PCB Design

RF PCB Design Guidelines From Experienced Radio Frequency Engineers

RF PCB Design

RF PCBs (Radio Frequency Printed Circuit Boards) are often indispensable in applications which require the transmission and reception of radio signals. While most of the PCB design procedures remain the same for RF PCBs, care has to be taken to follow the RF PCB design guides to ensure the reliability and robustness of the performance in RF PCBs.

In this article, very experienced radio frequency engineers will give you some first-hand and very practical RF PCB design guidelines.

RF circuit board design

Here are some main considerations for RF PCB design.

  1. RF PCB design requires special considerations since they operate at high frequencies   (impedance of 50 ohms.)
  2. They require specialized materials which has reduced dielectric loss. (FR4, PTFE, Rogers laminate, ceramic-based materials)
  3. Materials with high conductivity are to be used to cater for the needs of high-frequency requirements.
  4. Maintaining signal integrity becomes essential to minimize errors in data transmission, particularly in high-speed devices. Impedance control and establishing transmission line structures are necessary to obtain this integrity in the RF circuit board design.

Now we are going to discuss something important in this RF PCB design guidelines

Below mentioned points are to be taken care of during schematic RF PCB design, RF PCB layout and manufacturing processes to ensure good quality boards.

Generic considerations for RF PCB design:

1. Frequency:

  • The challenges of high-frequency PCB design which includes electro-magnetic interference, cross-talk and signal losses should be addressed.
  • This can be done by establishing proper stack-up RF circuit board design.

2. Impedance Matching:

  • Degradation of signal and EMI (electromagnetic interference) becomes a common issue with high-frequency circuits as the wavelength is much shorter.

3. Electromagnetic Interference (EMI):

  • Electromagnetic interference is a major concern for the PCBs in general. The interference can occur due to internal devices or external environments. However, it is necessary to mitigate the effects of the EMI in any system to ensure the proper functioning of the devices.

4. Choice of PCB material:

  • While a few substrates like FR4 are considered to be an optimal choice, due to its ease in manufacturing processes and availability, the choices mostly rely on factors such as low dielectric constant, low dissipation factor, and high thermal conductivity.

PCB materials

5. Use of Decoupling Capacitors:

  • The optimal use of decoupling capacitors for the RF system becomes inevitable to isolate power and ground planes as it has adverse effects on the performance of the devices.
  • A sufficient number of decoupling capacitors to near larger ICs or per square inch of the boards are calculated to maintain its efficiency.

It is essential to remember that these RF PCB design guidelines should be used in conjunction with other design and manufacturing recommendations specific to your components, PCB manufacturer, and material sets.

Floor planning and component placement

Schematic RF PCB Design Guidelines

Floor planning:

  • To begin with, planning the placement of the components in RF PCBs involves segregating the design into functional blocks and creating a block diagram.
  • The high-frequency design prototypes involve mixed signals and a complex combination of both analog and digital devices to be placed on the same circuit. Hence careful planning is needed to avoid lengthier trace lines passing to and from in the circuit.
  • This can support proper routing and optimal signal paths, thereby minimizing cross-talk, parasitic interferences and signal issues associated with complex circuits.

Placement of high-frequency components:

  • It is highly crucial in RF circuit board design, unlike low-frequency circuit designs, to consider placing the high-frequency components like amplifiers, high-Q inductors and noise filters to be placed in special positions in the design, as failing to incorporate this, would result in several confusions and performance issues during RF PCB layout. This is one of the important RF PCB design rules to be implemented.
  • The placement of components should take care to minimize the loop around in the circuits during routing. It should also offer equal trace lengths.

 Precautions for schematic design:

  • While the schematic RF PCB design guides are common across PCBs, we should always remember that the best schematics lead to the best layout. Hence it is important to minimize parasitic components to optimize the RF board design.
  • Also, the netlists are automatically generated from the design software. One tip to maintain a clear and accessible design is to keep schematic sheets segregated to specific functionalities, thereby supporting the debugging process and easy error tracking.
  • An advanced simulation tool can be used along with schematic RF board design, before launching the layout design, to test and validate the design. This can provide us with good confidence to carry out the layout and routing tasks.
  • A good schematic can lessen the time to market as the entire layout and manufacturing process depends on the design.

RF PCB Layout Design Guidelines

Layout considerations- A comparison of general PCBs with RF PCBs

  • Also, specific considerations for transmission line placements come into the picture when the frequency of operation increases, to maintain stability and signal integrity.
  • The grounding requirements are unique to RF PCBs, where a separate ground place stacked between the layers is required to minimize EMI and other signal issues.
  • The signal crossing should be avoided by proper routing techniques. Several crosslines can lead to poor routing, and they can damage signal stability, generally paving the way to attenuation and noises.
  • While FR-4 is the general consideration for conventional PCBs, PTFE is considered the most preferred substrate for RF PCBs.
  • Board stacking up is preferred with a top metal layer assigned for chip connection, followed by a dielectric layer of suitable thickness. This layer is followed by a ground plane typically. The last layer, however, can be dedicated to the additional digital and power lines, if any.

RF PCB Stack-up

  • The conventional single point grounds are replaced by a properly dedicated ground plane in case of RF PCB design, to prevent oscillations.
  • The transistors used in the design should be grounded well and through vias.
  • The source-to-ground connections should be wide and not lengthy, to prevent ground inductance.
  • A crowded layout is often a trouble for RF systems. Hence the RF PCB layout is expected to be simple and neat (With fewer crossovers and neat signal paths)

RF PCB layout design

  • Avoiding corners of PCBs for trace lines is another necessity during routing, as it has a significant impact on the performance of a few PCBs under high-frequency category (depending on the range of operating frequency), due to reflection occurring at corners. This is because the corners of PCBs have higher metal deposits hence the distortion. However, it can be simulated to check, and calculate the values of excess capacitance and based on the values, the decision can be taken whether to avoid the corners of PCBs.
Conclusion

Ultimately, high-frequency design considerations can be challenging with too many design constraints, yet not unachievable with suitable RF PCB design guidelines and sound knowledge of RF systems. With rising requirements for RF-based devices in smart homes and IOT systems, the complexity can be solved by developing highly accurate technologies in EDA tools.

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