RF Passives › Component Behaviour
Non-Ideal R, L, C at High Frequencies
Real resistors, inductors and capacitors carry parasitic elements that dominate at RF. Understand equivalent circuits, self-resonant frequency (SRF) and why your 10 nH inductor becomes a capacitor above 500 MHz.
The core problem: At DC and low frequencies, lumped components behave as their label says. But at RF, every component has parasitic inductance, capacitance and resistance that change its impedance completely. A capacitor can look like an inductor. An inductor can look like a capacitor. A resistor can resonate. Understanding this is fundamental to any RF design above ~50 MHz.
Real Capacitor
Equivalent Circuit & SRF
SRF = 1/(2π√(ESL·C))
Q at SRF = —
|Z| actual
Ideal 1/ωC
SRF point
Below SRF: acts capacitive (|Z| falls with frequency).
At SRF: ESL resonates with C → |Z| = ESR (minimum, purely resistive).
Above SRF: acts inductive — your capacitor is now an inductor!
At SRF: ESL resonates with C → |Z| = ESR (minimum, purely resistive).
Above SRF: acts inductive — your capacitor is now an inductor!
Real Inductor
Equivalent Circuit & SRF
SRF = 1/(2π√(L·Cp))
Q peak = —
|Z| actual
Ideal ωL
SRF point
Below SRF: acts inductive (|Z| rises with frequency, Q peaks).
At SRF: Cp resonates with L → |Z| = DCR (maximum, purely resistive).
Above SRF: acts capacitive — your inductor is now a capacitor!
At SRF: Cp resonates with L → |Z| = DCR (maximum, purely resistive).
Above SRF: acts capacitive — your inductor is now a capacitor!
Real Resistor
Equivalent Circuit & Behaviour
Resonance = —
Q at 100MHz = —
|Z| actual
Ideal R
Resonant point
Low frequency: |Z| ≈ R as expected.
At resonance: lead inductance resonates with Cp → impedance peaks or dips.
High frequency: Cp dominates → resistor looks like a short circuit!
At resonance: lead inductance resonates with Cp → impedance peaks or dips.
High frequency: Cp dominates → resistor looks like a short circuit!
What is Self-Resonant Frequency (SRF)?
SRF Definition: The frequency at which a component's parasitic reactance exactly cancels its intended reactance. At SRF, the component looks purely resistive.
For a capacitor: f_SRF = 1/(2π√(ESL·C))
For an inductor: f_SRF = 1/(2π√(L·Cp))
Rule of thumb: Never use a component above 50% of its SRF. Above SRF it behaves as the opposite component type.
For a capacitor: f_SRF = 1/(2π√(ESL·C))
For an inductor: f_SRF = 1/(2π√(L·Cp))
Rule of thumb: Never use a component above 50% of its SRF. Above SRF it behaves as the opposite component type.
Practical implication: A 100 nF decoupling capacitor with 3 nH ESL has SRF ≈ 9 MHz. At 100 MHz it looks like a 3 nH inductor — completely useless for decoupling. This is why RF decoupling uses multiple capacitors in parallel (different SRFs stagger across the band).
Typical SRF Values
| Component | Value | Typical SRF | Usable to |
|---|---|---|---|
| SMD Cap (0402) | 100 nF | ~9 MHz | 4 MHz |
| SMD Cap (0402) | 10 nF | ~30 MHz | 15 MHz |
| SMD Cap (0402) | 100 pF | ~300 MHz | 150 MHz |
| SMD Cap (0402) | 1 pF | ~3 GHz | 1.5 GHz |
| SMD Inductor | 100 nH | ~100 MHz | 50 MHz |
| SMD Inductor | 10 nH | ~500 MHz | 250 MHz |
| SMD Inductor | 1 nH | ~3 GHz | 1.5 GHz |
| Resistor (0402) | 50 Ω | ~1 GHz | 500 MHz |
| Resistor (0402) | 1 kΩ | ~200 MHz | 100 MHz |
RF Design Rules for Lumped Components
1. Always check SRF in datasheets
RF component datasheets specify SRF. Never use a component above 50% of its SRF in a signal path.
RF component datasheets specify SRF. Never use a component above 50% of its SRF in a signal path.
2. Smaller package = higher SRF
0201 > 0402 > 0603 for SRF. Smaller packages have less lead inductance and less parasitic capacitance.
0201 > 0402 > 0603 for SRF. Smaller packages have less lead inductance and less parasitic capacitance.
3. Parallel decoupling caps
Use 100 nF ∥ 10 nF ∥ 100 pF ∥ 1 pF in parallel to cover a wide frequency range. Each handles a different band.
Use 100 nF ∥ 10 nF ∥ 100 pF ∥ 1 pF in parallel to cover a wide frequency range. Each handles a different band.
4. High-value resistors are capacitive at RF
A 10 kΩ resistor has significant body capacitance. At GHz frequencies it looks like a short. Use resistor dividers carefully in RF feedback paths.
A 10 kΩ resistor has significant body capacitance. At GHz frequencies it looks like a short. Use resistor dividers carefully in RF feedback paths.
5. Bond wire and via inductance matters
A 1 mm bond wire ≈ 1 nH. A PCB via ≈ 0.3–1 nH. At 5 GHz, 1 nH = 31 Ω reactance — significant in matching networks.
A 1 mm bond wire ≈ 1 nH. A PCB via ≈ 0.3–1 nH. At 5 GHz, 1 nH = 31 Ω reactance — significant in matching networks.
6. Use transmission line elements instead
Above ~2–3 GHz, replace lumped L and C with microstrip stubs, coupled lines and λ/4 sections which have predictable distributed behaviour.
Above ~2–3 GHz, replace lumped L and C with microstrip stubs, coupled lines and λ/4 sections which have predictable distributed behaviour.