RF (Radio Frequency) circuits are primarily responsible for transmitting, receiving, and processing wireless signals. Their performance directly determines the communication distance, stability, and anti-interference capability of a product. However, RF circuit design is not merely about the correctness of the schematic; the true determinant of success lies in the layout and routing on the PCB. It can be said that 70% of the skill in excellent RF design is demonstrated in the layout and routing stage. This article will delve into the core requirements and practical principles of RF circuit layout and routing, providing readers with a detailed practical guide.

I. RF Circuit Layout

Layout is the cornerstone of RF PCB design, and its importance cannot be overstated. A poor layout cannot be salvaged even with the most sophisticated routing techniques. The core principles of layout are: "zoning, shortest path, and robust grounding."

1. Strict Zoning and Isolation

Functional Zoning: The PCB must be clearly divided into different functional areas: RF area, high-speed digital area (such as CPU, memory), analog area (such as sensors), and power management area. Each area must be separated by a clear isolation strip (a blank area without any traces or vias) to prevent energy coupling. Ensure that the RF signal path is unidirectional, and avoid cross or parallel traces between input and output loops to prevent self-oscillation. For example, the output of a power amplifier (PA) should be far away from the input of a low-noise amplifier (LNA).

2. RF Main Signal Path: Straight and Short Traces

From the RF transceiver chip (RFIC), to the power amplifier (PA) or low-noise amplifier (LNA), and then to the antenna interface, this path is the "lifeline" of the RF signal. To minimize this path length, no effort must be spared. Every additional 1mm of trace introduces additional losses and uncertainties. All components (such as matching networks and filters) should be placed close to the RFIC pins, arranged in straight lines or smooth curves. Avoid right-angle bends; all bends must use 45-degree angles or rounded edges. 90-degree right-angle traces cause effective changes in trace width, resulting in impedance discontinuities, which in turn cause signal reflection and radiation.

3. Maintain a Stable Ground

Grounding is the "earth" of RF circuits, the path for all current return. A poor grounding system is the root cause of the vast majority of RF problems. There must be a complete, unsegmented ground plane (usually located on an adjacent layer) directly beneath the RF area. This ground plane provides a stable reference ground for RF signals and acts as natural shielding. RF signal lines are absolutely prohibited from crossing any cracks or slots in the ground plane. Otherwise, the signal return path is forced to take a long detour, forming a huge loop antenna, leading to serious radiation and electromagnetic interference (EMI) problems.

4. Power Supply Decoupling

RF chips are both energy consumers and significant noise generators. The purpose of a power supply decoupling network is to handle instantaneous high current demands and prevent noise contamination of the power network. A strategy of parallel connection of large, medium, and small capacitors is required. Typically, a large capacitor handles low-frequency fluctuations, a ceramic capacitor handles intermediate frequencies, and a small high-frequency capacitor handles RF noise. These capacitors must be placed as close as possible to the chip's power supply pins. The capacitor ground terminals should be directly connected to the main ground plane through multiple vias to minimize return inductance.

II. RF Circuit Routing

The core of RF routing is "impedance control, parasitic reduction, and effective shielding."

1. Impedance Control

In the RF field, we deal with electromagnetic waves, not simple currents. Ensuring a constant characteristic impedance along the signal path is paramount for signal integrity. The characteristic impedance standard for most RF systems is 50 ohms. PCB impedance calculation tools (such as SI9000) must be used to accurately calculate the required trace width based on the layer stack-up (dielectric thickness, dielectric constant, copper thickness).

RF traces must be controlled impedance transmission lines. The two most common types are: 1. Microstrip lines: Located on the outer layer with only one reference ground plane. Simple to fabricate, but susceptible to external interference. 2. Striplines: Sandwiched between two ground planes. Good shielding, but slightly higher loss than microstrip lines, suitable for complex high-density boards. RF traces must maintain consistent trace width, and the "teardrop" reinforcement commonly used by autorouters is strictly prohibited. Minimize the use of vias, as they introduce parasitic inductance and capacitance, causing impedance abrupt changes and signal reflections.

2. Via Usage

RF link vias are necessary but must be used with caution. Vias should be avoided as much as possible on critical RF signal paths. If unavoidable, back-drilling (depth control) techniques should be used to remove residual studs at the via ends, or a dedicated RF via model should be used for simulation optimization. Grounding vias should be used extensively and densely on ground pads, shielding mounts, and ground plane edges; this is called "via stitching." The purpose is to provide the shortest, lowest impedance return path for ground current and to prevent the ground plane from resonating like an antenna. Via spacing should typically be less than 1/20 of the highest frequency wavelength.

3. Shielding and Isolation

When space and isolation requirements are insufficient, physical shielding is necessary. Encapsulating sensitive RF components (such as LNAs) or strong radiation sources (such as PAs and VCOs) with a metal shield is the most effective way to suppress radiated and received interference. The shield itself must be well grounded through dense vias around its perimeter. For parallel lines prone to crosstalk (such as RF lines and digital control lines), a grounding wire can be laid next to them, and a grounded copper layer can be laid on top with vias to form a "quasi-coaxial" shielding structure, effectively suppressing crosstalk.

III. Practical Checklist

After completing the layout and routing, please review the following checklist:

1. Layout: Is the RF path the shortest and straightest? Are the functional zones clearly defined? Are decoupling capacitors close to the chip power pins?

2. Impedance: Have all RF traces undergone 50-ohm impedance calculations and simulations? Are the trace widths consistent?

3. Vias: Are vias avoided in the main RF path? Are there sufficient grounding vias?

4. Grounding: Is there a complete ground plane below the RF area? Do signal lines cross ground plane divisions?

5. Isolation: Are the RF input/output ports adequately isolated? Has the installation location of the shielding cover been considered?

RF circuit layout and routing is a rigorous engineering science, requiring RF engineers to have a deep understanding of electromagnetic fields and transmission line theory. It is also an art of trade-offs, requiring optimal choices between performance, cost, and manufacturing processes. By adhering to the aforementioned core principles, combining them with the datasheet recommendations for specific components, and conducting preliminary verification using modern electromagnetic simulation software (such as ADS and HFSS), one can navigate the complexities of the high-frequency world and design stable, efficient RF products. Remember, in the RF field, "the devil is in the details," and success lies in the relentless pursuit of perfection in every minute aspect.