Coax Launch Calibration Fixture

Key Takeaways

Definition: A coax launch calibration fixture is a specialized hardware interface used to characterize and remove errors introduced when a signal transitions from a coaxial cable to a planar PCB transmission line.
Criticality: Without proper calibration fixtures, the losses and reflections from the connector launch mask the true performance of the Device Under Test (DUT).
Metrics: The most vital metrics are Return Loss (VSWR), Insertion Loss, and Phase Stability across the target frequency band.
Calibration Methods: TRL (Thru-Reflect-Line) is the gold standard for high-frequency launch characterization, while SOLT (Short-Open-Load-Thru) is common for lower frequencies.
Manufacturing: Precision in etching, plating, and connector soldering is non-negotiable; even a 0.1mm deviation can ruin performance at mmWave frequencies.
Validation: Time Domain Reflectometry (TDR) is essential to visualize impedance discontinuities at the launch point.
Advanced Applications: Quantum computing requires specialized approaches like a cryogenic compatible SMT process to ensure the fixture survives near-zero Kelvin temperatures.

What coax launch calibration fixture really means (scope & boundaries)

To understand why a coax launch calibration fixture is necessary, we must first address the physics of signal transitions. In the world of RF and high-speed digital design, signals travel through coaxial cables in a TEM (Transverse Electro-Magnetic) mode. However, once that signal reaches a Printed Circuit Board (PCB), it must transition into a planar mode, such as microstrip, stripline, or Coplanar Waveguide (CPW).

This physical transition point—the "launch"—is a major source of impedance discontinuity. If the launch is not perfectly matched, energy reflects back to the source. This reflection creates noise, reduces signal power, and corrupts data.

A coax launch calibration fixture serves two primary purposes. First, it acts as a physical test vehicle to validate the design of the launch itself. Engineers design a specific footprint, build the fixture, and measure it to ensure the transition is smooth. Second, it serves as a "de-embedding" tool. By measuring known standards (like a Thru line or a Reflect short) built onto the fixture, a Vector Network Analyzer (VNA) can mathematically subtract the effects of the connector and the launch. This leaves only the data for the actual circuit you want to test.

At APTPCB (APTPCB PCB Factory), we see this as the bridge between simulation and reality. A perfect simulation means nothing if the physical launch introduces a -10dB return loss at your operating frequency. The fixture is the reality check. It defines the boundary between the measurement equipment and the device being measured.

Metrics that matter (how to evaluate quality)

Having defined the scope of the fixture, we must now quantify what makes a "good" launch design using specific, measurable data points.

A high-performance coax launch calibration fixture is defined by its transparency. Ideally, it should be invisible to the signal. Since invisibility is impossible, we minimize its impact. The following table outlines the critical metrics engineers must track during the design and validation phases.

Metric	Why it matters	Typical Range / Factors	How to measure
Return Loss (S11)	Indicates how much signal is reflected at the launch. High reflection means poor energy transfer.	> 20 dB (Excellent) > 15 dB (Good) < 10 dB (Poor)	VNA (Frequency Domain)
VSWR (Voltage Standing Wave Ratio)	Another way to express reflection. A ratio of 1:1 is perfect. High VSWR can damage transmitters.	< 1.2:1 (Precision) < 1.5:1 (Standard) > 2.0:1 (Unacceptable)	VNA or Power Meter
Insertion Loss (S21)	Measures signal power lost as it passes through the launch. Includes dielectric and conductor losses.	< 0.5 dB per launch (depends heavily on frequency and material).	VNA (Thru measurement)
TDR Impedance Profile	Visualizes the impedance at every millimeter of the path. Shows exactly where the mismatch occurs.	50 Ohms ± 2 Ohms (High Precision) 50 Ohms ± 10% (Standard)	TDR Oscilloscope or VNA with Time Domain option
Phase Stability	Critical for phased arrays and differential pairs. The launch must not distort the signal phase.	< 5 degrees variation across the band.	VNA (Phase plot)
Bandwidth	The frequency range where the launch maintains acceptable VSWR.	DC to 110 GHz (Connector dependent).	VNA sweep
Passive Intermodulation (PIM)	Crucial for cellular/5G. Nonlinearities in the launch create interference frequencies.	< -150 dBc (High performance).	PIM Analyzer

Selection guidance by scenario (trade-offs)

Once you understand the metrics, the next step is choosing the right fixture architecture based on your specific application environment.

Not all fixtures are created equal. A coax launch calibration fixture designed for a 2.4 GHz Wi-Fi module is vastly different from one designed for a 77 GHz automotive radar or a quantum processor. APTPCB recommends evaluating the following scenarios to balance cost, performance, and complexity.

1. Standard RF & IoT (< 6 GHz)

Context: Wi-Fi, Bluetooth, Zigbee.
Fixture Type: Edge-mount SMA connectors on FR4 or mid-range laminates.
Trade-off: Cost is the driver here. You do not need expensive vertical launch connectors. Standard edge launchers are sufficient.
Calibration: Simple SOLT (Short-Open-Load-Thru) calibration is usually adequate.

2. High-Speed Digital (SerDes / PCIe)

Context: 25 Gbps to 112 Gbps data links.
Fixture Type: Compression mount connectors (solderless) to preserve signal integrity.
Trade-off: Solderless connectors are expensive and require precise mechanical footprints, but they allow for reuse and avoid soldering variability.
Calibration: TRL (Thru-Reflect-Line) is often required to de-embed the long trace lengths typical in these boards.

3. mmWave & 5G (> 20 GHz)

Context: Radar, 5G backhaul, Satellite comms.
Fixture Type: 2.92mm (K), 2.4mm, or 1.85mm connectors. Grounded Coplanar Waveguide (GCPW) is the preferred launch topology.
Trade-off: Material selection is critical. You must use PTFE-based substrates (like those found in our Rogers PCB materials section). FR4 is too lossy.
Calibration: Advanced TRL with multiple line lengths to cover the wide bandwidth.

4. Quantum Computing & Cryogenics

Context: Qubits operating at mK temperatures.
Fixture Type: Non-magnetic connectors (often beryllium copper) with a cryogenic compatible SMT process.
Trade-off: Standard solder becomes brittle and fails at cryogenic temperatures. You may need indium-based solders or specialized mechanical clamping.
Special Note: The PCB material must have a matched Coefficient of Thermal Expansion (CTE) to the connector to prevent cracking during cool-down.

5. High-Volume Production Test

Context: End-of-line testing for thousands of units.
Fixture Type: "Pogo pin" style RF probes or quick-disconnect coaxial interfaces.
Trade-off: Durability is key. The fixture must withstand 100,000+ mating cycles. The electrical performance is often sacrificed slightly for mechanical robustness.

6. Research & Characterization Lab

Context: Validating a new chip or material.
Fixture Type: Precision vertical launch connectors placed as close to the DUT as possible.
Trade-off: Performance is everything. Cost is secondary. The fixture often uses a flux free soldering quantum PCB approach to ensure no residue impacts the dielectric properties at high frequencies.

From design to manufacturing (implementation checkpoints)

After selecting the right scenario, the focus shifts to the rigorous execution of the design and manufacturing process.

Designing a coax launch calibration fixture is not just about drawing lines in CAD. It requires a holistic approach where the PCB stackup, the connector footprint, and the manufacturing tolerances are aligned. Below is a checklist APTPCB uses to ensure the final product matches the simulation.

1. Stackup Definition

Recommendation: Use a symmetric stackup with tightly controlled dielectric thickness.
Risk: If the dielectric varies, the impedance shifts.
Acceptance: Verify stackup with an Impedance Calculator before layout.

2. Connector Footprint Optimization

Recommendation: Do not rely solely on the connector vendor's datasheet. Vendor footprints are often generic. Optimize the anti-pad (ground cutout) size using 3D EM simulation (HFSS/CST).
Risk: A generic footprint often results in a capacitive dip in the TDR profile.
Acceptance: Simulation must show Return Loss > 20dB.

3. Ground Via Placement

Recommendation: Place "fencing" vias as close to the signal pad as manufacturing rules allow. This contains the field and prevents leakage.
Risk: If vias are too far away, the launch becomes inductive, ruining high-frequency performance.
Acceptance: Vias should be within 1/8th wavelength of the highest operating frequency.

4. Material Selection

Recommendation: Choose low-loss materials (Df < 0.003) for frequencies > 10 GHz.
Risk: Using standard FR4 will result in massive signal attenuation and phase distortion.
Acceptance: Confirm material availability (e.g., Rogers 4350B, Megtron 6).

5. Surface Finish

Recommendation: Use ENIG (Electroless Nickel Immersion Gold) or Immersion Silver. Avoid HASL.
Risk: HASL creates uneven surfaces, making the connector sit at an angle, causing air gaps.
Acceptance: Surface flatness check.

6. Etching Tolerances

Recommendation: Specify "RF Etch" or strict impedance control (±5% or better).
Risk: Over-etching the signal conductor increases impedance; under-etching decreases it.
Acceptance: Cross-section analysis (microsection) on coupons.

7. Backdrilling (for Thru-hole connectors)

Recommendation: Backdrill any unused via stubs on the connector signal pin.
Risk: Stubs act as antennas, creating resonance spikes that kill specific frequencies.
Acceptance: TDR measurement to confirm stub removal.

8. Soldering Process

Recommendation: For sensitive applications, specify a flux free soldering quantum PCB process or ensure rigorous cleaning.
Risk: Flux residue is hygroscopic and conductive, altering the dielectric constant at the launch point.
Acceptance: Ionic contamination test.

9. Solder Mask Application

Recommendation: Remove solder mask from the RF line (Solder Mask Defined vs. Non-Solder Mask Defined). Usually, bare dielectric is better for high frequency.
Risk: Solder mask adds loss and unpredictable dielectric constant variations.
Acceptance: Visual inspection of mask clearance.

10. Final Assembly Validation

Recommendation: 100% TDR testing on the fixture before use.
Risk: Assuming the fixture is good can lead to scrapping good DUTs (false failures).
Acceptance: TDR plot must be flat within limits.

Common mistakes (and the correct approach)

Even with a checklist, engineers often fall into specific traps that compromise the integrity of the coax launch calibration fixture.

Here are the most frequent errors we see at APTPCB and how to avoid them.

Ignoring the Reference Plane:
- Mistake: Assuming the calibration ends at the connector interface.
- Correction: The calibration plane must be moved to the end of the launch (where the uniform transmission line begins) using TRL or de-embedding.
Neglecting Surface Roughness:
- Mistake: Using standard copper foil for 50 GHz+ designs.
- Correction: At high frequencies, the "skin effect" forces current to the surface. Rough copper increases resistance. Use VLP (Very Low Profile) or HVLP copper.
Thermal Relief on RF Pads:
- Mistake: Using thermal relief spokes on the connector ground pads to make soldering easier.
- Correction: Never use thermal relief on RF grounds. It adds inductance. Use solid connections and pre-heat the board for soldering.
Wrong Connector Torque:
- Mistake: Hand-tightening connectors or over-torquing them.
- Correction: Always use a calibrated torque wrench (e.g., 8 in-lbs for SMA). Incorrect torque changes the contact resistance and air gap.
Overlooking the "Ground Return Path":
- Mistake: Focusing only on the signal trace and forgetting how the ground current returns to the connector outer shell.
- Correction: Ensure the top-layer ground pour connects immediately and robustly to the connector body.
Using the Wrong Calibration Kit:
- Mistake: Using a mechanical cal kit when an E-Cal (Electronic Calibration) module is needed, or vice versa, without accounting for the fixture length.
- Correction: Match the calibration method to the fixture topology.
Forgetting Cryogenic Contraction:
- Mistake: Designing a fixture for room temperature and putting it in a dilution refrigerator.
- Correction: Account for the fact that PTFE shrinks more than copper. Use a cryogenic compatible SMT process designed to handle thermal stress.

FAQ

Q: What is the difference between an edge launch and a vertical launch? A: An edge launch connects to the side of the PCB, aligning with the signal layer. A vertical launch (compression or solder) mounts on top and uses a via or pin to transition down to the signal layer. Vertical launches are often better for high-density boards but require more complex design.

Q: Can I use FR4 for a coax launch calibration fixture? A: Only for low frequencies (typically < 2 GHz) or short traces. For anything critical or high-speed, the dielectric loss and inconsistency of FR4 make it unsuitable for calibration standards.

Q: What is "De-embedding"? A: De-embedding is a mathematical process performed by the VNA or software. It subtracts the S-parameters of the fixture (the connector and the launch trace) from the total measurement, leaving only the results for the device you actually want to test.

Q: Why is TRL calibration better than SOLT for fixtures? A: SOLT relies on defining the "Short," "Open," and "Load" perfectly at the connector reference plane. TRL (Thru-Reflect-Line) relies on the characteristic impedance of the transmission lines on the PCB itself. This makes TRL much more accurate for removing the effects of the launch transition.

Q: How long should the "Thru" line be? A: In a TRL kit, the "Thru" is usually a zero-length connection (direct connection of reference planes). If a non-zero length Thru is used, its length must be known precisely.

Q: What connector should I use for 40 GHz? A: You should use a 2.92mm (K) connector (rated to 40 GHz) or a 2.4mm connector (rated to 50 GHz). Standard SMA connectors are typically only good up to 18 GHz or 26.5 GHz.

Q: How does solder mask affect the launch? A: Solder mask has a higher dielectric constant than air or most RF laminates. Placing it over the RF trace slows the signal and adds loss. It is best to remove it from the high-frequency path.

Q: What is a "launch taper"? A: A taper is a gradual change in the width of the signal conductor at the connector interface. It helps smooth out the impedance step between the connector pin width and the PCB trace width.

To successfully design and manufacture your fixture, utilize these resources from APTPCB:

Impedance Calculator: Verify your trace widths and stackup before starting layout.
Rogers PCB Materials: Explore the technical specs of high-frequency laminates suitable for calibration fixtures.
Get a Quote: Ready to manufacture? Submit your Gerbers for a DFM review.

Glossary (key terms)

Term	Definition
Coax Launch	The physical transition point where a signal moves from a coaxial connector to a planar PCB trace.
VSWR	Voltage Standing Wave Ratio. A measure of how efficiently radio-frequency power is transmitted from a power source, through a transmission line, into a load.
TDR	Time Domain Reflectometry. A measurement technique used to determine the characteristics of electrical lines by observing reflected waveforms.
VNA	Vector Network Analyzer. An instrument that measures the network parameters (S-parameters) of electrical networks.
SOLT	Short-Open-Load-Thru. A common calibration method for VNAs using defined mechanical standards.
TRL	Thru-Reflect-Line. A high-precision calibration method that uses transmission lines on the PCB itself as standards.
De-embedding	The mathematical process of removing the effects of test fixtures (cables, connectors, launches) from measurement data.
CPW	Coplanar Waveguide. A type of electrical transmission line which can be fabricated using printed circuit board technology, featuring a central conductor separated from ground planes by a gap.
GCPW	Grounded Coplanar Waveguide. A CPW structure with an additional ground plane underneath the dielectric.
Skin Effect	The tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface.
Dielectric Constant (Dk)	A measure of a material's ability to store electrical energy in an electric field. Affects signal speed and impedance.
Loss Tangent (Df)	A measure of signal power loss as it propagates through a dielectric material.
DUT	Device Under Test. The component or circuit that is being measured.

Conclusion (next steps)

The coax launch calibration fixture is the unsung hero of high-frequency electronics. It bridges the gap between theoretical design and physical reality. Whether you are working on 5G infrastructure, high-speed data centers, or quantum processors, the quality of your data depends entirely on the quality of your launch.

By focusing on the metrics of Return Loss and Phase Stability, selecting the right architecture for your scenario, and adhering to strict manufacturing checkpoints, you can eliminate measurement uncertainty.

Ready to build your fixture? When submitting your design to APTPCB for a quote, please provide:

Gerber Files: Including drill files for backdrilling if required.
Stackup Details: Specify the exact material (e.g., Rogers 4350B) and copper weight.
Impedance Requirements: Clearly mark the target impedance (usually 50 Ohms) and the specific layers.
Connector Datasheet: So we can verify the footprint and stencil design.
Special Process Notes: Mention if you need a cryogenic compatible SMT process or specific plating requirements.

Precision manufacturing is the final variable in the equation. Let us help you solve it.