How large should the DC blocking capacitor be?

The capacitor's reactance at the lowest operating frequency must be much less than the system impedance (typically 50 ohms). A rule of thumb is Xc less than Z0/10, giving Cmin = 1 / (2 pi f_low times 5) for a 50 ohm system. At 100 MHz: Cmin = 318 pF. At 1 GHz: Cmin = 32 pF. At 10 GHz: Cmin = 3.2 pF. However, larger capacitors have lower self-resonant frequencies due to parasitic inductance. Above the SRF, the capacitor becomes inductive and its impedance rises. Select a capacitor whose SRF is above your highest operating frequency.

What happens above the self-resonant frequency?

Every real capacitor has parasitic series inductance (ESL) from its leads and internal structure. At the self-resonant frequency (SRF = 1 / (2 pi sqrt(C times ESL))), the capacitive and inductive reactances cancel and the impedance is minimum (just the ESR). Above the SRF, the component behaves as an inductor with rising impedance, and the DC block becomes a band-stop filter that attenuates the signal. A 100 pF 0402 capacitor has an SRF around 2 to 3 GHz. A 1 pF 0201 capacitor has an SRF around 15 to 20 GHz. For broadband applications, use multiple capacitors of different values in parallel to cover the full band.

Do coaxial DC blocks work differently than chip capacitors?

Coaxial DC blocks are packaged components with SMA, N, or 2.92 mm connectors that contain an internal series capacitor optimized for the connector's impedance. They provide better impedance match than a chip capacitor on a PCB because the transition geometry is controlled. Inner-only DC blocks have a capacitor only on the center conductor, while inner-outer DC blocks have capacitors on both the center and outer conductor, providing galvanic isolation between both conductors. Inner-outer types are needed when the ground potential differs between the two sides.

RF Design

DC Block

Two amplifier stages running at different drain voltages need to exchange an RF signal without shorting their bias supplies together. The solution is a series capacitor: infinite impedance at DC, near-zero impedance at RF. It sounds trivial until you realize the capacitor has parasitic inductance that turns it into a resonator, a self-resonant frequency above which it stops being a capacitor entirely, and a physical size that disrupts the transmission line impedance. Choosing the right DC block means balancing capacitance (for low-frequency coverage), package size (for high SRF), and layout (for maintained 50 Ω match).

Category: RF Design

Function: Pass AC, block DC

Key Spec: SRF (self-resonant frequency)

Selecting the Right Capacitor for Your Frequency

Minimum capacitance (for X_C < Z₀/10):
C_min = 1 / (2π × f_low × Z₀/10)
= 10 / (2π × f_low × 50) for 50 Ω systems

Self-resonant frequency:
SRF = 1 / (2π√(C × ESL))

Insertion loss from reactance mismatch:
IL ≈ 20·log₁₀(1 + X_C²/(4Z₀²)) ≈ X_C²/(4Z₀²) × 8.686 dB (for X_C << Z₀)

Capacitor Package vs. SRF and Frequency Coverage

Package	Typical C	ESL	SRF	Usable Range	Application
0402 (100 pF)	100 pF	~0.4 nH	~2.5 GHz	10 MHz to 2 GHz	Sub-6 GHz signal chain
0402 (10 pF)	10 pF	~0.4 nH	~8 GHz	100 MHz to 7 GHz	C-band, X-band
0201 (1 pF)	1 pF	~0.15 nH	~13 GHz	500 MHz to 12 GHz	X/Ku-band MMIC
0201 (0.3 pF)	0.3 pF	~0.15 nH	~24 GHz	2 GHz to 22 GHz	K-band, 5G FR2
Coaxial (SMA)	Internal	Minimal	18 to 26 GHz	10 MHz to 18 GHz	Test equipment, bench

Common Questions

Frequently Asked Questions

How large should the blocking capacitor be?

X_C must be < Z₀/10 at the lowest frequency. For 50 Ω at 100 MHz: C_min = 318 pF. At 1 GHz: 32 pF. At 10 GHz: 3.2 pF. But larger caps have lower SRF, so choose the smallest value that satisfies the low-frequency requirement.

What happens above the SRF?

The capacitor becomes inductive (impedance rises with frequency). The DC block turns into a band-reject filter. A 100 pF 0402 has SRF ~2.5 GHz; above that, it attenuates the signal. Use smaller capacitors (lower C, higher SRF) or parallel multiple values for broadband coverage.

Coaxial vs. chip DC blocks?

Coaxial DC blocks (SMA, N, 2.92 mm) have controlled impedance transitions for better match. Inner-only types block DC on the center conductor. Inner-outer types isolate both conductors for different ground potentials between sides.

Related Terms

← dBi vs. dBd DC IV Curve →

Component Selection

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