RF & Microwave, , Component Distributors Inc. (CDI) A Broadband DC Block is a specialized form of capacitor.  The capacitor is used in series within a Radio Frequency (RF) circuit to prevent DC current from flowing between two RF circuits while still allowing the RF signal to pass.  The capacitor design is optimized to allow a wide range of RF frequencies to pass, making it a "broadband".  They are further designed to minimize resonances across a wide range of frequencies, so they will not alter (distort) the RF signal as it passes through. 

They can also be used as decoupling elements when connected between a signal line and ground.  This is usually done as part of a DC bias network that is connected to an active RF device, such as an Amplifier.
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