This tool calculates the value of a DC blocking capacitor.
Enter the minimum frequency that has to pass through the cap and the value of the impedance. All frequencies lower than this frequency (including DC) will be attenuated.
Formula
C > 1/(2*π*f*XC)
Example Calculation
A DC blocking capacitor that has a 10 Ω impedance at 50 kHz, has a minimum value of 318 nF.
What is a DC Block?
It is a passive component or connectorized module that’s used to block DC while allowing the flow of RF signals above a specified frequency. It is essentially a capacitor with key specifications that include
- Frequency of operation
- Insertion Loss
- Maximum DC voltage
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 receivers.
Where are DC blocks are used?
Some electronic instruments such as spectrum analyzers cannot tolerate DC voltages at their input. If you try to measure noise on a DC line with this instrument, it will damage the equipment.
The use of a DC block is highly recommended in such situations.
It’s good practice 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.
Many radio receivers 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 receivers from damage it is recommended that a DC block with known specifications be used.
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. Use the calculator to determine the minimum capacitance value.