Microstrip RF Power Splitter/Combiner Technology and Application Analysis

　　I. Core Definition and Characteristics of Microstrip Structure

　　Microstrip RF power splitters/combiners are passive devices that distribute and combine signals based on microstrip transmission lines. Their core structure consists of a "top layer microstrip line (signal transmission), an intermediate dielectric substrate (support and impedance matching), and a bottom layer ground metal (return path)," possessing three significant characteristics:

　　Planar Integration Advantage: Microstrip lines can be directly integrated with other RF circuits on the PCB (such as filters, amplifiers, and antennas) without additional interconnect structures, reducing interface loss (0.2~0.5dB lower than coaxial interconnects), and adapting to miniaturized devices (such as mobile phone RF front-ends and micro base station modules).

　　Low-Cost Mass Production: Manufactured using PCB photolithography, with an accuracy of up to 0.1mm, suitable for large-scale mass production (single batch capacity of over 100,000 units), with costs only 1/3 to 1/5 of cavity-type devices, especially suitable for low-cost scenarios such as consumer electronics and the Internet of Things.

　　Flexible frequency band adaptation: By adjusting the microstrip linewidth and substrate dielectric constant, it can cover the 100MHz~60GHz frequency band, with 1~20GHz being the mainstream application range. Higher frequency bands (20~60GHz) require special processes to suppress radiation loss.

　　It should be noted that the microstrip structure is a semi-open transmission system. Higher frequency bands (>10GHz) are prone to radiation loss and edge field interference, requiring structural optimization (such as adding a shielding layer and using a low-displacement substrate) to compensate for these shortcomings.

　　II. Core Performance Indicators of Microstrip Devices

　　1. Basic Electrical Performance Indicators (Adapted to Microstrip Characteristics)

　　Insertion Loss (IL): Affected by substrate dielectric loss, conductor loss, and radiation loss, it needs to be controlled according to frequency band:

　　Low Frequency Band (100MHz~2GHz): 2-way splitter IL≤0.3dB (total IL≤3.3dB when including the theoretical 3dB shared loss), 4-way splitter ≤6.5dB;

　　Mid-High Frequency Band (2~20GHz): 2-way splitter IL≤0.5dB (total IL≤3.5dB), radiation loss ≤30%;

　　Millimeter Wave Band (20~60GHz): 2-way splitter IL≤0.8dB (total IL≤3.8dB), low dielectric loss substrate (such as Rogers RO5880, tanδ=0.0009) is required to suppress loss.

　　Isolation: Conventional microstrip Wilkinson structures offer ≥20dB isolation (2 channels), which can be improved to 25~30dB by increasing the number of isolation resistors and optimizing microstrip line spacing. Multi-port (e.g., 8 channels) devices typically offer ≥18dB isolation, suitable for multi-channel scenarios with mild interference.

　　VSWR: Typical values for input/output ports are ≤1.4, and for high-frequency bands (>10GHz), it needs to be controlled at ≤1.5. Impedance matching deviation mainly stems from microstrip linewidth error (allowable ±0.05mm) and substrate dielectric constant fluctuations (±0.2).

　　2. Microstrip-Specific Structural and Environmental Specifications

　　Dimensional Accuracy: Microstrip linewidth tolerance ≤ ±0.05mm (ensuring impedance deviation ≤ 5%), line spacing ≥ λ/8 (λ is the lowest operating frequency wavelength), avoiding coupling interference between adjacent lines;

　　Temperature Stability: Within a temperature range of -40℃ to +85℃, IL fluctuation ≤ ±0.15dB, mainly dependent on the substrate's thermal expansion coefficient (e.g., FR-4 thermal expansion coefficient 13ppm/℃, Rogers RO4350 17ppm/℃);

　　Mechanical Strength: Microstrip copper foil adhesion ≥ 1.5N/mm (compliant with IPC-TM-650 standard), avoiding copper foil detachment due to bending or vibration.

　　III. Mainstream Topology Design and Optimization of Microstrip Devices

　　1. Microstrip Implementation Schemes of Classic Topologies

　　(1) Microstrip Wilkinson Topology (Mainstream in Mid-to-High Frequency)

　　Structural Principle: Replacing the traditional coaxial λ/4 transmission line with a microstrip line, the isolation resistor is soldered between the two output microstrip lines, and the substrate is made of a material with a dielectric constant εr = 2.2~4.4 (balancing impedance and size);

　　Performance Optimization:

　　Adopting a "double-section λ/4 microstrip line" design, the isolation is increased from 20dB to 28dB (2~8GHz band);

　　The isolation resistor is a surface-mount high-frequency thin-film resistor (such as 0402 package, operating frequency ≥20GHz), reducing parasitic inductance;

　　The output microstrip line has a gradual transition (length ≥λ/16), reducing VSWR to below 1.3. Applicable scenarios: 2~20GHz frequency band, such as WiFi 6E (5.925~7.125GHz), 5G micro base stations (3.5GHz).

　　(2) Microstrip branch line coupler (broadband scenario)

　　Structural principle: Composed of 4 λ/4 microstrip lines forming a "田" (field) shaped structure, the input signal is coupled and distributed to the output, supporting 0.5~18GHz ultra-wideband;

　　Key optimizations:

　　Adopting a "stepped microstrip line" to adjust the characteristic impedance, extending the bandwidth to 50% of the center frequency (e.g., center frequency 5GHz, bandwidth 2~8GHz);

　　The bottom ground layer has a "defective grounding structure (DGS)" to suppress high-frequency harmonics and improve isolation by 1~2dB;

　　Performance indicators: 2-way splitter IL≤0.4dB (2~8GHz), isolation ≥22dB, VSWR≤1.4. (3) Microstrip Resistive Topology (Low Frequency, Low Cost)

　　Structural Principle: A T-type or star-type network is formed by surface-mount resistors, with microstrip lines serving as signal input/output paths. FR-4 substrate (εr=4.4) is used to reduce costs.

　　Limitations: Resistor losses lead to high inductance (IL) (IL≥4dB for a 2-way splitter), limiting its application to low-frequency scenarios below 1GHz (e.g., RFID 915MHz). 2. High-Frequency Loss Optimization of Microstrip Structure

　　Radiation Loss Suppression: Adding a metal shielding cap above the microstrip line (distance from the substrate ≥ λ/4), with a shielding effectiveness ≥ 40dB, reduces radiation loss in the 20GHz band from 0.3dB to 0.1dB;

　　Conductor Loss Reduction: Using thicker copper foil (thickness ≥ 35μm, conventional thickness 18μm) reduces skin effect loss, lowering conductor loss in the 20GHz band by 20%;

　　Dielectric Loss Control: For high-frequency bands (>10GHz), a low dielectric loss substrate is selected, such as Rogers RO5880 (tanδ=0.0009), reducing dielectric loss by 95% compared to FR-4 (tanδ=0.02).

　　IV. Material Selection and Process Specifications for Microstrip Devices

　　1. Core Material Selection (Matching by Scenario)

　　Material Type Selection Basis Typical Model / Parameters Applicable Scenarios

　　Dielectric Substrate Dielectric Constant εr, Dielectric Loss Tangent tanδ, Thermal Stability

　　Low Frequency, Low Cost: FR-4 (εr=4.4, tanδ=0.02); Mid-to-High Frequency, Low Loss: Rogers RO4350 (εr=3.48, tanδ=0.0037); Millimeter Wave: Rogers RO5880 (εr=2.2, tanδ=0.0009)

　　Consumer Electronics / IoT; 5G Micro Base Stations; Millimeter Wave Radar

　　Conductor Material Conductivity, Adhesion, Corrosion Resistance

　　Copper Foil (Conductivity 5.8×10⁷) S/m), surface gold plating (thickness ≥1μm) or tin plating (thickness ≥3μm)

　　General applications; high reliability applications (e.g., industrial equipment)

　　Isolation resistors

　　Operating frequency, power, temperature coefficient

　　High-frequency thin-film resistors (0402/0603 package, power 0.1~0.5W, temperature coefficient ≤50ppm/℃)

　　Mid-to-high frequency splitters/combiners

　　Shielding materials

　　Shielding effectiveness, weight

　　Aluminum alloy shielding cover (thickness 0.5~1mm); conductive foam (shielding effectiveness ≥30dB after compression)

　　High-frequency anti-interference applications; portable devices

　　2. Key process specifications

　　Photolithography: microstrip line pattern accuracy ≤±0.05mm, edge burrs ≤0.02mm, avoid impedance abrupt changes;

　　Welding process: isolation resistors use reflow soldering (temperature 230~250℃), solder joint pull force ≥0.5N, no cold solder joints;

　　Surface treatment: copper foil surface gold plating (thickness 1~3μm) or immersion gold plating (thickness ≥3μm) 0.1~0.2μm), improving corrosion resistance (no oxidation after ≥96 hours of salt spray testing).

　　V. Testing, Verification, and Integration Scheme for Microstrip Devices

　　1. Specific Testing Points

　　* Electrical Performance Testing: Connect a vector network analyzer (VNA) using a microstrip test fixture (SMA/microstrip transition interface). The fixture must first undergo "de-embedding calibration" to eliminate fixture loss (test error ≤0.05dB after calibration);

　　* Radiated Loss Testing: Test the device's radiated field strength (30MHz~6GHz) in a microwave anechoic chamber. The radiated power must be ≤-50dBm (compliant with CISPR 22 Class B);

　　* Temperature Cycling Testing: Cycle 50 times (30 minutes each) from -40℃ to +85℃. After testing, the IL change should be ≤±0.15dB, and VSWR ≤1.5.

　　2. Integrated Application Solutions

　　Antenna Integration: The microstrip splitter is directly integrated with the microstrip antenna array (such as a 4×4 MIMO antenna) on the same PCB, reducing feeder loss (0.3~0.5dB lower than coaxial feeders), making it suitable for portable devices such as mobile phones and tablets.

　　Filter Integration: A microstrip bandpass filter (such as a 5G 3.5GHz band filter that suppresses 2.4GHz WiFi interference ≥40dB) is connected in series at the input port of the microstrip splitter, forming an integrated "splitter-filter" module.

　　Multi-Layer Integration: Using multi-layer PCB technology, the microstrip splitter is placed on the top layer, the ground and power layers are placed on the bottom layer, and control circuitry (such as an MCU) is placed on the middle layers, achieving a "RF-baseband" co-design, reducing the size by 40% compared to single-layer solutions.

　　VI. Typical Application Scenarios and Case Studies

　　1. WiFi 6E Microstrip Two-Splitter (5.925~7.125GHz)

　　Application Requirements: WiFi 6E routers need to distribute one RF signal to two antennas, requiring IL ≤ 3.5dB (including theoretical 3dB), isolation ≥ 22dB, dimensions ≤ 10mm × 8mm × 1.6mm, and cost ≤ $1.

　　Technical Solution: Utilizing a microstrip Wilkinson topology, Rogers RO4350 substrate (0.8mm thickness), 1.2mm microstrip linewidth (50Ω impedance), and 0402 packaged isolation resistors (0.1W, 100Ω), without shielding (to reduce cost).

　　Application Results: IL = 3.4dB (5.925~7.125GHz), isolation = 23dB, VSWR = 1.35, mass production yield ≥ 98%, suitable for mass production requirements of consumer-grade routers. 2. 5G Microbase Station Microstrip Quad Divider (3.3~3.6GHz)

　　Application Requirements: 5G microbase stations need to distribute the output signal of one RRU to four antennas, requiring IL ≤ 6.8dB (theoretical 6dB), isolation ≥ 20dB, support for filter integration, and an operating temperature range of -40℃ to +85℃.

　　Technical Solution: Dual-stage microstrip Wilkinson structure, Rogers RO4350 substrate (1.6mm thickness), 2.4mm microstrip linewidth, integrated 3.3~3.6GHz microstrip bandpass filter (suppressing out-of-band interference ≥ 45dB), gold-plated surface (corrosion resistance).

　　Application Results: IL = 6.7dB, isolation = 21dB, IL change after temperature cycling = 0.1dB, module size after filter integration = 25mm × 20mm × 1.6mm, suitable for the miniaturization requirements of microbase stations.

　　3. Millimeter-Wave Radar Microstrip Combiner (24GHz)

　　Application Requirements: Vehicle-mounted millimeter-wave radar needs to combine two received signals, requiring IL ≤ 0.8dB, isolation ≥ 25dB, dimensions ≤ 5mm × 5mm × 0.8mm, and vibration resistance (10~2000Hz/10g).

　　Technical Solution: Microstrip branch line coupler topology, Rogers RO5880 substrate (tanδ=0.0009), microstrip linewidth 0.6mm, with an added aluminum alloy shielding cover (0.5mm thick), and the bottom fixed with conductive adhesive (vibration resistance).

　　Application Results: IL = 0.7dB, isolation = 26dB, no performance improvement after vibration testing.

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