In the previous article, we discussed PDN design and plane resonance. In this article, which is FC36, we will cover reference planes. We will now directly and comprehensively organize the concept of the "reference plane," which has been repeatedly mentioned in FC25 (Return Current), FC31 (Stack-up), and FC34 (Crosstalk).
What is a Reference Plane?
A reference plane is a conductor plane that serves as the potential reference for signal traces. In most cases, this refers to a GND plane, but as explained in FC25, a properly decoupled power plane can also function as a reference plane.
As the term "reference" implies, the essence of this concept is its dual role: it is both a reference point for measuring signal voltage and a path for return current. The phenomena we have covered individually in previous articles—return current (FC25), crosstalk (FC34), plane resonance (FC35), and characteristic impedance (detailed in FC39)—all arise from their relationship with this reference plane.
Why Must It Be a "Plane"?
We will physically organize the reasons why the reference must be a plane (surface) rather than a single wire (trace).
Ensuring a low-impedance path is the greatest advantage of a plane. While a trace (a linear conductor) restricts current to a specific path, a plane (a planar conductor) allows return current to flow along the shortest distance anywhere directly beneath the signal trace. The plane most naturally satisfies the physical law explained in FC25: "Return current chooses the path of least inductance."
Providing a uniform impedance environment is another advantage of a plane. As long as the distance between the signal trace and the plane (dielectric thickness) remains constant, the characteristic impedance will remain nearly constant regardless of where the signal trace runs on the plane. This ensures that signal quality remains within a predictable range even if the trace routing is changed.
Independence between multiple signals is also important. By having the plane function as a common reference, the mutual influence between individual signal traces (crosstalk) can be controlled using the relatively simple method of managing trace spacing (see FC34).
The Relationship Between Three Transmission Line Structures and the Reference Plane
Signal traces on a PCB are classified into different transmission line structures based on their positional relationship with the reference plane. This will be covered in detail in FC38 (Transmission Lines), but in this article, we will summarize the overview from the perspective of the reference plane.
A Microstrip Line is a structure where the signal trace is on the outermost layer of the board (top or bottom) and has a single reference plane directly beneath it. The stack-up from FC31 (Layer 1 signal, Layer 2 GND) falls into this category. Because the environment is asymmetric—with air (or a board protective film) above the trace and a dielectric below it—a portion of the propagating electromagnetic wave exists in the air (this is why radiation from a microstrip line is slightly higher than that of a stripline).
A Stripline is a structure where the signal trace is on an inner layer and has reference planes on both sides (above and below). In a 4-layer configuration like Pattern B from FC31 (Signal-GND-GND-Signal), creating a stricter stripline structure requires 6 or more layers (because GND is needed on both sides of the signal layer). Since the electromagnetic wave is completely confined within the dielectric, radiation is lower and crosstalk is reduced.
A Coplanar Waveguide is a structure where a GND trace is placed on the same plane as the signal trace, and in addition to the reference plane, the surface GND trace also functions as a potential reference. It is sometimes used in high-frequency RF circuits (such as GNSS RF front-ends).
In a 4-layer board, which is the standard for FC design, it is common for Layer 1 and Layer 4 to be microstrip lines (with Layers 2/3 serving as their respective references), and if inner layer routing is required, a structure close to a stripline (though often with single-sided referencing) is used.
Reference Plane Discontinuities: Integrated Effects
We will organize how the "GND plane splits," "layer changes," and "power plane boundaries," which have been explained individually in previous articles, are related under the unified concept of the reference plane.
Regarding the impact on signal quality, if there is a break in the reference plane, the characteristic impedance changes abruptly at that location (impedance discontinuity). This causes signal reflections, which manifest as ringing or overshoot in digital signals, and distortion of frequency characteristics in analog signals.
Regarding the impact on EMI, as explained in detail in FC25, when return current bypasses a break, the loop area increases, which worsens radiated EMI.
Regarding the impact on crosstalk, in areas where the reference plane is discontinuous, the coupling path between adjacent traces changes from one mediated by the plane to a more direct trace-to-trace coupling (electromagnetic field coupling through space), which can result in unpredictable crosstalk patterns.
The understanding that these three effects (signal reflection, EMI, and crosstalk) arise simultaneously from a single physical cause—a break in the reference plane—forms the foundation for prioritization in PCB design. Many of the issues emphasized in FC30 that 'cannot be detected by DRC' are consolidated into this single principle of 'reference plane continuity'.
The 'Switching' Problem of Reference Planes
When a signal moves from one layer to another, the reference plane also switches. We will reorganize the via issues covered in FC25 and FC32 from the perspective of the reference plane.
Layer changes with the same reference (GND to GND) are the simplest case. When moving from Layer 1 (GND reference) to Layer 4 (another GND, provided it is internally connected to the Layer 2 GND), the return current can smoothly transition to the new reference if both GND planes are connected with low impedance (sufficient via stitching).
Layer changes with different references (GND to Power) are the 'return via power plane' cases explained in FC25. Decoupling capacitors function as the switching point for the reference, and their placement and capacitance determine the efficiency of the switch.
Layer changes without a reference plane are the most problematic cases. If a signal passes through a layer without a clear reference plane directly beneath it (e.g., in complex stack-ups of 6 or more layers where signal layers are adjacent), the return current path becomes unclear, which can lead to unpredictable behavior. In FC design, stack-up designs (FC31) that fundamentally avoid this situation are recommended.
Prioritization for Selecting Reference Planes
When there are multiple candidates (GND, multiple power voltages), we will organize the criteria for deciding which one to choose as the reference plane.
The GND plane is always the top priority. GND is typically the single potential reference for the entire system and functions as a direct, low-impedance return path without needing decoupling. As a principle, high-speed and high-sensitivity signals should be designed to be adjacent to a GND plane whenever possible.
Power planes are the next best choice. As explained in FC23 and FC35, power planes can function equivalently to GND at high frequencies if proper decoupling is present. However, if the placement density and capacitance of the decoupling capacitors are insufficient, performance as a reference, especially in the low-frequency range, will be inferior.
Layers where multiple different power voltage planes are intertwined should be avoided. If a signal passes through such a layer, it is highly likely to cross multiple voltage boundaries, making the design to ensure decoupling at each boundary complex. Planning for signal layers to be adjacent to a single, clear reference plane during the FC31 stack-up design phase prevents problems in later stages.
Reference Plane Design for Sensor Signals
We will organize the practical application of reference plane design around sensors (IMU, GNSS, compass, barometer), which are the core of FC design.
IMU SPI signals (FC6–FC8) are high-speed and noise-sensitive. Maintaining a single, continuous GND plane directly beneath the entire SPI bus routing is the top priority. Avoid the aforementioned 'layer changes without a reference plane,' and if a layer change is unavoidable, ensure that the GND stitching vias from FC32 are placed.
GNSS antenna feed lines (see previous series) require strict management of characteristic impedance (usually 50Ω), so it is important that the distance to the reference plane (dielectric thickness) remains constant throughout the routing. Carefully plan the routing path so that the feed line does not pass near breaks in the GND plane or vias.
Barometer signal lines (FC10) are relatively low-speed, but because absolute accuracy is important, stability of the reference plane (an independent area not subject to interference from other high-speed signals) is required.
3D Visualization Review of Reference Planes
We will organize the 'visual confirmation in 3D view' emphasized in FC30 into concrete practical steps from the perspective of the reference plane.
Individual layer display is a basic step in tools like KiCad to display the GND plane layer and power plane layer separately to check for unintended splits or breaks.
In the overlay display of signal layers and reference layers, display the routing layer for high-speed/high-sensitivity signals and the reference plane layer directly beneath it together, and visually trace whether the reference plane is continuous throughout the entire routing path.
In the cross-check with via locations, compare the positions of layer transitions (vias) against the reference plane structure to verify that the GND stitching vias explained in FC32 and FC33 are appropriately placed.
This series of review processes is a core verification task in the PCB Design series, integrating and applying the knowledge of power design, ground design, return current, EMI countermeasures, and PDN design explained from FC21 through FC36.
Summary
The reference plane serves as both the potential reference for signals and the path for return current; this dual role forms the foundation of transmission line structures such as microstrip lines and striplines. Breaks in the reference plane are a common cause that simultaneously triggers three problems: signal reflection, EMI, and crosstalk, which is the key to a unified understanding of the individual phenomena explained in FC21 through FC35. The priority of using the GND plane as the primary reference and supplementing it with sufficient decoupling when using power planes is the fundamental policy for the entire FC design, including sensor signals. In the next installment, FC37, we will explain S-parameters—a method for describing high-frequency circuit characteristics in the frequency domain and how to read them practically.