DC blocks prevent the flow of direct current (DC) through radio frequency (RF) circuits. They serve as high-pass filters that prevent DC voltages, which have a frequency of zero Hertz (Hz), from interfering with sensitive RF components such as receivers. In other words, DC blocks are capacitors in series with a transmission line. They prevent the flow of DC energy while allowing RF signals to pass with little (if any) attenuation.
Types of DC Blocks
There are three types of DC blocks: inner, outer, and inner/outer.
- Inner blocks have a capacitor in series with the inner, or 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 blocks have a capacitor 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 blocks prevent DC from passing along both conductors, typically in a coaxial connection.
DC blocks can also be incorporated into connector blocks or block connectors.
Electrical and Material Specifications
Electrical specifications for DC blocks include frequency range (for example, from DC to 10 GHz or 0.01 to 3 GHz), maximum voltage standing wave ratio (VSWR), insertion loss, maximum DC voltage, breakdown voltage, and power rating (peak or CW). The electrical specifications can vary depending on the type of connector (N, BNC, SMA, 2.9 mm, and 2.4 mm).
Material specifications for DC blocks include body material (e.g., stainless steel), substrate (e.g., alumina), solder type (e.g., SN62, lead free, tin/lead), insulator (e.g., Teflon), and contacts (e.g., gold-plated beryllium copper)
Applications for DC Blocks
DC blocks are used to eliminate ground loops, suppress signal-source modulation leakage, and improve the signal-to-noise ratio. They are used to block current surges in rail systems and subway tunnels, route DC to tower-mounted amplifiers (TMA), and provide DC isolation as required. DC blocks are sometimes used in test setup isolation, and in other applications where the flow of DC between different sections of an RF system is undesirable.
Standards and Approvals
DC blocks may be manufactured to meet U.S. military specification (MIL-SPEC) MIL-A-3933, general requirements for radio and microwave frequency fixed attenuators; or MIL-PRF-39012, general requirements and tests for radiofrequency connectors used with flexible RF cables and certain other types of coaxial transmission lines.