RF Capacitor Dielectric Types
| Dielectric | Q at 1 GHz | Temperature Coeff | RF Use |
|---|---|---|---|
| C0G (NP0) | 500–2000 | 0 ±30 ppm/°C | Matching, filter, resonator |
| U2J | 300–1000 | −750 ppm/°C | Matching, RF bypass |
| X7R | 50–200 | ±15% over −55 to +125°C | Bypass only (not matching) |
| X5R | 30–100 | ±15% over −55 to +85°C | Power bypass (bulk) |
Series Resonance in Capacitors
Real capacitor model: C in series with ESL (package lead inductance) Series resonant frequency: f_SRF = 1/(2π√(ESL·C)) Below SRF: capacitive (expected behavior) At SRF: minimum impedance = ESR only (use as RF bypass here) Above SRF: inductive (capacitor acts as inductor!) Example: 100 pF C0G (0402), ESL ≈ 0.5 nH: f_SRF = 1/(2π√(0.5nH × 100pF)) = 712 MHz Above 712 MHz: behaves as 0.5 nH inductor → do NOT use as RF bypass at 2.4 GHz! At 2.4 GHz: need ≤ 10 pF to have SRF above 2.4 GHz
C0G vs X7R for RF Matching Networks
Matching network at 900 MHz, C = 6.8 pF:
C0G: Q ≈ 1000 at 900 MHz → ESR = 1/(2π·900MHz·6.8pF·1000) = 0.026 Ω → IL <0.01 dB
X7R: Q ≈ 100 at 900 MHz → ESR = 0.26 Ω → IL ≈ 0.1 dB
Rule: ALWAYS use C0G/NP0 capacitors in RF matching networks above 100 MHz
X7R is only acceptable for bypass/decoupling where IL doesn't matter
Analysis in RF View
- Download C0G and X7R .s2p files from SimSurfing.com for same capacitance value
- Load both into RF View simultaneously
- S11 Smith chart: C0G stays tightly on the capacitive arc; X7R drifts with temperature
- S21 view (if measured in series fixture): C0G shows flat passband; X7R shows more loss
- Find SRF: where S11 trace crosses the real axis (top of Smith chart), or where |S11| = minimum
RF View Capacitor Comparison: Load C0G and X7R .s2p files simultaneously. Smith chart comparison immediately shows the stability difference. Find SRF for both. Select the right type for your application. Free on Android.