Why use CPW instead of Microstrip?

Microstrip requires a ground plane on the bottom layer of the PCB. If you want to connect a surface-mount component to ground, you must drill a via completely through the board. At microwave frequencies (above 20 GHz), the physical length of that via acts like an inductor, ruining the impedance match and causing instability. Because CPW has ground planes right next to the signal trace on the top layer, surface-mount components can be soldered directly to ground with zero vias, eliminating via inductance entirely.

How is the impedance of a CPW controlled?

In Microstrip, impedance is heavily dependent on the thickness of the PCB substrate. If the manufacturer uses a slightly thicker core, the impedance changes. In CPW, the electromagnetic fields are tightly concentrated in the gap between the center conductor and the top ground planes. Therefore, the impedance is almost entirely dictated by the ratio of the center trace width (W) to the gap width (G). This makes CPW highly insensitive to substrate thickness variations, which is critical for mass production.

What are the disadvantages of CPW?

It consumes significantly more surface area than Microstrip because you must route massive ground planes on the top layer alongside the signal. Furthermore, if the two top ground planes ever experience a slight voltage difference, they will excite an unwanted 'slotline mode,' which causes massive radiation and signal loss. To prevent this, designers must stitch the two ground planes together with air-bridges (in ICs) or via-fences (in PCBs) to force them to the exact same electrical potential.

Simulation & Design

Coplanar Waveguide (CPW)

An engineer is designing an active RF amplifier for a 40 GHz millimeter-wave system. They attempt to route the signal using standard Microstrip. However, to ground the emitter of the transistor, they have to use a plated via hole to reach the bottom layer. At 40 GHz, the physical length of that via hole acts as a massive parasitic inductor, causing the amplifier to violently oscillate. They switch the design to a Coplanar Waveguide (CPW). In CPW, the ground plane is brought up to the exact same surface layer as the signal trace, hugging it tightly on both sides across a microscopic gap. Now, the transistor's emitter can be soldered directly to the adjacent surface ground without needing a via. The parasitic via inductance is eliminated, stability is restored, and the electromagnetic fields are tightly confined in the surface gap, minimizing dispersion at extreme frequencies.

Category: Simulation & Design

Geometry: Ground - Gap - Signal - Gap - Ground (All Top Layer)

Primary Advantage: Zero via inductance for surface-mount components

Transmission Line Comparison

Feature	Microstrip	Stripline	Coplanar Waveguide (CPW)
Ground Location	Bottom layer only	Top and Bottom (Sandwiched)	Top layer (Adjacent to signal)
Component Grounding	Requires inductive vias	Requires inductive vias	Direct surface solder (No vias)
Dispersion at mmWave	High (Fields spread into air)	Zero (Pure TEM mode)	Low (Fields confined in gap)
Sensitivity to Board Thickness	Extreme	Moderate	Very Low

CPW Impedance Control:
Unlike Microstrip, which relies on the substrate height (h), a pure CPW's impedance is governed by the ratio of the trace width (W) to the total gap width (W + 2G).
If a manufacturer changes the core thickness of the PCB from 20 mil to 30 mil, a Microstrip trace will instantly jump from 50 ohms to 65 ohms, ruining the circuit. A CPW will remain perfectly at 50 ohms because its fields exist almost entirely in the horizontal surface gap (G), not the vertical substrate (h).

The Effective Dielectric Constant (ε_eff):
For a standard CPW without a bottom ground, exactly half the electric field travels through the air, and half travels through the substrate.
ε_eff ≈ (ε_r + 1) / 2

Common Questions

Frequently Asked Questions

What is the 'Slotline Mode' problem?

CPW has two separate ground planes on the top layer. For the transmission line to work properly (in the 'even mode'), those two ground planes must be at the exact same zero-volt potential. If an asymmetrical bend or a nearby component causes one ground plane to experience a slightly different voltage or phase than the other, it excites an odd mode called the 'Slotline Mode.' This parasitic mode radiates energy into space, destroying the signal.

How do you prevent the Slotline Mode?

You must forcefully tie the two ground planes together electrically. In monolithic microwave integrated circuits (MMICs), designers use microscopic 'air bridges'—tiny loops of gold wire that arch over the center signal trace to connect the left ground to the right ground. On printed circuit boards, designers use via fences, dropping vias from the top grounds down to a solid internal ground plane to ensure equal potential.

Can you make the gap as small as you want?

Theoretically yes, making the gap smaller confines the fields tighter and improves high-frequency performance. Practically, you are limited by PCB manufacturing tolerances. Most standard board houses cannot accurately etch a gap smaller than 4 mils (0.1 mm). If you specify a 3-mil gap and the etching acid over-etches by 1 mil, your impedance will shift drastically. Extreme CPW designs require high-precision semiconductor lithography.

Related Terms

← Conjugate Match CPWG →

PCB Design

CPW Impedance Calculator

Input your substrate dielectric constant and operating frequency. Adjust the trace width (W) and gap width (G) to find the perfect 50-ohm geometry that fits within your manufacturer's minimum etching tolerances.

Calculate CPW Dimensions