How does CPW compare to microstrip?

CPW has several advantages over microstrip at RF and millimeter-wave frequencies. No vias required: ground is on the same surface as the signal conductor, so shunt components (capacitors, inductors, varactors) can be connected directly to ground without through-substrate vias. In microstrip, every grounded component requires a via, which adds parasitic inductance (0.1 to 0.5 nH) and complicates fabrication. On-wafer probing: CPW's ground-signal-ground (GSG) configuration matches standard RF probe tips directly, making it the standard for on-wafer MMIC measurements. Microstrip requires special probe transitions. Less dispersion: CPW's effective permittivity is less frequency-dependent than microstrip, providing better wideband performance. The quasi-TEM mode has a more uniform field distribution. Substrate thickness independence: CPW impedance is determined primarily by the center conductor width and gap, not the substrate thickness. This makes it easier to design on thin GaAs or InP substrates used in MMICs. Disadvantages: CPW lines require wider ground planes and air bridges to suppress the unwanted slotline mode. The two-ground geometry can support both the desired CPW mode and the parasitic slotline mode (odd mode), requiring periodic air bridges or bond wires to equalize ground potentials.

What determines CPW impedance?

CPW impedance is determined by the ratio of the center conductor width (W) to the total width (W + 2G), where G is the gap between the center conductor and each ground plane. The key parameter is k = W / (W + 2G), and the impedance is: Z0 = (30 pi / sqrt(epsilon_eff)) times K'(k) / K(k), where K(k) is the complete elliptic integral of the first kind and K'(k) = K(sqrt(1 minus k squared)). For CPW on an infinitely thick substrate: epsilon_eff = (1 + epsilon_r) / 2. Practical ranges: W = 10 to 200 micrometers, G = 5 to 100 micrometers on GaAs (epsilon_r = 12.9) or alumina (epsilon_r = 9.8). Impedance range: 20 to 120 ohms achievable by varying W/G ratio. 50 ohms on GaAs: W approximately equals 70 micrometers, G approximately equals 45 micrometers. For CBCPW (conductor-backed CPW), the bottom ground plane also influences impedance and must be accounted for. The bottom ground lowers the impedance relative to standard CPW with the same W and G.

What is the slotline mode problem?

CPW supports two fundamental modes: the desired CPW mode (even mode, both ground planes at the same potential) and the parasitic slotline mode (odd mode, ground planes at opposite potentials). The CPW mode has the electric field primarily in the gaps, symmetric about the center conductor. This is the mode used for signal transmission. The slotline mode has the electric field across the full width between the two ground planes, with the center conductor acting as a floating element. This mode is unwanted because it has different impedance, velocity, and radiation characteristics. Any asymmetry in the circuit (bends, T-junctions, discontinuities) can excite the slotline mode, causing resonances, radiation, and signal degradation. Suppression methods: air bridges connecting the two ground planes every quarter to half wavelength. These are short metal strips that bridge over the center conductor, physically connecting the two ground planes. They enforce equal ground potential, killing the slotline mode. In PCB CPW: vias connecting both ground planes to a common ground. In MMIC: deposited air bridges are a standard process step. Without air bridges, CPW circuits exhibit mysterious resonances and loss at frequencies where the slotline mode becomes a half-wavelength resonant.

Transmission Line

CPW

/see-pee-double-you/ — Coplanar Waveguide

Center conductor + coplanar grounds (same surface). Z₀ = f(W,G,ε_r). 20-120Ω achievable. ε_eff = (1+ε_r)/2. No vias for shunt components. GSG probing standard. Quasi-TEM, low dispersion. Air bridges every λ/4 suppress slotline mode. CBCPW: bottom ground added. Standard for GaAs/InP MMICs and on-wafer test.

Z₀: 20-120Ω

Probing: GSG

Mode: quasi-TEM

Understanding CPW

CPW is the transmission line of choice for MMIC (Monolithic Microwave Integrated Circuit) design and on-wafer testing. Its ground-signal-ground topology eliminates the need for substrate vias, which are difficult and expensive to fabricate on thin III-V semiconductor wafers (GaAs, InP). The GSG configuration also matches standard RF probe tips, making CPW the natural choice for wafer-level characterization.

At PCB level, CPW is used in high-speed digital and mmWave designs where via inductance would compromise signal integrity. The key design challenge is suppressing the parasitic slotline mode with periodic air bridges or ground-equalization vias.

CPW Equations

CPW impedance (infinite ground):
Z₀ = 30πK(k′)/(K(k)√ε_eff)
k = w/(w+2s), k′ = √(1−k²)
K = complete elliptic integral

Effective permittivity:
ε_eff = 1+(ε_r−1)K(k′)K(k₁)/(2K(k)K(k₁′))

Planar Transmission Line Comparison

Property	CPW	Microstrip	Advantage	Notes
Ground access	Top surface	Bottom only	CPW	Easy shunt
Via need	None	Required	CPW	Lower cost
Dispersion	Lower	Higher	CPW	Better at mmW
Radiation	Lower	Higher	CPW	With ground
Probe test	Excellent	Difficult	CPW	GSSG pads

Common Questions

Frequently Asked Questions

CPW vs microstrip?

No vias: shunt components ground directly (no via inductance). GSG probing: standard for on-wafer. Less dispersion: more uniform ε_eff vs freq. Substrate-independent Z₀. Disadvantages: wider layout (ground planes), air bridges needed to suppress slotline mode, two-ground asymmetry issues at discontinuities.

Impedance?

Z₀ set by W/(W+2G) ratio and ε_r. Elliptic integrals K(k)/K'(k). 50Ω on GaAs: W=70μm, G=45μm. Wider W or narrower G: lower Z₀. CBCPW: bottom ground lowers Z₀ vs standard CPW. Range 20-120Ω practical. ε_eff = (1+ε_r)/2 for thick substrate.

Slotline mode?

CPW supports CPW mode (even, desired) + slotline mode (odd, parasitic). Any asymmetry excites slotline: bends, T-junctions. Causes resonances, radiation, signal loss. Fix: air bridges every λ/4 connecting grounds. MMIC: deposited air bridges. PCB: ground vias. Without suppression: mysterious resonances appear.

Related Terms

← Coupler Cross-Mod →

MMIC Design

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