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Update July 2024. Thanks to Zhongzhu for providing way more content on this subject! Scroll to the bottom of the page to see an example matching network for a DC block.
The DC block is a passive device that blocks DC signals on an RF signal path. It appears as open (disconnected) at DC and short (connected) at RF. It differs from a regular low-pass filter in that, a DC block is used to prevent DC signals from getting into RF signals, or to provide DC isolation, on an unbalanced transmission line.
A DC block is most often implemented as a lumped element capacitor with low series reactance at RF frequencies, connected in series. Or less commonly, it can be implemented in distributed elements, such as parallel coupled lines.
In the industry, the term "DC block" often refers to the type used for coax cables. Such types usually look like typical RF cable connectors, and have capacitors built-in.
DC blocks for coax cables
DC blocks on an unbalanced transmission line (such as coax cable) have three arrangements:
- Signal only. Also called "Inside only". (Signal is the inside conductor of a coax cable.)
- Ground only. Also called "Outside only". (Ground is the outside conductor of a coax cable.)
- Both signal and ground, "Inside / Outside".
The three types are shown in the circuit diagram symbols below:
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Three types of dc blocks
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Many vendors offer coaxial dc blocks in all three arrangements.
Applications and use cases of DC blocks
DC blocks can be found in many applications:
Provide RF coupling between blocks or stages
Most RF signal chains require RF pass-through but no DC coupling between individual blocks or stages, for proper RF operation, DC biasing, and to prevent overvoltage damage. This is achieved using DC blocks, often as a single capacitor in series.
Antenna input / output
Depending on the design, some antennas have a DC block to connect to the RF front-end circuits.
AC coupling on high speed serial digital communication interfaces
Some high-speed serial digital communication interfaces such as PCI-Express (PCIe), Serial-ATA (SATA) require DC blocking / AC coupling, for the purposes of isolating different DC biasing levels at TX and RX, overcurrent protection, channel isolation / reducing cross-talk, and improving noise margin.
The operating principles are similar to the DC blocks in RF circuits. However, their design requirements focus more on time domain aspects such as noise margin and rise/fall time of digital signals. They are also frequently called "AC coupling", instead of "DC blocking".
Make a bias tee
Every bias tee contains a DC block as one of its branches.
Reduce low frequency interference on RF paths and improve SNR
They are used in wireless audio systems, to prevent audio frequency signal from interfering with RF signal.
DC power / DC ground isolation
Some RF systems with high DC voltage may need a DC block on the RF lines for safety considerations.
Eliminate ground loops
Inserting a DC block on a GND path will block DC return current on that path. This is useful in preventing ground loops at DC.
RF test equipment setup
Some RF test setups with multiple ports or multiple pieces of connected equipment, will require DC blocks at some points, to eliminate ground loops or provide DC isolation.
Some test equipment has DC blocks (or bias tees) built in to prevent damage from DC voltage. For example, VNAs are almost always DC blocked. What about noise sources and power heads? Consult data sheets before you inject any DC signals into any test equipment!
Special case of power delivery into RF system
For power delivery designs with a single combined power + RF input line, and separate power and RF returns (grounds), a DC block is required on the RF return (RF ground).
Things to consider when designing and selecting DC blocks
As simple as it may seem, there are many factors involved in order to get a DC block properly.
Whether designing a new DC block, or selecting an existing one off-the-shelf, all the factors below apply.
Application scenario
Will your DC block be used in a coax cable? As a module in a box container? On a circuit board? On an RFIC / MMIC? What types of DC / low frequency signals will it block? What types of RF signals will it pass?
Form-factor
What size / dimension/shape is required?
If for coax connector, what type? BNC? SMA? Male or female? Does it require straight connection or 90 degree bending? etc...
If for circuit board, will it be through-hole or surface mount? What are the limits on footprint size and height? etc...
DC and AC voltage rating
As the capacitor blocks DC signal, there is a voltage difference between its terminals. The capacitor's voltage rating must be substantially higher than the SUM of (DC voltage it blocks) plus (RF peak voltage it passes). Otherwise you may risk turning the DC block into a single-use DC+RF spark plug!
Cutoff frequency
In addition to isolating direct current at 0 Hz, sometimes DC blocks are used to isolate low frequency signals, such as audio or baseband. In this case, the cutoff frequency must be set higher than the low frequency to be isolated, instead of setting it as close to DC as possible.
Capacitance
The capacitance value is chosen based on both the highest "low frequency" signal it needs to block, and the lowest frequency RF signal it needs to pass through.
The minimum capacitance is determined by the lowest frequency of the pass-through RF signal. Larger capacitance is required to pass lower frequency RF signals.
The maximum capacitance is determined by the highest frequency of the signal it needs to block.
Self-resonance frequency
For details about RF passive component self resonance frequency (SRF) look here.
The capacitor's self resonance frequency is where it will act close to ideal to pass RF signals. Ideally it will be inside the frequency band you are trying to pass. Below self resonance the capacitor will add capacitive reactance to the network, above SRF it acts more like an inductor.
Obtain the frequency response plot from the capacitor manufacturer. And don't forget there is a non-trivial device variation on self resonance frequency.
Parasitics and loss
For more information on RF passive component parasitics look here. Real capacitors have parasitics such as Effective Series Resistance (ESR) and Effective Series Inductance (ESL), which determine the impedance / reactance at RF, which in turn determine important figures of merits like insertion loss, and self-resonance frequency.
The ESR and ESL should be made as low as possible to reduce loss and other unwanted effects, just like designing other RF capacitors. This also means the DC blocking capacitor should have a high Q-factor. Be aware that capacitor ESR is frequency-dependent. The ESR figure should be taken at RF, not at DC. Learn more about ESR here.
Also note, do not choose capacitance larger than necessary, because parasitic resistance and inductance increase with capacitance as well. This means the self resonance frequency will also be lower for larger capacitors.
Another important thing is to minimize any parasitic shunt capacitance when the DC block is placed into the network. For DC block surface mounted on PCB, one of such sources is the capacitance of component pads. This can cause instability problems.
Type of capacitor
The type of capacitor is determined by all the factors above. The desire for high Q-factor means that in most cases, only choose special RF capacitors.
Many capacitor types may not be a good choice. Electrolytic or tantalum capacitors are unsuitable for DC blocks due to their very high ESR and ESL. "General purpose" multi-layer ceramic capacitors (MLCC) with X5R / X7R / X8R dielectrics typically used for power decoupling are not suitable either, despite their small size and low ESR, Their low ESR number is quite misleading, because it's usually only limited to low frequencies. They also have generally poor Q-factors so the manufacturers don't even list the figure. When used as RF components, these MLCC capacitors will degrade SNR, and too much unwanted parasitics at RF.
Impedance matching
The series capacitor of a DC block causes an impedance discontinuity. To use it on an RF signal path, impedance should be matched to the characteristic impedance Z0. It's your job to do your best to match the DC block to Z0 any way you can. Here is a page on impedance matching but admittedly it does not offer any suggestions for compensating for series capacitance.
Here we have created a simple example of impedance-matching a capacitor that includes self-resonance. Below is a model of a 10 pF cap that has series resonance at 4 GHz (we looked in a Kyocera chip-cap catalog, this seems typical).
Here is the frequency response of the cap by itself. Above self-resonance it is on a one-way trip to inductance-ville.
Next, we added some short impedance transformers to input and output and let the optimizer loose.
Here is the result. We were able to get almost 16 dB return loss from 1 to 30 GHz.
Note that our model of the cap does not include all parasitics, but you can add them in yourself and continue this work. If anyone has developed a DC block that works well and can provide a photo, please send it to us, if you are able to!