Design of RF circulator and isolator for LTE network

　　As the core architecture of 4G communication, LTE network has strict requirements on signal isolation and transmission efficiency in its RF front end - it needs to achieve low-loss signal flow in multiple frequency bands (700MHz-2600MHz) and resist the interference of high-power transmission signals on the receiving end. RF circulators and isolators designed for LTE network characteristics need to take into account broadband coverage, high isolation, high power tolerance and miniaturization requirements. The following are specific design points.

　　I. Band adaptation design: covering LTE multi-band requirements

　　(I) Selection of broadband ferrite materials

　　LTE network involves multiple frequency bands (such as China's B1/B3/B7/B38 frequency bands, covering 700MHz-2600MHz), and the core of circulators/isolators is the optimization of broadband characteristics of ferrite materials. Modified garnet ferrite (YIG) is used, and the magnetic permeability (μ'=15-20, loss tangent tanδ<0.001@1GHz) is adjusted by doping Ga³⁺ ions, so that it maintains stable magnetic properties in the 700MHz-2600MHz frequency band. Compared with traditional spinel ferrite, its broadband insertion loss fluctuation can be controlled within ±0.2dB (spinel ferrite fluctuation reaches ±0.5dB), meeting the signal consistency requirements during LTE multi-band switching.

　　For frequency band design (such as covering only the 700MHz-900MHz low frequency band), Ni-Zn ferrite can be selected, which has lower loss in the low frequency band (IL<0.4dB@800MHz), and the cost is 30% lower than that of YIG material, which is suitable for low-frequency dedicated channels of macro base stations.

　　(II) Multi-band matching structure design

　　Use the "step impedance matching" structure: design a gradual impedance transition section (smooth transition from 50Ω to ferrite characteristic impedance) at the three ports of the circulator (or the two ports of the isolator), optimize the transition section length (λ/4≈10.7cm for the 700MHz band, λ/4≈2.8cm for 2600MHz) through electromagnetic simulation (such as HFSS simulation), and ensure that the standing wave ratio (VSWR) is ≤1.2 in the full frequency band.

　　For scenarios that need to cover multiple discrete frequency bands (such as supporting B1 (2100MHz) and B3 (1800MHz) at the same time), the "band switching" function can be designed: switch the ferrite bias magnetic field through the micro-electromechanical system (MEMS) switch (adjust the magnetic field strength of the permanent magnet ±5%), so that the device is in a resonant state in different frequency bands, and the switching response time is ≤10μs, meeting the fast frequency band switching requirements during LTE carrier aggregation.

　　2. Performance parameter optimization: meeting LTE communication indicators

　　(I) High isolation design suppresses transmission interference

　　The transmission power of LTE base stations can reach 43dBm (20W), and the sensitivity of the receiving end must be below -110dBm. If the transmission signal leaks to the receiving end, it will directly submerge the useful signal. Therefore, the reverse isolation of the isolator needs to be ≥25dB (typical value 30dB), and the port isolation of the circulator (such as the transmitting end to the receiving end) needs to be ≥22dB.

　　The design is:

　　Optimize the absorption load (key component of the isolator): adopt a composite structure of carbon film resistor and ceramic matrix, the internal impedance is stable at 50Ω±2Ω in 700MHz-2600MHz, the power capacity is ≥50W (continuous), and the reflected signal is efficiently absorbed (absorption loss ≥25dB);

　　Optimize the magnetic field distribution of the three-port circulator: adjust the gap (0.1-0.3mm) between the permanent magnet (such as NdFeB magnet, remanence 1.2T) and the ferrite through finite element simulation, so that the magnetic field of the forward transmission path (transmitter→antenna) is uniform, and the magnetic field of the reverse path (antenna→receiver) is disordered, which enhances the isolation effect.

　　(II) Low insertion loss improves signal efficiency

　　Every 0.1dB increase in the loss of LTE signal during transmission will reduce the base station coverage by 1-2%. Therefore, the forward insertion loss of the circulator/isolator needs to be controlled within ≤0.4dB (700MHz band) and ≤0.5dB (2600MHz band).

　　The implementation paths include:

　　Ferrite thin-film design: the thickness is reduced from the traditional 1.5mm to 0.8-1.0mm, reducing high-frequency eddy current loss (loss is reduced by 0.2dB at 2600MHz);

　　Gold-plated port process: the inner conductor uses oxygen-free copper (purity 99.99%), and the surface is gold-plated (thickness ≥3μm), which reduces contact resistance (≤0.01Ω) and interface loss.

　　(III) High power tolerance to adapt to base station requirements

　　The transmission power of the macro base station reaches 20W (43dBm), and the RF device needs to withstand peak power ≥100W (400μs pulse) and average power ≥50W. Structural design:

　　Use copper-tungsten alloy (CuW70) heat sink with thermal conductivity ≥180W/m・K to control the ferrite working temperature to ≤85℃ (exceeding 120℃ will cause permanent degradation of magnetic properties);

　　Inner conductor and shell are integrated by die-casting (aluminum alloy ADC12) to reduce thermal resistance nodes (total thermal resistance ≤0.5℃/W) and ensure that the shell temperature is ≤60℃ under 20W continuous power.

　　III. Structural design: taking into account miniaturization and engineering adaptation

　　(I) Miniaturization design of micro base station/small base station

　　LTE micro base station (covering radius of 500m) needs to integrate multi-channel RF modules, and the size of circulator/isolator needs to be ≤15mm×15mm×8mm. Microstrip line structure is used to replace the traditional coaxial structure:

　　The ferrite chip (5mm×5mm×0.8mm) is mounted on the PCB substrate (Rogers 4350, dielectric constant 3.48) and connected to the port through a microstrip line;

　　The permanent magnet adopts a thin-film design (thickness 2mm) and is integrated on the top of the shell, reducing the overall height by 40%, which is suitable for embedded installation (such as inside the RRU module of a micro base station).

　　This type of design has an insertion loss of about 0.6dB in the 2600MHz frequency band and an isolation of ≥20dB, which meets the low-power (5W) scenario requirements of micro base stations.

　　(II) High reliability structure of macro base station

　　Macro base station needs to operate for a long time in outdoor environment (MTBF≥100,000 hours), and the structural design emphasizes:

　　Sealing protection: IP65 waterproof shell is used, and fluororubber sealing ring (temperature resistance - 40℃-125℃) is used at the interface to avoid rain and salt spray erosion (coastal areas need to pass 96 hours salt spray test);

　　Anti-vibration design: The inner conductor and the shell are connected by elastic support (beryllium copper shrapnel), which can withstand vibration of 10-2000Hz and 10g acceleration (in compliance with ETSI EN 300 019-2 standard), to prevent poor contact caused by vibration during long-distance transportation or base station operation.

　　4. Integrated design: Adapting to LTE RF front-end architecture

　　(I) Multi-channel integrated module

　　The MIMO technology of LTE base stations (such as 4T4R) requires multiple independent RF channels, and a "4-channel circulator array" can be designed: 4 three-port circulators are integrated in the same metal cavity (size 50mm×50mm×10mm), sharing a permanent magnet group (magnetic field uniformity ±2%), and the channel isolation is ≥30dB (to avoid channel interference). This design reduces space occupancy by 30% compared to independent devices, and improves heat dissipation efficiency through a common heat sink (aluminum, thermal resistance 0.3℃/W).

　　(II) Co-design with filters

　　In LTE RF front-ends, circulators are often used in series with cavity filters (such as transmitter: PA→circulator→filter→antenna). The impedance and frequency band characteristics of the two need to be matched during design:

　　The output port impedance of the circulator (50Ω) is strictly matched with the input port of the filter (50Ω) to avoid reflection loss caused by impedance mutation (≤-25dB);

　　The temperature characteristics of the two are complementary: the insertion loss of ferrite increases with temperature (+0.002dB/℃), and the loss of the filter decreases with temperature (-0.001dB/℃). After coordination, the loss fluctuation at full temperature (-40℃-85℃) can be controlled within ±0.3dB.

　　5. Testing and Verification: Comply with LTE Standard Requirements

　　After the design is completed, it needs to pass strict testing to ensure that it meets the 3GPP specifications:

　　Band coverage test: Within 700MHz-2600MHz, test the insertion loss and isolation every 1MHz to ensure that the parameters of the entire frequency band meet the standards;

　　Power tolerance test: Work at 20W continuous power for 1000 hours, the repeated isolation change is ≤0.5dB, and there is no physical damage;

　　Intermodulation distortion test: Under the stimulation of dual-tone signal (1MHz interval, 20W each), the third-order intermodulation product (IMD3) is ≤-100dBc (avoid interference with adjacent frequency channels).

　　Devices that meet the test requirements can be applied to LTE macro base stations, micro base stations, CPE (customer front-end equipment) and other scenarios, among which macro base stations focus on high power and wide bandwidth, while micro base stations and CPE focus on miniaturization and low power consumption (static power consumption ≤10mW).

　　The RF circulators and isolators designed for LTE networks, through the coordinated optimization of materials, structure and integration, can not only meet the efficient transmission of multi-band signals, but also resist high-power interference, providing core guarantee for the stable operation of LTE networks, while reserving performance redundancy for the evolution to 5G NR (compatible with some LTE bands).

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