings must be selected based on the actual power requirements of the system.

　　Bandwidth: This refers to the operating frequency range within which a device maintains low loss (e.g., IL fluctuation ≤ 0.5dB). When selecting a device, ensure that the bandwidth covers the actual operating frequency band of the system, such as the 3.5GHz and 2.6GHz bands of 5G NR.

　　III. Common Topologies and Low-Loss Design Features

　　1. Wilkinson Topology (Mainstream Low-Loss Solution)

　　Structural Principle: Consists of a λ/4 transmission line and an isolation resistor, supporting equal power distribution/combination;

　　Low-Loss Advantages:

　　Ideally, there is no reflection loss (good impedance matching);

　　The isolation resistor only functions when there is port mismatch, and has no power consumption during normal operation, reducing additional losses;

　　Applicable Scenarios: Mid-to-high frequency bands (1-18GHz), scenarios requiring both isolation and low loss (such as 5G base stations, satellite communications);

　　Optimization Directions: Use high-conductivity metals (such as copper, silver) to fabricate the transmission line to reduce conductor loss; select a low-dissipation substrate (such as PTFE, dielectric loss tangent tanδ < 0.001).

　　2. Resistive Topology

　　Structure Principle: Composed of a star or T-type resistor network, simple design and low cost;

　　Loss Disadvantages: Resistors themselves consume power, resulting in high insertion loss (IL ≥ 4dB for a 2-way splitter), suitable only for low-frequency bands (<500MHz) and scenarios with low loss requirements (such as test instruments);

　　Low-Loss Improvement: Uses high-precision, low-temperature-coefficient metal film resistors to reduce resistor self-loss.

　　3. Branch Line Coupler Topology

　　Structure Principle: Composed of 4 λ/4 branch transmission lines, enabling equal or unequal power distribution;

　　Low-Loss Characteristics: Stable performance at high frequencies (>10GHz), low radiation loss, but narrow bandwidth (typically 10%-20% of the center frequency), requiring wideband matching design to extend bandwidth.

　　IV. Key Considerations for Low Insertion Loss Design

　　1. Material Selection

　　Transmission Line Material: Copper (conductivity 5.8×10⁷ S/m) or silver (6.1×10⁷ S/m) are preferred, with gold plating (thickness ≥1μm) to reduce conductor loss due to oxidation;

　　Substrate Selection:

　　Low Frequency Band (<2GHz): FR-4 (tanδ≈0.02), low cost;

　　Mid-High Frequency Band (>2GHz): PTFE (tanδ<0.001), Rogers high-frequency substrates (e.g., RO4350, tanδ=0.0037), to reduce dielectric loss;

　　Isolation Components: Thin-film resistors or thick-film resistors should be used, avoiding carbon film resistors (high loss, poor stability).

　　2. Structural Optimization

　　Transmission Line Size: Design the characteristic impedance of the λ/4 transmission line (typically 50Ω) according to the operating frequency. The line width must match the substrate dielectric constant (e.g., with a dielectric constant εr=4.4, the 50Ω transmission line width is approximately 1.8mm, and the substrate thickness is 1.6mm) to reduce reflection loss caused by impedance abrupt changes.

　　Port Design: Use low-loss RF connectors such as SMA and N-type connectors. Implement a gradual matching at the transition between the connector and the transmission line (e.g., chamfering, gradual line width) to avoid signal abrupt changes.

　　Miniaturization and Integration: Use multilayer substrates (e.g., LTCC low-temperature co-fired ceramic) to reduce transmission line length and radiation loss; integrate splitters/combiners with filters and amplifiers to reduce external connection losses.

　　3. Simulation and Testing Verification

　　Simulation Tools: Use electromagnetic simulation software such as HFSS and CST to simulate transmission line loss and radiation loss, optimize structural parameters (such as line width, spacing, and substrate thickness), and ensure that the insertion loss (IL) meets the standard.

　　Testing Methods: Use a vector network analyzer (VNA) to test the insertion loss. The test environment must be shielded from electromagnetic interference, and cable loss must be calibrated (to eliminate the influence of the test cable's own IL on the results).

　　V. Typical Application Scenarios

　　1. 5G Mobile Communication System

　　Application Location: Base station antenna feeder system, distributed base station RRU (Remote Radio Unit);

　　Requirements: 5G signals operate at high frequencies (2.6GHz, 3.5GHz, millimeter wave), resulting in rapid signal attenuation. Low-loss splitters are needed to distribute the RRU output signal to multiple antennas, while combiners are used to synthesize the received signals from multiple antennas before transmitting them to the BBU (Baseband Processing Unit). The IL must be ≤3.5dB (2 channels) to ensure coverage and signal quality.

　　2. Satellite Communication System

　　Application Scenario: Satellite ground station receiver, combining multi-channel satellite signals and transmitting them to a demodulator;

　　Requirements: Weak satellite signals (typically below -100dBm), requiring an extremely low-loss combiner (IL≤0.5dB) and high isolation (≥30dB) to avoid inter-channel crosstalk.

　　3. RF Test and Measurement

　　Application Scenario: Signal distribution for test instruments (e.g., signal generators, spectrum analyzers), distributing a standard signal to multiple devices under test;

　　Requirements: Low loss to ensure stable test signal power, IL fluctuation ≤0.2dB, guaranteeing test accuracy.

　　4. RF Energy Harvesting System

　　Application Scenario: Collecting and combining environmental RF signals (e.g., base station, WiFi signals) to power low-power devices;

　　Requirements: Low environmental RF signal power (typically -20~-60dBm), requiring a combiner IL≤0.3dB to maximize energy harvesting efficiency.

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