What causes frequency dependent loss in a DC block and how do I select one for wideband applications?

A DC block is a series capacitor that passes RF signals while blocking DC voltage. The frequency-dependent loss arises from the capacitor impedance and parasitic elements: (1) Low-frequency rolloff: at low frequencies, the capacitive reactance X_c = 1/(2×pi×f×C) becomes high, reflecting the signal. The -3 dB cutoff: f_low = 1/(2×pi×C×Z0). For C = 100 pF in 50 ohms: f_low = 1/(2×pi×100e-12×50) = 31.8 MHz. Below this frequency: the insertion loss increases at 20 dB/decade (the DC block acts as a high-pass filter). (2) High-frequency resonance: every capacitor has a self-resonant frequency (SRF) determined by its parasitic series inductance (ESL): SRF = 1/(2×pi×sqrt(L_parasitic × C)). At the SRF: the capacitor impedance is minimum (ESR only) and the DC block has minimum insertion loss. Above the SRF: the capacitor becomes inductive, and the impedance increases. The insertion loss increases. For a 100 pF 0402 chip capacitor: ESL ≈ 0.4 nH, SRF ≈ 25 GHz. For a 100 nF 0402: SRF ≈ 800 MHz. (3) ESR loss: the equivalent series resistance (ESR) of the capacitor causes resistive loss at all frequencies. For ceramic capacitors: ESR = 0.1-0.5 ohms (contributing 0.01-0.05 dB insertion loss in 50 ohms). For wideband DC block selection: choose a capacitor value that provides low reactance (< 5 ohms) at the lowest operating frequency AND has an SRF above the highest operating frequency. For 100 MHz to 18 GHz: C = 22-47 pF in 0402 package (SRF > 20 GHz). For 10 MHz to 6 GHz: C = 100-220 pF in 0402 (SRF > 8 GHz). For broadband (10 MHz to 40 GHz): use a thin-film or MIM (metal-insulator-metal) capacitor with very low ESL (< 0.1 nH), SRF > 50 GHz.