1. Core Design Points of Microstrip RF Circulator/Isolator

　　1.1 Material Selection: Foundation of Performance

　　Substrate Material: The dielectric constant (Dk) and loss tangent (Df) directly determine insertion loss and frequency stability. For medium-frequency scenarios (1–3 GHz), FR-4 with Dk 4.2–4.8 and Df 0.014–0.02 is cost-effective, while high-frequency applications above 10 GHz (e.g., Ka-band) require low-loss materials like Teflon (Dk 2.55–2.94, Df 0.001) or Rogers series to minimize dielectric loss . The substrate thickness (h) should match impedance requirements—thinner substrates reduce characteristic impedance but increase parasitic coupling, while thicker substrates improve isolation but may raise insertion loss .

　　Ferrite Core: As the core of non-reciprocal propagation, ferrite materials must balance magnetic and dielectric properties. For Ka-band devices, TDK’s 140M lithium ferrite (saturation magnetization 500 mT, relative permittivity 15) is preferred for its low magnetic loss, while BaM-type hexagonal ferrite enables self-biased designs, eliminating external magnets and reducing size . The normalized saturation magnetization (P = γMₛ/f₀) should be controlled between 0.4–0.7 to ensure optimal bandwidth and loss performance .

　　Conductor Material: Gold-plated copper is typically used for microstrip lines. Gold plating reduces skin effect loss at high frequencies, while copper ensures conductivity. The conductor thickness (t) is optimized to balance current-carrying capacity and fabrication feasibility—thicker conductors reduce resistance but may increase radiation loss .

　　1.2 Topology Structure: Key to Signal Directionality

　　Basic Structure: Full ferrite substrate topology is widely adopted for its advantages in bandwidth extension, low insertion loss, and miniaturization . The core consists of a central ferrite disk and three Y-shaped microstrip lines with 120° symmetry. The disk diameter (d) and substrate thickness (h) are calculated via classical field theory formulas—for Ka-band (31–40 GHz), the disk diameter is typically 1.5–2.5 mm, and substrate thickness 0.2–0.5 mm .

　　Y-Junction Optimization: The length and width of the Y-junction directly affect impedance matching and isolation. In the 26–28 GHz range, both return loss (S₁₁) and isolation (S₁₂) first increase and then decrease with larger Y-junction dimensions. Optimal junction length is 0.8–1.2 mm and width 0.3–0.5 mm, ensuring VSWR ≤1.2 and isolation ≥20 dB .

　　Transition Design: Microstrip line transitions to SMA connectors require gradual impedance tapering. The transition length is set to 1/4 wavelength at the center frequency to minimize reflection, reducing S₁₁ by 5–10 dB compared to abrupt transitions .

　　1.3 Magnetic Circuit System: Guarantee of Non-Reciprocity

　　Bias Magnetic Field: Traditional designs use NdFeB permanent magnets to provide a 100–500 Oe bias field. The field strength must be calibrated to match ferrite properties—insufficient bias reduces isolation, while excessive bias increases magnetic loss . Self-biased designs based on BaM ferrite eliminate external magnets, shrinking device size to 7mm×7mm×0.4mm for Ka-band applications .

　　Magnetic Shielding: Permalloy shields are added around the magnet to reduce external field leakage (<0.5 G at the device surface). This is critical for integrated systems to avoid interfering with adjacent components .

　　2. S-Parameter Optimization Strategy

　　2.1 Insertion Loss (S₂₁) Optimization

　　Loss Source Mitigation: Insertion loss arises from dielectric, conductor, and radiation losses. Dielectric loss is reduced by selecting low-Df materials (e.g., Teflon instead of FR-4), which lowers loss by 30–50% at 3 GHz . Conductor loss is minimized via gold plating and increasing line width (within impedance constraints)—a 0.1mm width increase reduces loss by ~0.1dB at 30 GHz . Radiation loss is suppressed by shortening microstrip line length and adding ground vias around the ferrite core .

　　Structural Fine-Tuning: Thicker ferrite substrates increase insertion loss—for 26–28 GHz, S₂₁ rises by 0.3dB when substrate thickness increases from 0.3mm to 0.5mm . Optimizing the central disk radius (R₀) to 1.0–1.2mm balances loss and bandwidth, achieving S₂₁ ≤0.3dB in Ka-band .

　　2.2 Isolation (S₁₂/S₃₂) Optimization

　　Magnetic Field Calibration: The bias field is tuned to align with ferrite resonance characteristics. For TDK 140M ferrite, a 300 Oe field achieves maximum isolation (>50dB) at 27 GHz . Deviations from the optimal field reduce isolation by 10–20 dB .

　　Topology Symmetry: Maintaining 120° symmetry of Y-junctions and uniform ferrite material properties (permittivity variation <±0.5) ensures balanced mode separation. Asymmetry of >5° in junction angles reduces isolation by >8dB .

　　Load Matching: Isolators require a 50Ω matched load at the isolated port. Using thin-film resistors with parasitic inductance <0.1nH minimizes reflection, improving isolation by 3–5dB .

　　2.3 Return Loss (S₁₁/S₂₂) Optimization

　　Impedance Matching: The characteristic impedance of microstrip lines is calculated via the Hammerstad formula, adjusting width (W) and substrate thickness (h) to 50Ω. For Rogers 5880 substrate (h=0.254mm, Dk=2.2), a line width of 0.6mm achieves 50Ω impedance .

　　Disk-Junction Matching: The normalized conductance (g) and susceptance slope (b) of the ferrite disk are optimized using the admittance slope method. For a 25% relative bandwidth, g=19.763 and b=70.196 ensure VSWR ≤1.2 .

　　Transition Optimization: Tapered transitions between microstrip lines and ports reduce impedance discontinuity. A 1/4-wavelength taper (length 2mm at 30 GHz) lowers S₁₁ from -15dB to -25dB .

　　3. Simulation and Measurement Verification

　　3.1 HFSS Simulation Optimization

　　Modeling Key Points: A 3D electromagnetic model includes the ferrite disk, microstrip lines, ground plane, and bias magnet. The ferrite is defined with tensor permeability, and the magnet is set as a permanent magnetic material with Bᵣ=1.2T . Radiation boundaries are set 3λ away from the device to avoid boundary reflection errors .

　　Parameter Sweep: Sweeping ferrite thickness (0.2–0.5mm) and disk radius (0.8–1.5mm) identifies optimal dimensions. For Ka-band, the optimal combination (h=0.3mm, R₀=1.1mm) achieves S₂₁≤0.3dB and S₁₂≥20dB .

　　Sensitivity Analysis: Simulating material parameter tolerances (Dk±0.2, Mₛ±50mT) ensures design robustness. Results show isolation variation <3dB within typical material tolerances .

　　3.2 Measurement Calibration and Adjustment

　　Calibration Method: SOLT (Short-Open-Load-Thru) calibration is performed on the network analyzer to eliminate cable and connector errors. Calibration frequency range is extended by 10% beyond the operating band (e.g., 25–29 GHz for 26–28 GHz devices) .

　　Real-World Adjustment: If measured S₁₁ exceeds specifications, trimming the Y-junction width by 0.05mm reduces VSWR by ~0.1. If isolation is insufficient, increasing the bias field by 50 Oe improves S₁₂ by 5–8dB .

　　Temperature Stability Test: Testing at -40°C to 85°C verifies performance. For Rogers substrates, S₂₁ variation is <0.2dB, meeting industrial application requirements .

　　The above content systematically presents design essentials and optimization methods without using tables. If you need to deepen specific sections (e.g., self-biased ferrite design, millimeter-wave simulation techniques) or add application cases (5G base stations, satellite communications), feel free to let me know.

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