How does SRF affect capacitor selection?

SRF (self-resonant frequency) is where the capacitor's parasitic inductance (ESL) resonates with the capacitance: SRF = 1/(2*pi*sqrt(ESL*C)). At SRF, the impedance reaches a minimum (series resonance, dominated by ESR). Below SRF, the component is capacitive (impedance decreases with frequency). Above SRF, the ESL dominates and the component is inductive (impedance increases with frequency). For coupling and DC blocking, the capacitor should be used near or at SRF where impedance is lowest. For matching networks, use below SRF/3 where the capacitance value is predictable. A 100 pF MLCC in 0402 package has SRF around 1.5 GHz and ESL of about 0.5 nH. For decoupling, multiple capacitors of different values are placed in parallel to cover a wide frequency range, with each capacitor providing low impedance near its SRF.

What dielectric types are used for RF capacitors?

C0G (NP0) is the premium RF dielectric: extremely stable with temperature (plus or minus 30 ppm per degree C), voltage, and frequency. Zero piezoelectric effect. Q factor of 200 to 1000 at 1 GHz. Used for matching networks, filters, VCO tuning, and any application requiring stable capacitance value. Available from 0.1 pF to about 10 nF. X7R is a high-K dielectric with much higher capacitance per volume but terrible RF properties: capacitance varies plus or minus 15 percent with temperature, decreases 30 to 80 percent with applied DC voltage, and has significant piezoelectric noise. Used only for low-frequency decoupling and energy storage, never for RF signal path. Porcelain (ATC, Johanson) achieves the highest Q (500 to 10000) with tight tolerance for precision filter applications. MIM (metal-insulator-metal) capacitors are fabricated in MMIC processes using thin silicon nitride or oxide dielectric between metal layers, achieving high density (1 to 2 fF per square micrometer) with SRF above 50 GHz.

How do you select the right capacitor for RF?

Step 1: Determine the required capacitance value from the circuit design (matching, coupling, or filtering calculation). Step 2: Check that the SRF of the capacitor is at least 3 times the operating frequency for matching applications, or near the operating frequency for coupling or decoupling. Step 3: Verify the Q factor is sufficient: for matching networks, Q greater than 100 is ideal (C0G dielectric); for coupling, Q greater than 20 is adequate. Step 4: Choose the smallest package size that meets the requirements, as smaller packages have lower ESL and higher SRF. A 0402 package has approximately 0.5 nH ESL; a 0201 has approximately 0.3 nH. Step 5: For DC blocking in the signal path, select the value so that X_C at the lowest operating frequency is much less than 50 ohms (less than 5 ohms, meaning C greater than 1/(2*pi*f*5)). At 1 GHz, this requires C greater than 32 pF. Step 6: Verify the voltage rating exceeds the maximum RF plus DC voltage.

Passive Components

RF Capacitor

/kah-pas-ih-ter/

Stores energy in an electric field with impedance X_C = 1/(2πfC). Parasitic ESL creates SRF = 1/(2π√(ESL×C)): below SRF = capacitive, above SRF = inductive. ESR determines Q = X_C/ESR. Key dielectric: C0G/NP0 (stable, Q 200-1000, for RF signal path), X7R (high-K, never for RF). Package ESL: 0402 ≈ 0.5 nH, 0201 ≈ 0.3 nH. Used for matching, DC blocking, coupling, decoupling, filtering.

X_C: 1/ωC

Use below: SRF/3

Q: 20-10000

Understanding RF Capacitors

At low frequencies, a capacitor is a simple energy storage element. At RF frequencies, it becomes a complex resonant structure. The lead inductance, electrode geometry, and dielectric properties all contribute to behavior that departs significantly from the ideal C = Q/V model. A 100 pF capacitor at 2 GHz may actually present an inductive impedance if its SRF is below 2 GHz, which completely defeats its intended purpose.

The golden rule of RF capacitor selection is: always check the SRF relative to your operating frequency. For signal-path applications (matching, coupling), use below SRF/3 where the capacitance is predictable. For decoupling (minimum impedance to ground), use at or near SRF where impedance is minimized. For broadband decoupling, place multiple values in parallel (100 pF + 1 nF + 100 nF) so that each value's SRF covers a different frequency range.

Capacitor Equations

Capacitance:
C = ε₀ε_rA/d farads

Impedance:
Z_C = 1/(j2πfC)
|Z_C| = 1/(2πfC)

Energy stored:
E = ½CV² joules

Charge:
Q = CV coulombs
I = C(dV/dt)

RF Capacitor Technology Comparison

Type	Range	Q @1GHz	SRF	Application
NP0/C0G MLCC	0.5 pF–10 nF	200–1000	1–10 GHz	RF matching/filter
X7R MLCC	100 pF–100 μF	10–50	10 MHz–1 GHz	Bypass/decoupling
Tantalum	1–1000 μF	<5	<10 MHz	Power bulk
Film (polypropylene)	100 pF–10 μF	500–5000	1–100 MHz	High-Q filter
Silicon (MIM)	0.1–100 pF	50–200	5–50 GHz	MMIC

Common Questions

Frequently Asked Questions

How does SRF affect selection?

SRF = 1/(2π√(ESL×C)). At SRF: min impedance (series resonance). Below: capacitive. Above: inductive. Matching: use <SRF/3. Coupling/DC block: can use near SRF. Decoupling: use AT SRF for min impedance. 100 pF 0402: SRF~1.5 GHz. Multiple values in parallel covers wide bandwidth.

What dielectrics for RF?

C0G/NP0: premium RF, ±30 ppm/°C, stable with voltage/frequency, Q 200-1000, no piezo effect. X7R: high-K, ±15% temp, -30 to -80% with DC voltage, piezo noise. NEVER for RF signal path. Porcelain (ATC): highest Q (500-10000), tight tolerance, precision filters. MIM: on-chip MMIC, SRF>50 GHz, mmWave.

Selection process?

1) Calculate C from circuit design. 2) Check SRF > 3×f_op for matching. 3) Verify Q sufficient (C0G for matching, Q>100). 4) Smallest package = lowest ESL = highest SRF. 0402: 0.5 nH. 0201: 0.3 nH. 5) DC block: X_C < 5 Ω at f_low (C>32 pF at 1 GHz). 6) Voltage rating > V_RF + V_DC.

Related Terms

← Calibration Carrier Aggregation →