4.2 dB. That's the signal-to-noise improvement we measured when an NDT inspector switched from a generic 75Ω video cable to a proper 50Ω TOFD cable on his corrosion mapping setup. He'd been using the video cable for two years—it "worked fine" for pulse-echo inspections because the strong back-wall echo didn't need much SNR headroom. But when he moved to TOFD for weld inspection, where the diffraction signals are 20-30 dB weaker, that impedance mismatch and the extra capacitance were eating into his detection margin. Different NDT methods put different demands on the cable. Using the same cable for everything is like using the same drill bit for wood, metal, and concrete—it'll technically make a hole, but you won't like the results.
This guide breaks down the actual cable requirements for the three primary ultrasonic NDT methods—conventional pulse-echo, TOFD, and phased array—and explains where you can get away with a general-purpose cable and where method-specific optimization actually matters.
Quick Reference: Cable Requirements by NDT Method
| Parameter | Pulse-Echo (Conventional) | TOFD | Phased Array (PAUT) |
|---|---|---|---|
| Cable type | Single coaxial | Dual coaxial (T + R) | Multi-channel coaxial bundle |
| Channels per probe | 1 | 1 per probe (2 total) | 16–128 |
| Impedance | 50Ω (standard) | 50Ω (critical) | 50Ω (critical) |
| Impedance tolerance | ±3–5Ω (forgiving) | ±2Ω (recommended) | ±1.5–2Ω (required) |
| Capacitance sensitivity | Low — △ marginal impact | High — ✗ bandwidth-critical | Moderate — ✓ per-channel consistency matters |
| Phase matching | N/A (single channel) | N/A (independent channels) | ±2–3% (standard), ±1–1.5% (TFM/FMC) |
| Crosstalk isolation | N/A | N/A (separate cables) | ≤ -50 to -55 dB |
| Typical AWG | RG-174/U (26 AWG) or 36 AWG | 36–40 AWG | 36–42 AWG |
| Connector (instrument side) | BNC or LEMO 00 | BNC or LEMO 00 | Proprietary multi-pin |
| Connector (probe side) | BNC, LEMO 00, Microdot | LEMO 00, Microdot | Proprietary multi-pin |
| Max practical length | 5–10 m | 2–5 m (shorter = better) | 1.5–5 m (manual), 5–10 m (automated) |
| Environmental demands | Moderate | Moderate to high | High (multi-channel reliability) |
| Relative cable cost | $ (commodity) | $$ (optimized coaxial) | $$$ (multi-channel assembly) |
Conventional Pulse-Echo: The Forgiving Method
Pulse-echo is the most cable-tolerant NDT method. The instrument sends a high-voltage pulse through the cable, it bounces off a reflector in the test piece, and the echo comes back through the same cable. The echo from a significant defect is typically only 10-20 dB below the initial pulse—a strong signal that doesn't require exceptional cable performance to detect.
A standard RG-174/U coaxial cable (26 AWG center conductor, solid PE dielectric, 50Ω nominal) is the workhorse of conventional pulse-echo NDT. It's cheap ($1-3 per meter), widely available, and rugged enough for field use. The impedance tolerance on commercial RG-174/U is typically ±5Ω—loose by medical standards, but perfectly adequate when you're looking for cracks that produce 20 dB echoes.
Where pulse-echo cable selection starts to matter is at higher frequencies. Standard RG-174/U has about 100 pF/m capacitance and attenuates roughly 3 dB/m at 10 MHz. For high-frequency immersion testing at 15-25 MHz—thin material inspections, bond-line evaluations, composite inspections—that capacitance acts as a low-pass filter that rolls off the high-frequency content you're trying to preserve. Switching to a lower-capacitance cable (60-70 pF/m, PTFE dielectric) extends the usable bandwidth and preserves axial resolution at higher frequencies.
TOFD: Where Cable Quality Actually Matters
Time-of-Flight Diffraction is the NDT method most sensitive to cable performance, and it's also the one where we see the most cable-related problems in the field. Here's why:
TOFD relies on diffraction signals rather than reflection signals. When an ultrasonic wave hits the tip of a crack, it diffracts—generating a weak spherical wave from the crack tip that propagates in all directions. This diffracted signal is typically 20-30 dB weaker than a direct reflection from the same crack. The TOFD receiver has to pull this weak diffraction signal out of the noise floor to measure the crack tip position and calculate defect size.
Every decibel of cable-induced signal loss directly reduces the system's ability to detect diffraction signals. A cable with 2 dB more attenuation than necessary at the operating frequency effectively reduces your detection sensitivity by 2 dB—equivalent to making the crack tip 25% smaller in terms of signal amplitude. In practical terms, this can mean the difference between detecting and missing a shallow surface-breaking crack in a critical weld.
TOFD also uses broadband transducers to achieve the short temporal pulse width needed for precise time-of-flight measurement. Broadband signals are more susceptible to cable-induced dispersion and bandwidth limiting than the narrowband signals used in conventional pulse-echo. A cable with high capacitance (100+ pF/m) will visibly broaden the received pulse, degrading the timing resolution and therefore the sizing accuracy of detected flaws.
TOFD Cable Optimization
For TOFD, specify a cable with PTFE or FEP dielectric (capacitance under 70 pF/m), 50Ω ±2Ω impedance, and the shortest practical length. We've found that moving from a standard RG-174/U to a 38 AWG PTFE-dielectric cable improved TOFD signal bandwidth by approximately 15% at 10 MHz—a meaningful improvement in sizing accuracy for defects under 3mm.
Since TOFD uses separate transmitter and receiver probes with separate cables, there's no multi-channel complexity. Two high-quality single coaxial cables with proper connector selection (LEMO 00 or Microdot for compact probe housings, BNC for standard probes) is the standard configuration.
Phased Array: Multi-Channel Complexity
Phased array cable requirements are covered in detail in our PAUT cable Engineering Guide , so here's the summary comparison with the other two methods.
PAUT cables are fundamentally different from pulse-echo and TOFD cables because they're multi-channel assemblies rather than individual coaxial cables. This changes everything: the connector interface moves from commodity BNC to proprietary multi-pin, the cable construction moves from single coaxial to bundled micro coaxial, and new requirements appear (phase matching, crosstalk isolation) that don't exist for single-channel methods.
The cost difference is dramatic. A pulse-echo cable might cost $15-30. A TOFD pair costs $30-60. A 64-channel PAUT cable assembly costs $200-600 depending on connector type and specification level. This price premium reflects the manufacturing complexity—each channel in a PAUT cable needs to be individually stripped, terminated, routed, and tested.
Despite the complexity, PAUT cables have one advantage: the channel count drives the use of smaller AWG conductors (38-42 AWG), which makes the cable more flexible than the bulkier RG-174/U used for pulse-echo. A 64-channel PAUT cable at 40 AWG has an overall OD of about 8-9mm and handles almost like a garden hose. A bundle of 64 individual RG-174/U cables would be unmanageable.
The Crossover: When Methods Share Equipment
Modern NDT instruments increasingly support multiple inspection methods. An Olympus OmniScan X3 can do pulse-echo, TOFD, and PAUT. A Zetec Topaz supports all three methods. This raises a practical question: do you need different cables for each method on the same instrument?
For PAUT versus single-element methods, yes—you always need separate cables because the connector interfaces are physically different. The multi-pin PAUT connector won't accept a single BNC cable and vice versa.
For pulse-echo versus TOFD, the same cable can technically be used for both. But if you're doing critical TOFD inspections to codes like BS EN ISO 10863 or ASME V Article 4 Mandatory Appendix III, the investment in optimized TOFD cables (lower capacitance, tighter impedance tolerance) pays for itself in improved detection sensitivity and sizing accuracy. The cost difference between a commodity pulse-echo cable and an optimized TOFD cable is maybe $15-25—insignificant relative to the cost of missing a defect in a critical weld.
Connector Selection by Method
BNC connectors dominate conventional pulse-echo NDT because they're cheap, fast to connect, and universally compatible. But they have limitations: the bayonet lock can loosen from vibration, and the physical size (15mm OD) limits how close probes can be positioned in dual-probe setups.
LEMO 00 connectors are increasingly preferred for TOFD and precision pulse-echo because they're more compact (10mm OD), provide positive locking engagement, and have better shielding effectiveness at the connector interface. The cost premium is about $8-12 per connector compared to BNC. For TOFD setups where two probes must sit close together on a weld cap, the compact LEMO form factor can be the difference between fitting the probe setup and not fitting it.
Microdot connectors are the smallest option (6mm OD) and are used on miniature probes for access-restricted inspections—pipe bore inspections, turbine blade roots, and similar confined geometries. They're fragile compared to BNC and LEMO, so they're not ideal for general field use.
For phased array, connector choice is dictated by the instrument—there's no cross-compatibility. If you're sourcing replacement NDT cables , always verify the connector by instrument model number and probe model number before ordering.
If you need cables for multiple NDT methods and want to consolidate your supplier, send us your instrument list and inspection methods —we'll provide a cable kit recommendation covering all your methods with optimized specifications for each.
One area we're currently evaluating: hybrid TOFD-PAUT cables that integrate two single-element TOFD channels alongside 32 or 64 phased array channels in a single cable bundle. Several new-generation instruments support combined TOFD+PAUT scanning in a single pass, and a unified cable eliminates the separate TOFD cable trailing behind the PAUT probe. We've built prototypes but haven't finalized qualification yet—the challenge is maintaining adequate isolation between the TOFD channels and the PAUT channels without making the cable excessively thick.
Related Products
FRS Technology supplies these constructions to medical, NDT and industrial OEMs worldwide, with engineering support at the design stage. Related products:
Have an existing cable to match or replace? Send us the sample or spec for a like-for-like quote.
