Coaxial Cables and Triaxial Cables

For many electrical measurements, the cable that connects your instrument to your device is a simple wire. But when you are working at the limits of sensitivity—measuring picoamps of current, sourcing from gigaohm impedances, or capturing fleeting signals—your choice of cable becomes a critical part of your experimental setup.

The wrong cable can introduce noise, leak current, and slow down your measurements. The right cable, on the other hand, preserves the integrity of your signal from source to destination. In this guide, we will explore the two most common types of high-performance cables: coaxial and triaxial.

The essential takeaway is this: Coaxial cables provide shielding from external noise, which is good for many applications. Triaxial cables provide active guarding, which is essential for the most sensitive low-current and high-impedance measurements. We will break down what shielding and guarding mean, when you need them, and how to connect everything together correctly.

Understanding Coaxial and Triaxial Cables

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At first glance, coaxial and triaxial cables look similar. They are both a significant step up from simple unshielded wires, but they serve different purposes rooted in the concepts of shielding and guarding.

	Coaxial Cable	Trixial Cable
Primary Function	Shielding	Guarding & shielding
Conductors	2 (signal, shield)	3 (signal, guard, shield)
Best For	General purpose, RF, currents >1 nA	Low current (<1 nA), high impedance
Key Advantage	Good EMI protection	Eliminate leakage, reduce settling time

Coaxial Cable: The Standard for Shielding

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A coaxial cable (or 'coax') is likely the shielded cable you are most familiar with. It consists of a central signal-carrying conductor surrounded by a layer of insulation (the dielectric), which is then enclosed by a braided or foil metallic shield. An outer plastic jacket protects the whole assembly.

Connectors: Common coaxial connectors in research labs include BNC, SMA, and N-type connectors. BNCs are ubiquitous for general lab use, while SMAs are valued for their compact size and excellent performance at high frequencies.

Function (Shielding): The primary job of the shield is to protect the signal on the center conductor from external electromagnetic interference (EMI), like the 50/60 Hz hum from power lines or radio frequency (RF) signals. The shield is typically connected to the 'low' or ground side of the signal, providing a path for noise currents to flow to ground instead of interfering with your measurement.

Technical Specifications:

• Impedance: Coaxial cables have a characteristic impedance, typically 50 Ω for most lab applications. For high-frequency or fast-pulsed signals, matching the cable impedance to the source and instrument impedance is critical to prevent signal reflections. For the sensitive DC measurements discussed here, impedance matching is less critical than insulation resistance, but it remains a fundamental property of the cable.
• Voltage Rating: Standard coaxial cables like RG-58 are typically rated for a few hundred volts (e.g., 300 V_RMS).
• Leakage: The insulation resistance is high but finite. For a standard polyethylene insulator, this can result in leakage currents in the nanoamp range, which is perfectly acceptable for many experiments but can be a significant source of error for more sensitive measurements.

Limitations of Coaxial Cable

While shielding is essential, it isn't a perfect solution for the most demanding measurements. Two issues can arise:

Leakage Current: If you are measuring a voltage from a high-impedance source, there is a large potential difference between the center conductor and the grounded shield. Even the best insulators are not perfect, and a small amount of current will leak across the dielectric. For low-current measurements (in the nanoamp range or lower), this leakage can be significant enough to corrupt your results.

Cable Capacitance: The center conductor and the shield act as a capacitor (typically 50-100 pF per meter). When measuring a high-impedance source, the RC time constant created by the source resistance and the cable capacitance can be very long. For a 1 GΩ source and a 1-meter cable, the time constant can be on the order of 100 ms, meaning you must wait a significant fraction of a second for the measurement to settle. For TΩ sources, this can extend to minutes.

Triaxial Cable: The Solution for Guarding

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A triaxial cable (or 'triax') adds an extra layer of protection to solve the limitations of coax. It features a center conductor, an inner dielectric, an inner shield (or ‘guard’), a second dielectric, an outer shield, and an external insulator.

Connectors: Triaxial connectors are designed to mate with all three conductors. The most common type is the 3-lug bayonet connector (called a TRB connector). These look very similar to BNC connectors but are physically incompatible to prevent incorrect connections.

Function (Guarding): In a triaxial setup, the outer shield provides the same EMI shielding as in a coax cable and is typically connected to a safe ground. The inner shield is used for guarding.

A guard is a low-impedance conductor that is actively driven by the measuring instrument to be at the exact same potential as the high-impedance center conductor. This simple but powerful setup has two profound benefits:

1. It Eliminates Leakage Current: Because the guard (inner shield) and the center conductor are at the same voltage, there is no potential difference across the insulator separating them. With no voltage drop, virtually no current can leak from the center conductor. The leakage current from the guard to the outer shield is supplied by the instrument's low-impedance guard buffer, not your sensitive measurement path. This is the key to making accurate measurements of picoamp and femtoamp currents.

2. It Increases Measurement Speed: By holding the inner shield at the same potential as the center conductor, the capacitance between them is effectively cancelled out. This dramatically reduces the overall RC time constant of your measurement setup. High-impedance measurements that might take a minute to settle using a coaxial cable can stabilize in under a second with a guarded triaxial cable.

Technical Specifications: High-quality triaxial cables use insulators like PTFE to achieve extremely high insulation resistance (often >1 TΩ per meter). The outer shield provides a safe, grounded enclosure, allowing the inner guard and center conductor to be used at high voltages (often up to 1 kV) without posing a shock hazard. For very low current measurements, it is important to use "low-noise" triaxial cable, which includes a layer of conductive carbon to reduce noise generated by friction within the cable (triboelectric noise).

Adapting Between Connector Types: A Practical Guide

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In a real-world lab, your setup might not be exclusively one type of connector. The key is to understand what happens inside an adapter and what trade-offs you are making.

The Common Problem: A High-End Triaxial Instrument and a Coaxial Test Fixture

You have a sensitive electrometer with triaxial outputs, but your probe station enclosure, cryostat, or device holder uses standard BNC or SMA feedthroughs. How do you make the connection without compromising your measurement?

The Goal: You want to leverage the low-leakage and low-noise properties of your triaxial cable for as much of the signal path as possible. The signal is most vulnerable along the length of the cable, so maintaining the guard here is paramount.

The Solution: Use a triaxial-to-coaxial adapter at the test fixture. However, the internal wiring of this adapter is critical.

Understanding Triaxial-to-Coaxial Adapter Wiring

Not all triax-to-coax adapters are wired the same. The internal connections determine how the guard and shield are handled, which has significant implications for your measurement and for safety. There are three common configurations:

• Type 1: Coaxial Shield connected to Triaxial Guard. In this setup, the coaxial cable's shield is used to carry the guard potential from the instrument. This can be effective for guarding, but it creates a major safety hazard: the outer metal shell of the BNC connector and the entire coax shield are now at the guard potential, which may be hundreds of volts. This configuration should only be used with extreme caution inside a fully interlocked, shielded enclosure.

• Type 2: Coaxial Shield connected to Triaxial Outer Shield (Ground); Triaxial Guard Floating. This is a common and safe general-purpose configuration. The coaxial shield is connected to the triaxial ground, maintaining a continuous, safe EMI shield. The instrument's guard terminal is left unconnected (floating) at the adapter. You lose the benefit of guarding from the adapter onwards, but you maintain proper shielding and safety. For this reason, Ossila recommends general-purpose adapters to be Type 2. This provides the best balance of performance and safety for most lab applications.

• Type 3: Coax Shield connected to both Triax Guard and Outer Shield. This adapter shorts the instrument's guard and ground terminals together at the point of connection. This completely defeats the purpose of guarding and is generally not recommended for sensitive DC measurements, though it may be used in some RF applications.

Other Connection Set Ups

Connecting Multiple Single-Ended Probes to a Differential Input: Many instruments use differential inputs. For example, the Ossila Source Measure Unit uses BNC connectors, while an Ossila Micromanipulator uses SMA-connections. A specialized interface is needed to correctly map the signals from single-ended probes. The Ossila Differential Interface, for example, takes three single-ended SMA inputs and routes them to two differential BNC outputs. One of the SMA inputs is the common ground reference point for the measurement, ensuring the signals are properly referenced for the SMU.

Extending a Triaxial Connection: If you need to connect two triaxial cables, you must ensure the connection maintains the separation of all three conductors. A straight-through female-to-female TRB triaxial adapter (often called a “barrel connector”) ensures the center conductor, inner guard, and outer shield are all passed through correctly, maintaining the full integrity of your guarded measurement path.

Ossila Triaxial and Coaxial Adaptors

Product Code	Diagram	Description	Grounding	Recommended Use
Triaxial Adapters (C2022A1)		TRB-M to BNC-F	Type 2	Triaxial instrument to BNC cable
Triaxial Adapters (C2022B1)		TRB-F to BNC-M	Type 2	Triaxial cable to BNC instrument
Triaxial Adapters (C2022C1)		TRB-F to BNC-F	Type 2	Triaxial cable to BNC cable
Triaxial Adapters (C2022D1)		TRB-F to TRB-F		Triaxial cable to Triaxial cable.
Triaxial Adapters (C2022E1)		TRB-F to BNC-F	Type 3	Triaxial cable to BNC cable (when guard layer of triaxial cable not driven by the instrument)
Triaxial Adapters (C2022F1)		TRB-M to BNC-F	Type 3	Triaxial instrument to BNC cable (guard tied to ground)
Triaxial Adapters (C2022G1)		TRB-M to SMA-F	Type 2	Triaxial cable to SMA cable
Triaxial Adapters (C2022H1)		TRB-F to SMA-F	Type 2	Triaxial instrument to SMA cable

Choosing a Cable

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Choosing the right cable is fundamental to achieving accurate, stable, and fast low-level electrical measurements.

• Coaxial cables provide essential shielding against external noise and are suitable for a wide range of everyday measurements.
• Triaxial cables add an actively driven inner shield to provide guarding. This is critical for high-impedance and low-current applications, as it eliminates leakage currents and dramatically reduces measurement settling times.
• Adapters allow you to build flexible test setups. Understanding their internal wiring is crucial for maintaining signal integrity and safety.

By understanding these principles, you can ensure that your cabling is helping, not hindering, your research.