A RF DC Block is a critical component of many RF systems where power is carried along the same paths as the RF signal. This function is essential for some devices. However, the presence of DC on a signal path is also detrimental to the performance of some DC components and devices. Hence, there is a need for a component that can be connected inline with the RF signal path that can block the DC voltage at that given point while still allowing for RF signals to pass through with minimal degradation.
Essentially, a DC block is a high pass filter, and are generally even constructed like them. The basic design of a DC block are capacitors in series with the transmission line conductors, either one of either or both. As many DC blocks are based on coaxial connectorized inline assemblies and modified coaxial transmission lines, the designations of the types of DC blocks are typically references to the inner and outer coaxial transmission line conductors. The inline capacitor can be in series with the inner conductor, outer conductor, or both. Inner DC Blocks have the capacitor inline with the inner conductor, while outer DC Blocks have the capacitor inline with the outer conductor. A DC Block with a capacitor inline with both the inner and outer conductor is commonly called an Inner and Outer DC Block.
An Inner DC Block is best suited to blocking DC and low-frequency audio currents, but only minimally impacts the transmission line performance. Outer DC Blocks directly prevent DC current on the outer conductor and can also be used to protect downstream RF devices and components from surges along the transmission line outer conductor/shielding. Inner/Outer DC Blocks are able to provide both benefits at marginally greater transmission line performance degradation than just an Inner or Outer DC Block.
In some cases, DC Blocks are also paired with attenuators or have different coaxial connectors on either side of the DC Block. This can be useful in reducing the BOM and footprint of a signal chain without sacrificing function or performance.
Just like with other passive RF components, there are a few basic performance parameters and specifications that are considered the most critical:
- Block type
- Characteristic impedance
- Connector type (in series and out series)
- Connector gender (in series and out series)
- Minimum frequency
- Maximum frequency
- Maximum VSWR
- Maximum Voltage (voltage withstand)
- Operating Voltage (DC)
- Input power maximum (CW)
- Insertion loss