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What is a DC Block?

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- everything RF

Aug 22, 2021
DC Blocks are components that prevent the flow of DC signals into systems while allowing higher frequency RF signals to pass through. DC Blocks are placed within a system to stop any signal with a frequency of Zero Hz (DC) from interfering with sensitive RF components. The DC block can be thought of as a high-pass filter allowing only the RF frequencies to pass through and are usually designed by placing capacitors in series with a transmission line. There are three types of DC Blocks:

Inner DC Blocks: Inner DC Blocks have a capacitor that is placed in series with the center conductor. They prevent the flow of DC and minimize the flow of low-frequency audio currents while providing minimum impedance to RF signals.

Outer DC Blocks: Outer DC Blocks have a capacitor that is placed in series with the outer conductor. They prevent the flow of direct current and low-frequency current surges along the outer conductors of transmission lines.

Inner/Outer DC Blocks: Inner/Outer DC Blocks have a capacitor placed in series with both the outer conductor and inner conductor. This prevents DC from passing along both conductors, typically in a coaxial connection.

Key parameters when looking for DC Blocks

Frequency: DC blocks are usually optimized for a Frequency range i.e they block all DC signals and allow RF signals in a specific frequency range.

Voltage Rating: Also known as Breakdown Voltage or Max Voltage, this is the maximum voltage that a DC Block can handle before it breaks down. The higher the voltage rating the better.

Insertion Loss: This is a measure of the loss of RF Signal that passes through the DC Block. In an ideal case, this would be zero.

Impedance: DC Blocks should be matched to the characteristic impedance of the system in which they are being used. Most RF systems are 50 ohms, however, 75 ohm DC blocks are also available.

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