Broadband RF Power Splitter/Combiner Technology and Application Analysis

　　I. Core Definition and Scenario Requirements of Broadband Devices

　　A broadband RF power splitter/combiner is a passive device capable of stable signal distribution and combining over a wide frequency range (typically covering two or more independent frequency bands, or a relative bandwidth ≥ 30%). Its core value lies in adapting to RF systems with multiple frequency bands, avoiding the complexity and loss accumulation caused by combining multiple devices.

　　Typical application scenarios place core requirements on broadband devices including:

　　* **Multi-band compatibility:** For example, communication systems need to simultaneously cover 4G (1.8GHz), 5G (3.5GHz), and WiFi 6E (6GHz) bands, requiring devices to maintain low loss and high isolation within the 0.8-6GHz range;

　　* **Stable performance across all frequency bands:** In scenarios such as radar and RF testing, signal frequencies dynamically change with the detection target or testing requirements. Devices need to maintain insertion loss fluctuations ≤0.5dB across a wide frequency band to avoid performance degradation caused by frequency switching;

　　* **Simplified system architecture:** Replacing combinations of multiple narrowband devices, reducing the number of interfaces and interconnection losses (combining a single broadband device with multiple narrowband devices can reduce total loss by 0.3-1dB), and reducing system size by 30%-50%.

　　II. Core Performance Indicators of Broadband Devices

　　1. Frequency Band Coverage and Bandwidth Characteristics

　　The frequency band coverage of broadband devices needs to be clearly defined according to the application scenario. Common broadband levels include:

　　Medium Broadband: Covering 0.5-6GHz (relative bandwidth approximately 186%), suitable for consumer electronics and small to medium-sized communication base stations;

　　Wideband: Covering 2-18GHz (relative bandwidth approximately 167%), suitable for microwave communication and general RF testing;

　　Ultra-Wideband: Covering 0.3-40GHz (relative bandwidth approximately 2653%), used in high-end scenarios such as radar and aerospace.

　　Bandwidth characteristics must meet the requirement of "consistent parameters across the entire frequency band," meaning that the fluctuations of core indicators (insertion loss, isolation, and VSWR) are controlled within a limited range throughout the entire operating frequency band. For example, a 0.5-6GHz two-splitter must guarantee: insertion loss fluctuation ≤ 0.4dB, isolation fluctuation ≤ 3dB, and VSWR fluctuation ≤ 0.15.

　　2. Basic Electrical Performance Requirements

　　Insertion Loss (IL): For medium-wideband devices, IL ≤ 0.5dB across the entire frequency band (excluding theoretically shared losses; for example, a 2-way splitter with a total IL ≤ 3.5dB); for ultra-wideband devices, IL can be relaxed to ≤ 0.8dB in the high-frequency band (>20GHz), but the overall frequency band fluctuation must be ≤ 0.6dB;

　　Isolation: For medium-wideband devices, the isolation ≥ 22dB for 2-way devices and ≥ 18dB for 4-way devices; for ultra-wideband devices, the isolation ≥ 15dB in the high-frequency band (>20GHz), and the difference in isolation between adjacent frequency bands ≤ 5dB;

　　VSWR: Input/output port VSWR ≤ 1.5 (medium-wideband), ≤ 1.6 (ultra-wideband), with the maximum value not exceeding [value missing] across the entire frequency band. 1.65, to avoid signal reflection caused by impedance mismatch;

　　Phase consistency: In phased array, radar, and other scenarios, the phase difference must be ≤±3° (medium-wideband) and ≤±5° (ultra-wideband) across the wide frequency band to avoid a decrease in beamforming or signal synthesis accuracy.

　　3. Environmental and reliability indicators

　　Temperature stability: Within the temperature range of -40℃ to +85℃, the insertion loss fluctuation across the entire frequency band is ≤±0.2dB, and the isolation fluctuation is ≤±2dB, suitable for outdoor communication base stations, vehicle-mounted, and other environments;

　　Power handling capacity: Continuous power (CW) ≥10W for medium-wideband devices, ≥5W for ultra-wideband devices (power capacity decreases slightly with increasing frequency in the high-frequency band), instantaneous overload withstand capacity is 2-3 times the rated power;

　　Interference immunity: In electromagnetic compatibility testing, radiated interference must comply with CISPR 22 Class A standards (electric field strength ≤40dBμV/m in the 30MHz-1GHz band), and conducted interference immunity is ≥30dB.

　　III. Topology Design and Optimization of Broadband Devices

　　1. Broadband Adaptation Schemes for Mainstream Topologies

　　(1) Multi-Section Wilkinson Topology (Mid-Broadband Core Scheme)

　　Traditional single-section Wilkinson structures have relatively narrow bandwidth (approximately 20% bandwidth), requiring optimization through "multi-section λ/4 transmission lines + graded isolation resistors":

　　Using 2-4 cascaded λ/4 microstrip or stripline lines, the characteristic impedance of each transmission line is allocated according to the Chebyshev or Butterworth polynomial to achieve wide-band impedance matching;

　　Isolation resistors use multiple parallel or series networks, adjusting the resistance value according to the frequency band (high resistance for low-frequency bands, low resistance for high-frequency bands) to improve the overall isolation to over 22dB;

　　Adapted frequency band: 0.5-6GHz, typical insertion loss fluctuation ≤0.4dB, suitable for 5G+4G compatible base stations and multi-band routers. (2) Multi-section Branch Line Coupler (Wideband Solution)

　　For the 2-18GHz wideband requirement, a multi-section branch line coupler topology optimization is adopted:

　　It consists of 3-5 λ/4 branch transmission lines, with the width and spacing of each transmission line designed according to a gradual change pattern to reduce impedance abrupt changes during frequency band switching;

　　Compensation capacitors are added at the branch nodes to offset the effects of parasitic inductance in the high-frequency band, ensuring a VSWR ≤1.5 within the wideband;

　　Performance characteristics: Insertion loss of the 2-way splitter ≤0.6dB (2-18GHz), isolation ≥18dB, suitable for general RF testing and microwave communication systems. (3) Gradiently Coupled Line Topology (Ultra-Wideband Solution)

　　Ultra-wideband (0.3-40GHz) scenarios require a gradually changing coupled line structure:

　　The coupling coefficient of the coupled line gradually changes exponentially or linearly along the transmission direction, achieving uniform energy distribution across the wide frequency band;

　　Multilayer dielectric substrates (such as LTCC technology) are used to arrange the coupled line and ground layer in a three-dimensional layout, reducing high-frequency radiation loss;

　　Performance characteristics: Insertion loss ≤0.8dB (0.3-40GHz), isolation ≥15dB, suitable for ultra-wideband signal processing scenarios such as radar and aerospace. 2. Broadband Loss and Interference Optimization Techniques

　　Radiation Loss Suppression: Broadband devices exhibit significant radiation loss at high frequencies (>10GHz). A metal shielding cover (≥λ/4 distance from the substrate, where λ is the lowest operating frequency wavelength) needs to be added above the transmission line, with a shielding effectiveness ≥40dB, reducing radiation loss in the 20GHz band from 0.3dB to 0.1dB.

　　Dielectric Loss Control: Low dielectric loss substrates (e.g., PTFE, dielectric loss tangent tanδ<0.001) are used for mid-band broadband devices, while multilayer ceramic substrates (tanδ<0.0008) are used for ultra-wideband devices to prevent a significant increase in dielectric loss with increasing frequency.

　　Crosstalk Suppression: Multi-port broadband devices require increased spacing between adjacent transmission lines (≥λ/8) or the installation of grounded metal partitions between ports to reduce inter-band crosstalk and keep isolation fluctuations within 3dB.

　　IV. Material Selection and Process Specifications for Broadband Devices

　　1. Material Selection Principles (Adapted to Bandwidth Levels)

　　Mid-Broadband Devices (0.5-6GHz):

　　Substrate: FR-4 (low cost for low-frequency bands, tanδ≈0.02) or Rogers series substrates (low loss for mid-to-high frequency bands, tanδ=0.003-0.005);

　　Conductors: Silver-plated copper foil (thickness ≥35μm) to reduce skin effect losses;

　　Isolation resistors: High-frequency thin-film resistors (operating frequency ≥20GHz, temperature coefficient ≤50ppm/℃).

　　Ultra-Wideband Devices (0.3-40GHz):

　　Substrate: LTCC low-temperature co-fired ceramic (multilayer structure, tanδ<0.0008) or aluminum nitride ceramic (high thermal conductivity, suitable for high-power applications);

　　Conductors: Pure silver plating (thickness ≥5μm) to improve high-frequency conductivity;

　　Shielding material: Aluminum alloy (thickness 1-2mm), anodized surface for enhanced corrosion resistance.

　　2. Key Process Requirements

　　Transmission Line Fabrication: High-precision photolithography is employed, with microstrip linewidth tolerance ≤ ±0.05mm and edge burrs ≤ 0.02mm, ensuring impedance deviation ≤ 5% across a wide frequency band;

　　Soldering Process: Isolation resistors, connectors, and other components are reflow soldered (temperature 230-250℃), with solder joint pull force ≥ 0.5N to avoid sudden performance changes within the frequency band caused by poor soldering;

　　Surface Treatment: Conductor surfaces are gold-plated (1-3μm thickness) or immersed in gold (0.1-0.2μm thickness) to improve salt spray resistance (no oxidation after ≥ 96 hours of salt spray testing), suitable for long-term outdoor operation.

　　V. Testing, Verification, and Application Scenarios of Broadband Devices

　　1. Core Testing Methods

　　* Wideband Sweep Test: Using a Vector Network Analyzer (VNA) to cover the entire operating frequency band of the device, sampling is performed at a density of ≤1% of the center frequency at band intervals (e.g., more than 550 test points are required for the 0.5-6GHz band) to verify the band consistency of insertion loss, isolation, and VSWR;

　　* Temperature Cycling Test: Cycles from -40℃ to +85℃ 50 times (30 minutes each time), with a full-band sweep test performed after every 10 cycles to ensure performance fluctuations meet requirements;

　　* Power Endurance Test: Continuously operate at rated continuous power for 1000 hours, testing the full-band insertion loss every 200 hours. The attenuation must be ≤0.2dB to avoid performance degradation caused by long-term high power.

　　2. Typical Application Scenarios Analysis

　　(1) Multi-band Communication Base Station

　　Application Requirements: The base station needs to simultaneously process 4G (1.8GHz) and 5G (3.5GHz) signals, requiring one RF signal to be distributed across two antennas. The device must cover 0.8-6GHz, with insertion loss ≤3.5dB (2 channels) and isolation ≥22dB.

　　Technical Solution: A 3-section Wilkinson topology is used, with Rogers series low-displacement materials selected for the substrate. The transmission lines are silver-plated, and a metal shield is added to suppress radiation loss.

　　Application Results: Insertion loss fluctuation across the entire frequency band is ≤0.4dB, isolation is ≥23dB, supporting parallel transmission of multi-band signals in the base station, reducing the number of components and system complexity. (2) RF Test System

　　Application Requirements: The test instrument needs to distribute one standard signal (2-18GHz) to four devices under test (DUTs). The device insertion loss must be ≤6.8dB (4 channels), VSWR ≤1.5, and phase consistency across the entire frequency band ≤±4°.

　　Technical Solution: A 4-section branch-line coupler topology is adopted, with a multi-layer PCB design and transmission line spacing optimized at λ/8 to reduce crosstalk.

　　Application Results: Insertion loss fluctuation across the entire frequency band ≤0.5dB, phase difference ≤±3.5°, and test signal power stability error ≤±0.2dB, meeting high-precision testing requirements.

　　(3) Ultra-Wideband Radar System

　　Application Requirements: The radar needs to cover an ultra-wideband detection signal range of 0.3-18GHz. The combiner needs to combine four received signals, with insertion loss ≤0.8dB, isolation ≥15dB, and vibration resistance (10-2000Hz/10g). Technical Solution: Employs a gradient coupled-line topology, LTCC multilayer ceramic substrate, and a unibody aluminum alloy casing with internal silicone rubber potting for vibration resistance.

　　Application Results: Insertion loss ≤0.7dB across the entire frequency band; performance remains stable after vibration testing; suitable for dynamic radar detection and harsh environment operation requirements.

　　VI. Technological Development Trends of Broadband Devices

　　* **Ultra-wideband Integration:** Developing full-band devices covering 0.1-40GHz, achieving integrated "splitting/combining-filtering-amplification" through heterogeneous integration technology (such as combining RF circuits with microwave photonic chips), further simplifying system architecture;

　　* **Intelligent Loss Compensation:** Integrating miniature power sensors and adjustable capacitor arrays, using algorithms to monitor loss fluctuations across the wide frequency band in real time, dynamically adjusting compensation parameters to ensure insertion loss fluctuations ≤0.3dB across the entire frequency band;

　　* **Miniaturization and Lightweighting:** Employing three-dimensional integration processes (such as SiP system-in-package), multi-layer transmission lines and components are arranged three-dimensionally, reducing volume by more than 60% compared to traditional solutions, adapting to lightweight applications such as aerospace and automotive;

　　* **High Power Compatibility:** Optimizing conductor materials (such as using copper-silver composite plating) and heat dissipation structures (such as integrated microchannel heat dissipation) to increase the power capacity of ultra-wideband devices to over 20W, meeting the needs of high-power radar, industrial heating, and other applications.

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