What is the frequency response of a DC block capacitor and how do I select the right value?

A DC block capacitor passes RF signals while blocking DC bias voltages. Its frequency response is determined by the capacitance value, self-resonant frequency (SRF), and package parasitics: (1) Low-frequency cutoff: below a certain frequency, the capacitor impedance is too high to pass the signal. The -3 dB cutoff: f_low = 1/(2×pi×C×2×Z_0), where Z_0 is the system impedance (50 ohms). For C = 100 pF: f_low = 1/(2×pi×100e-12×100) = 15.9 MHz. Below 15.9 MHz: the insertion loss exceeds 3 dB (signal is attenuated). For C = 10 nF: f_low = 159 kHz (much lower cutoff, passes lower frequencies). For C = 100 nF: f_low = 15.9 kHz. Rule: larger capacitance = lower cutoff frequency. (2) Self-resonant frequency (SRF): the frequency at which the capacitor series inductance (ESL) resonates with the capacitance: SRF = 1/(2×pi×sqrt(C×ESL)). At the SRF: the capacitor impedance is minimum (essentially just the ESR). Below SRF: the component behaves as a capacitor (impedance decreases with frequency). Above SRF: the component behaves as an inductor (impedance increases with frequency). For a 0402 package, 100 pF: ESL ≈ 0.4 nH. SRF = 1/(2×pi×sqrt(100e-12×0.4e-9)) = 796 MHz. The cap is useful as a DC block from 16 MHz to about 800 MHz. Above 800 MHz: insertion loss increases. For a 0402, 10 pF: ESL ≈ 0.4 nH. SRF = 2.52 GHz. Useful from 159 MHz to about 2.5 GHz. For a 0201, 1 pF: ESL ≈ 0.2 nH. SRF = 11.3 GHz. Useful from 1.59 GHz to about 11 GHz. (3) Insertion loss: at the SRF and slightly below: the insertion loss is minimum (0.05-0.2 dB). Well below SRF: insertion loss increases (high capacitive reactance). Above SRF: insertion loss increases (inductive impedance). For broadband DC blocking (e.g., 100 MHz to 6 GHz): use multiple capacitors in parallel (a large cap for low-frequency coverage + a small cap for high-frequency coverage).