PIN Diode Biasing and S-Parameter States
A PIN diode's RF behavior changes dramatically with DC bias:
| Bias State | Carrier Distribution | RF Impedance | Use |
|---|---|---|---|
| Forward bias (If >1 mA) | I-region flooded with carriers | Low resistance: 1–10 Ω | Switch ON, low IL |
| Zero/reverse bias | I-region depleted | Parallel RC: high Z | Switch OFF, high isolation |
PIN Switch S-Parameter Specifications
| State | S21 | S11 | Typical Values |
|---|---|---|---|
| Forward (ON) | Low loss | Moderate | S21=−0.5 to −1 dB, S11=−10 to −20 dB |
| Reverse (OFF) | High isolation | −∞ (open-ish) | S21=−30 to −50 dB, S11=0 to −2 dB (reflects) |
Analyzing PIN Diode .s2p Files
Forward bias file (PIN_ON.s2p): S21: insertion loss when switch is ON → target <0.7 dB at operating freq S11: input match → lower is better, depends on circuit topology Reverse bias file (PIN_OFF.s2p): S21: isolation when switch is OFF → target >35 dB S11: input reflection (most power reflected back) Load both files in RF View → overlay S21 dB: ON: trace near 0 dB (small loss) OFF: trace drops to −35 to −50 dB (high isolation) Visual gap = switch isolation = OFF_level − ON_level [dB]
Frequency Dependence
PIN diode isolation degrades with frequency due to parasitic package capacitance C_pkg (0.1–0.5 pF) which provides a bypassing path:
Isolation at frequency f: Isolation_dB ≈ 20·log₁₀(2π·f·C_pkg·Z₀) [approximate upper limit] At 2.4 GHz with C_pkg=0.3pF: Isolation_max ≈ 20·log₁₀(2π×2.4e9×0.3e-12×50) = 20·log₁₀(0.226) ≈ −13 dB → Only 13 dB from capacitance alone → actual may be higher with series L compensation
RF View PIN Diode: Load PIN_ON.s2p and PIN_OFF.s2p simultaneously. S21 overlay clearly shows ON-state IL and OFF-state isolation. Delta marker reads isolation vs insertion loss difference directly. Free on Android.