The most apparent distinction between radio frequency (RF) circuits and traditional circuits lies in the fact that RF circuits are distributed parameter circuits, where components are dispersed in the circuit layout. The physical positioning and layout of these components significantly influence circuit performance. Due to the high-frequency nature of RF circuits, they are particularly susceptible to skin effect and coupling effects during practical operation. This poses challenges for designers when it comes to controlling interference and radiation. These challenges include issues like mutual interference between digital and analog circuits, power supply noise, and ground design flaws. To address these challenges, TechSparks offers an RF PCB design guide that explores and optimizes various potential defects while striving to find compromises to mitigate interference issues.
What is Radio Frequency (RF)
Radio Frequency refers to an electromagnetic frequency within the range of 300 kHz to 300 GHz that can radiate into space. It is a type of electromagnetic wave characterized by its ability to transmit over long distances and generate high-frequency alternating current changes (more than 10^4 cycles per second). In comparison to low-frequency circuits, RF circuits are more complex due to the wavelength of electromagnetic waves being comparable to the size of the circuit or device. In RF circuits, the relationship between device size and conductor size requires the use of distributed parameter theory.
The table below provides a frequency band division, illustrating the corresponding frequency ranges and wavelengths. According to the definition of RF, the RF range extends from intermediate frequency to extremely high frequency.
|Corresponding Wavelength of Electromagnetic Wave
|Very Low Frequency (VLF)
|3 kHz – 30 kHz
|Very Long Wave: 100 km – 10 km
|Low Frequency (LF)
|30 kHz – 300 kHz
|Long Wave: 10 km – 1 km
|Medium Frequency (MF)
|300 kHz – 3000 kHz
|Medium Wave: 100 m – 100 m
|High Frequency (HF)
|3 MHz – 30 MHz
|Short Wave: 100 m – 10 m
|Very High Frequency (VHF)
|30 MHz – 300 MHz
|Meter Wave: 10 m – 1 m
|Ultra High Frequency (UHF)
|300 MHz – 3000 MHz
|Decimeter Wave: 100 cm – 10 cm
|Super High Frequency (SHF)
|3 GHz – 30 GHz
|Centimeter Wave: 10 cm – 1 cm
|Extremely High Frequency (EHF)
|30 GHz – 300 GHz
|Millimeter Wave: 10 mm – 1 mm
|300 GHz – 3000 GHz
|Submillimeter Wave: 1 mm – 0.1 mm
From a technical perspective, the emergence of RF technology is a milestone in human development, revolutionizing the way information is obtained in society and fundamentally changing the interaction between people and objects. Currently, four main frequencies exist in RF communication:
Baseband Frequency: The original signal frequency range, typically ranging from DC to a certain frequency. In RF communication, the baseband signal is the actual information to be transmitted, not the RF signal. Therefore, the baseband frequency involves signal modulation and data transmission in digital communication.
Transmission Frequency: The carrier frequency used for data transmission. This is the frequency of the RF signal, ranging from 500 MHz to 38 GHz. This frequency is used to carry the baseband signal and transmit it to remote locations.
Intermediate Frequency (IF): The frequency used during the modulation and demodulation process. In RF communication, the received high-frequency signal is first converted to an intermediate frequency for processing, and then further converted to the baseband for demodulation. IF is typically used for filtering and signal processing to avoid interference and improve system performance.
Reception Frequency: The frequency used for receiving signals. This is the final step where the received signal is converted to the baseband, and the original information is extracted.
Key Performance Parameters of RF PCB Materials
This important parameter describes the medium’s influence on electromagnetic wave propagation, affecting critical properties such as signal propagation speed and characteristic impedance. Although a lower dielectric constant typically indicates faster signal propagation, in practical circuit design, it doesn’t necessarily mean that lower is always better for RF PCB materials. It’s essential to balance various characteristics. For instance, for smaller circuit layouts, a material with a higher dielectric constant may be preferred, while lower dielectric constant materials may be more suitable for designs like power amplifiers.
Here are some examples of dielectric constants for common materials at 1MHz:
- Vacuum: 1.0
- Pure PTFE: 2.1
- Cyanate Ester/Glass: 3.2
- Polyimide-Glass (FR4): 4.4-5.2
- Ceramic-Filled PTFE: 6.0-10.2
- Water: 70.0
Also known as tangent loss or loss factor, it describes the energy loss of a signal in the medium. Losses typically increase with higher frequencies. Choosing materials with low loss is advantageous for improving sensitivity to RF signals and ensuring clarity.
Here are some examples of dielectric loss for common materials:
- FR-4: Approximately in the range of 0.02 – 0.05.
- PTFE: Typically in the range of 0.001 – 0.002.
- RO4000 Series: Approximately in the range of 0.002 – 0.003.
- RO3000 Series: Approximately in the range of 0.002 – 0.004.
- Fiberglass: In the range of 0.02 – 0.03.
- Ceramic-Filled PTFE: Approximately in the range of 0.003 – 0.01.
In many RF applications, especially in high-power scenarios, the material’s thermal conductivity is crucial for system reliability. Therefore, thermal conductivity is a key factor to consider.
Here are examples of thermal conductivity for common materials (in W/mK):
- PTFE/Glass Cloth Diclad, Cuclad: 0.26
- PTFE/Ceramic Powder Glass Cloth CLTE: 0.5
- AR1000: 0.65
- AD350i: 0.45
- Ceramic Powder-Filled Thermosetting Material 25N/FRO45: 1.2
- Thermal Conductive Material 99N: 1.2
- FR-4: 0.24-0.26
RF Circuit Component Layout
This discussion focuses on the component layout of multi-layer RF PCB. The arrangement of components significantly influences the transmission path of RF signals and the direction of current flow. The key takeaway here is the importance of adjusting component orientations for optimal RF circuit performance.
In RF circuit component layout, a linear layout (Figure 1) is the most ideal technique. By arranging components in a row, the signal path’s length can be minimized, avoiding signal interference caused by crossovers and close proximity between two signal lines, as well as mitigating signal reflection and standing wave issues. However, in practical applications, space limitations can hinder a linear layout.
In such cases, an L-shaped layout (Figure 2, left) can be employed. This involves placing components in an L shape, where the signal may need to follow one path, make a right-angle turn, and then continue along another path. While an L-shaped layout may not match a linear layout in terms of signal performance, it can compensate for space limitations.
For some beginners who don’t pay attention to details, might arrange components in a U-shaped layout (Figure 2, right), where the shape of the signal path resembles the letter “U.” This is a poor practice. Firstly, RF circuits are already quite challenging, and a U-shaped design makes the circuit messy and difficult to maintain and troubleshoot. Secondly, the curved path results in a longer signal path. Most critically, it introduces unnecessary signal interference and electromagnetic coupling.
Although the U-shaped design should be avoided, if there’s no other option, try to increase the distance between the input and output, at least by 1.5 cm or more.
Same or Symmetrical Layout
If your RF PCB board uses the same modules, a practical tip is to employ an identical or symmetrical layout. “Same modules” refer to configurations of circuit portions in different areas that are identical. This approach has several advantages. Firstly, designers can create one area first and then replicate it, significantly reducing workload and speeding up the design process. Secondly, this RF PCB design technique makes maintenance easier. When issues arise, engineers can compare a functioning module with a problematic one to pinpoint the root cause of the problem, without needing to learn different maintenance methods for each module.
In RF and microwave circuit environments, we often use feedthrough inductors to carry current or signals. Typically, inductors are coil-shaped components, primarily used to transfer current or signals from one circuit to another while providing isolation. However, the high-speed current flow brought by high frequencies leads to the generation of electromagnetic fields, resulting in mutual inductance effects. To mitigate the adverse impact of this mutual inductance effect on the circuit, a wise strategy is to place the feedthrough inductors for the bias circuit perpendicular to the RF channel (Figure 5).
The advantages of vertically placing the inductors are that when two inductor components are close and oriented in parallel, the magnetic field lines they generate overlap or couple with each other, increasing undesired energy transfer. In contrast, placing them vertically reduces the overlap of magnetic field lines. Additionally, the direction of signal transmission is determined by the connecting lines or wires between circuit components. When inductors are laid out vertically, signals propagate horizontally. This means that the primary direction of propagation for magnetic field lines is horizontal, which helps reduce mutual inductance with neighboring inductors. Moreover, a vertical inductor layout can provide a certain degree of physical isolation, further improving performance and reducing potential interference.
45° Component Placement
The application areas of RF PCB often involve higher-end electronics, where stringent space constraints necessitate the use of smaller circuit boards. An excellent strategy is to position the components on the board at a 45° angle (Figure 6). This approach not only increases the surface utilization of the board but also allows for more direct routing of RF lines, reducing the length of wires or circuit traces.
RF Trace Design
Short and direct signal routing, minimizing vias and avoiding intersections with other signal lines, remains fundamental rule in PCB design, even in RF circuits.
As mentioned earlier, in layouts with L-shaped and U-shaped configurations, corners are inevitable. However, these abrupt turns can lead to signal reflections, which are especially noticeable in RF circuits. To mitigate this, you can employ two approaches:
Firstly, you can introduce a short section of trace before the corner (Figure 7). Although we’ve mentioned that traces should be as short as possible, this added short segment can act as a buffer zone for signal transmission, gradually transitioning it to the new direction and reducing reflections.
Secondly, you can use curved traces (Figure 8), substituting smooth curves for sharp 90-degree angles. In practice, this has proven to be the most effective measure for significantly reducing signal losses.
Tapered Trace Handling
Chip is securely mounted on the PCB by soldering their pins, allowing current to flow through traces to reach the pins and ultimately activate the chip to perform its functions. However, in RF circuits, a situation may arise where the trace width connected to the chip is significantly wider than the width of the chip’s pins. This can potentially have adverse effects on circuit performance. The width of a chip’s pins is uniformly determined by the manufacturer, while trace width is complex to modify for current-carrying capacity on the circuit board. To address this scenario, an effective strategy is to implement tapered trace handling (Figure 9). Gradually reduce the trace width before it approaches the chip’s pins. This can prevent sudden changes in trace width, which can pose risks such as heat dissipation issues and impedance mismatches.
Power and Ground
Generally, it is not advisable to introduce plane splits in the power plane of an RF PCB. In other words, the power plane should be continuous. This is because introducing plane splits can cause electromagnetic fields generated by RF circuit currents to escape into space through the gaps or discontinuities. Secondly, to avoid loop issues, it’s better to use elongated power traces or planes instead of square ones. Square power traces or planes have multiple corners and edges, which can lead to current concentration in these areas, hindering proper dissipation. Using elongated shapes ensures that current flows from the power source end to the load (such as chips or components) and then returns to the power source end to complete the circuit.
Where possible, designers should incorporate ground traces on every layer of an RF PCB and connect these traces to the primary ground. For RF signals, sources of interference, sensitive signals, and other critical signals, adopting ground enclosure measures (Figure 10) means connecting a portion of the circuit to the ground to establish a reliable ground connection or shielding. This is done to ensure that the ground points of signal circuits can effectively connect to the ground plane on the PCB to maintain the circuit’s reference potential and reduce electromagnetic interference. Grounding techniques can include directly connecting ground traces to component pins, using ground planes, and other methods.
In the above two RF PCB design guidelines, most solutions to layout design issues have been covered. After addressing these potential internal interference problems, it’s recommended to use a shielded enclosure (Figure 11) to protect the circuit from external interference. This enclosure is made of metal and sealed on all sides to prevent external electromagnetic radiation from entering.
To establish connections between the internal circuitry and the outside world within the shielded enclosure, wires need to be introduced. Since RF circuits typically exhibit high-frequency characteristics, it’s advisable to use microstrip lines for these connections. Microstrip lines are specialized transmission lines designed for high-frequency signal transmission, characterized by their flat, ribbon-like shape, and they use narrow conductive strips to carry electrical signals.
Inside the enclosure, different RF modules should be isolated, especially sensitive circuits and strong radiation sources. This can be achieved by slotting the partition board, with the slots typically positioned at the center of the partition and having a width of approximately 3mm, allowing microstrip lines to pass through. This design ensures both electromagnetic interference avoidance and signal connectivity.
Finally, at the corners of the enclosure, it’s recommended to place 3mm metalized holes for securing the shielding shell. This ensures the stability and maintainability of the shielded enclosure.
Common Applications of RF PCB Board
The rapid development of RF technology has led to the widespread application of RF PCB in various fields. Here is an overview of some key domains and frequency bands:
- C-Band (4/6GHz) and Ku-Band (12/15GHz): Used for satellite communication and television broadcasting, enabling efficient data transmission.
- Inter-Satellite Communication (36GHz): Provides reliable communication links between satellites.
Microwave Relay Communication:
- Mainline Microwave (2.1GHz, 8GHz, 11GHz): Utilized for long-distance communication transmission.
- Branch Microwave (6GHz, 8GHz, 11GHz, 36GHz): Plays a role in rural multi-point communication.
Radar, Meteorology, Ranging, Positioning:
- Radar Surveillance (P, L, S, C): Different frequency bands used for remote monitoring.
- Precision Guidance (X, Ka): Enables high-precision navigation and guidance.
- Meteorology (1.7GHz, 0.1375GHz): Used for weather forecasting and monitoring.
- Automotive Collision Avoidance, Automatic Tolling (36GHz, 60GHz): Improves vehicle safety and toll collection systems.
- Global Positioning System (1227.60MHz and 1575.42MHz): Provides accurate location information for navigation.
- Radio Astronomy (36GHz, 94GHz, 125GHz): Used for astronomical observations.
Computer Wireless Networks:
- 2.5GHz, 5.8GHz, 36GHz: Provides high-speed communication in computer wireless networks.
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