An RF PCB design can be quite different from a traditional board. It is distinguished by parameters such as impedance matching, type traces (preferably coplanar), elimination via (to avoid reflecting), ground planes and vias, and power supply decoupling. These boards also have important aspects like stack-up and material selection.

These factors make RF design more complex due to elements such as EMI interference and high-frequency signal channeling. 911EDA PCB design services team are experts in RF PCB design. We will be discussing all of these issues in detail in this article. Let us start with impedance matching.

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A controlled-impedance radio circuit allows maximum power transfer from source to load without distortion when the impedance is constant throughout the trace. This is called the characteristic impedance for the trace (Z ). The geometry of the trace determines the characteristic impedance. This includes trace width, the dielectric constant of PCB material, and trace thickness. Also, the height from the reference plane. PCB designers can match these impedances by designing matching circuits.

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Certain materials are used to make RF PCBs that meet high-frequency operation requirements. These materials must have low signal losses and be stable in high-frequency operation. They also need to be capable of absorbing high amounts of heat. Consistency across wide frequency ranges is required for the dielectric constant (DK), loss tangent (tan-d), and coefficient of thermal expansion(CTE). These boards have 3 and HTML3.5 as the typical values for the dielectric constant. For the frequency range 10-30GHz, the loss tangent values range from 0.0022 - 0.0095.

These are just some of the requirements. Also, consider the cost and ease of manufacturing.

Materials made of Polytetrafluoroethylene (PTFE), ceramics, and hydrocarbons mixed or with a form of glass are commonly used. Rogers material is a popular choice for RF circuit board design. Rogers material comes in wide different varieties. Below are a few examples:

  • RT/duroid
  • RO3000
  • RO4000
  • Rogers TMM

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Radio frequency board stacking requires careful attention to details like isolation between components and traces, power supply decoupling, the number of layers, arrangement, and placement. The figure below shows a standard 4-layer RF stack-up.

a standard four layer RF pcb stackup

The top layer is where the radio-frequency components and trace are placed. The top layer is followed immediately by a ground and power plane. The bottom layer contains all non-RF parts and trace information. This arrangement minimizes interference between RF and non-RF components. The ground plane is the shortest path for ground return current. This stack-up is ideal for small radio-frequency boards.

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High-frequency signals propagate RF traces and are therefore susceptible to interference and transmission losses. The traces are transmission lines in radio-frequency boards. Coplanar waveguide, microstrip, and stripline are the most commonly used transmission lines. Designers are concerned about the characteristic impedance of these lines. Here are some radio frequency trace design considerations that ensure proper operation and minimal losses.

  • It is crucial to keep the trace as short as possible. This reduces attenuation.
  • Never place RF and standard traces parallel in a layout. Interference will result if the two are placed in this manner.
  • Ground planes are needed to provide signal return routes.
  • It is not recommended that test points be placed on the trace. It will disrupt the trace's impedance matching value.
  • For trace performance, it is better to gradually curve bends than to keep sharp right turns.

a graphic showing bad, good, and best trace angles

When right-hand bends are impossible, PCB designers can use the metering process to reduce their effects. Below is an illustration of how to measure a trace.

how to measure a trace for metering

The following formula gives M:

formula to determine M in trace metering

Design ground planes

A return path is required for any radio-frequency trace or component or to allow current propagating through it. This is done by a ground plane. Ground planes require additional design considerations. Let us take a look at them.

  • Each RF layer should have a dedicated ground plane. To make the current flow path as short as possible, PCB designers should place the ground plane directly below each layer.
  • The ground plane must be continuous. Breaks are not allowed. These breaks could open the way to shorter routes for the current to return.
  • Two grounding vias must be installed for every shunt component placed in an RF transmission cable.

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Avoiding vias in RF trace should be a top priority. However, if PCB designers cannot avoid these, you must follow specific via dimensions and lengths. Parasitic capacitance is caused by a via in a circuit board. This capacitance can affect high-frequency operation in the case of radio frequency boards. To reduce interference at these frequencies, vias must be designed with the following guidelines in mind:

  • To decrease the parasitic capacitance by introducing more parallel vias
  • A dedicated via must be attached to each pin or pad of a component.
  • Use ground plane stitching whenever possible. This creates a shorter ground return route for the current.
  • Vias reduce the routing of RF traces from one layer to the next.
  • The design allows you to use as many vias between the inner and top layer ground planes as possible. These vias must be at least 1/20th of signal wavelength.

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Radio-frequency boards require noise reduction to be effective. These boards are susceptible to noise at high frequencies. Noise removal is, therefore, a complex task. One of these methods is power supply decoupling.

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This filter removes any noise introduced to the circuit by the power supply. These capacitors are known as decoupling capacitors. These capacitors are connected to the power supply.

Impedance matching should be an integral part of every RF circuit board. The impedance of the circuit should remain constant after connecting decoupling capacitors. To avoid impedance changes, follow the following design considerations:

  1. For decoupling, always connect capacitors with the minimum impedance
  2. To achieve minimum impedance, operate the capacitors at the self-resonant frequency (SRF). The SRF value of a capacitor will be inversely proportional to its capacitance.
  3. Look for capacitors with a close SRF to the noise frequency.

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It is vital to place decoupling capacitors correctly. Below is a circuit that shows two decoupling capacitors being placed in parallel with an IC.

a pcb schematic showing decoupling capacitor placement

The higher capacitor is used to filter low-frequency noise and store energy. The lower capacitor filters out the high-frequency noise. These are other placement guidelines:

  • The components and the decoupling capacitors should be placed on the same surface.
  • Place the capacitors parallel with the signal flow path.
  • Each capacitor should have its own ground via.
  • Place the capacitors in ascending order according to their capacitance. The capacitor with the lowest capacitance is the closest to the power supply.

Both manufacturers and designers need to pay more attention to RF boards' design and fabrication process. The DFM team should follow the design checklist. These boards are susceptible to interference and high-frequency noises, so even the slightest mistake can significantly impact the operation. These aspects and other methods will assist us in improving our design.