A DC block is a passive device that’s used to prevent DC from crossing across the points between which it is connected while allowing the flow of RF signals. It is essentially a capacitor with key specifications that include frequency of operation and maximum DC voltage that it can withstand.
When included in an RF circuit, a DC block takes the form of a surface mount capacitor. Alternatively it is often designed into a cylindrical enclosure with SMA connectors for example, as shown in the picture below. The connectorized component can be easily used with test equipment and SDRs.
Where are DC blocks are used?
Many instruments such as spectrum analyzers cannot tolerate DC voltages at their input. So for instance, if you try to measure noise on a DC line with such a spectrum analyzer it will damage the instrument.
The use of a DC block is highly recommended in such situations. In fact, you might want to leave it on the instrument permanently to prevent accidental damage. And if you do, just remember to account for the insertion loss or attenuation due to the DC block in your measurements.
Benchtop Spectrum analyzers have N-type connectors and the DC block shown below will connect directly to the RF input of a spec analyzer.
Many software-defined radios have an internal DC block by way of a capacitor at the input. However most of them do not list any specifications on the DC voltage that can be tolerated. To prevent SDRs from damage it is recommended that a DC block with known specifications be used. Most SDRs have SMA-F input connectors. Here is an SMA-connectorized DC block that would be useful in this situation. As well here are a series of DC blocks with a variety of different connector types.
What are the key specifications of a DC block?
Every DC block has a frequency range over which it operates. This is the range across which the insertion loss is minimal. Ideally a DC block attenuates the input signal by no more than 1 dB. A DC block also has a maximum DC voltage specification.
Summary
A DC block is passive component that can be used to stop DC voltage entering the circuit while allowing RF signals to flow across. It can be used to prevent damage to RF instruments and Software-defined radios.
