How is impedance matched across a coax-to-CPW transition?

Both coaxial cable and CPW support 50-ohm characteristic impedance, so the fundamental matching is geometric: the coax center conductor diameter and dielectric determine its impedance, while the CPW center strip width and gap spacing determine its impedance. The transition region where the coax geometry transforms into the CPW geometry creates a parasitic reactance (typically inductive from the current path discontinuity plus capacitive from the exposed dielectric). This reactance causes reflections that limit bandwidth. Compensation techniques include stepped impedance sections (a short section of higher or lower impedance to cancel the reactive discontinuity), chamfered or tapered ground plane connections (gradually transitioning the coax outer conductor to the CPW ground planes), and via fences around the transition (suppressing the parallel-plate mode between the substrate and a ground plane below). Full-wave EM simulation (HFSS, CST) is used to optimize the transition geometry, typically achieving return loss below -20 dB over 3:1 or wider bandwidth and below -15 dB over 10:1 bandwidth.

What is the difference between CPW and GCPW transitions?

CPW (coplanar waveguide) has a center strip with ground planes on both sides, all on the same surface of the substrate, with no ground plane on the back side. GCPW (grounded coplanar waveguide, also called conductor-backed CPW or CBCPW) adds a ground plane on the back side of the substrate, connected to the top ground planes through via holes. For coax-to-CPW transitions, GCPW is more common in PCB applications because the back-side ground provides a return path for the coax outer conductor, enables heat sinking, and improves mechanical mounting. However, GCPW introduces a parasitic parallel-plate mode between the top metallization and the back-side ground plane, which can propagate above the cutoff frequency determined by the substrate thickness. Via fences (rows of plated through-holes connecting top and bottom grounds) suppress this mode by creating an effective waveguide below cutoff for the parasitic mode. Via spacing must be less than lambda/4 at the maximum operating frequency. For a 10 mil (254 micrometer) substrate at 60 GHz, via spacing must be less than 1.25 mm.

How are coax-to-CPW transitions used for on-wafer probing?

On-wafer probing is the primary application of precision coax-to-CPW transitions. RF wafer probes (manufactured by FormFactor, MPI, and others) convert the coaxial connector interface (typically 1.0 mm for DC to 110 GHz or 0.8 mm for DC to 145 GHz) to CPW probe tips with ground-signal-ground (GSG) pitch of 50 to 250 micrometers. The transition occurs within the probe body: the coax center conductor connects to a precisely machined or thin-film transmission line that tapers to the probe tip dimensions. The probe tips make contact with the device under test's CPW pads at controlled contact force (typically 5 to 20 grams per tip). Calibration standards (SOLT, TRL, or multiline TRL) on an impedance standard substrate (ISS) de-embed the probe transition's parasitic effects, establishing the measurement reference plane at the probe tip. This enables S-parameter measurement of on-wafer devices (transistors, MMICs, passive components) with accuracies of plus or minus 0.1 dB insertion loss and plus or minus 1 degree phase to 110 GHz.

Transmission Lines

Coax-to-CPW Transition

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A coax-to-CPW transition converts coaxial TEM mode to coplanar waveguide quasi-TEM mode. Center conductor connects to CPW center strip; outer conductor to ground planes. End-launch designs achieve return loss < -20 dB to 67 GHz (1.85 mm connectors) or 110 GHz (1.0 mm). Critical for on-wafer probing with GSG pitches of 50 to 250 μm, enabling S-parameter accuracy of ±0.1 dB to 110 GHz.

Category: Transmission Lines

Return loss: < -20 dB

Bandwidth: DC to 110+ GHz

Understanding Coax-to-CPW Transitions

Coplanar waveguide is the dominant transmission line for MMICs and high-frequency test fixtures because all conductors (signal and ground) are on the same surface, eliminating the need for via holes to a back-side ground plane and enabling direct probing of circuits. However, test equipment (network analyzers, signal generators, spectrum analyzers) use coaxial cables with standardized connectors (SMA, 2.92 mm, 2.4 mm, 1.85 mm, 1.0 mm). The coax-to-CPW transition bridges this interface, converting the rotationally symmetric TEM mode of coax into the planar quasi-TEM mode of CPW while maintaining impedance continuity.

The electromagnetic challenge is managing the mode conversion in a region where the field distribution changes dramatically. In coax, the electric field is radial between center and outer conductors, uniformly distributed around the circumference. In CPW, the electric field spans the gaps between the center strip and ground planes, concentrated at the strip edges. The transition region must smoothly transform one field distribution into the other without exciting higher-order modes (which cause resonances and radiation) or creating abrupt impedance discontinuities (which cause reflections). The design space includes the connector body geometry, the PCB pad layout, the ground via placement, and any matching structures (stubs, tapers, or stepped impedance sections).

Coax-to-CPW Transition Design

CPW Impedance:
Z₀ = (30π/√ε_eff) × K(k')/K(k)

Where:
k = W/(W + 2G) ; k' = √(1-k²)

Via Fence Spacing (GCPW):
p < λ_eff/4 = c/(4f_max√ε_r)

W = center strip width, G = gap to ground, K = complete elliptic integral of the first kind. For 50 Ω on 10 mil alumina (ε_r=9.6): W = 100 μm, G = 60 μm. Via fence at 60 GHz on ε_r=3.5: p < 670 μm.

Coax-to-CPW Transition Types

Type	Orientation	Bandwidth	Return Loss	Application
End-launch (SMA)	Horizontal	DC to 18 GHz	< -20 dB	PCB test fixtures
End-launch (2.92 mm)	Horizontal	DC to 40 GHz	< -20 dB	Module testing
End-launch (1.85 mm)	Horizontal	DC to 67 GHz	< -18 dB	V-band modules
End-launch (1.0 mm)	Horizontal	DC to 110 GHz	< -15 dB	W-band, mmWave
GSG probe	Vertical (angled)	DC to 145 GHz	< -15 dB	On-wafer MMIC test

Common Questions

Frequently Asked Questions

How is impedance matched across the transition?

Both coax and CPW support 50 Ω; the challenge is the transition region's parasitic reactance (inductive from current path change + capacitive from exposed dielectric). Compensation: stepped impedance sections, chamfered ground connections, via fences. EM simulation (HFSS/CST) optimizes geometry for < -20 dB return loss over 3:1+ bandwidth.

What is the CPW vs GCPW difference?

CPW has no back-side ground. GCPW adds back-side ground connected by vias, enabling heat sinking and mechanical mounting but introducing parasitic parallel-plate mode. Via fences (< λ/4 spacing) suppress this mode. For 10 mil substrate at 60 GHz: via spacing < 1.25 mm.

How are transitions used for on-wafer probing?

RF probes convert coax (1.0 mm or 0.8 mm) to GSG tips at 50 to 250 μm pitch. Transition occurs in probe body via precision thin-film line. Contact force: 5 to 20 g/tip. Calibration (SOLT/TRL/mTRL) on ISS de-embeds probe parasitics, achieving ±0.1 dB and ±1° accuracy to 110 GHz.

Related Terms

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