About one in three custom ultrasound cable assembly RFQs we receive at FRS Technology is missing at least one critical specification. Not obscure details—fundamental parameters like test frequency for impedance, or whether the phase matching tolerance applies to the cable assembly or just the raw cable. Incomplete specs lead to misquoted projects, prototype delays, and sometimes assemblies that technically meet the written spec but don't actually work in the probe. The spec is the contract between the probe designer and the cable manufacturer. Getting it right up front saves weeks of back-and-forth and prevents costly re-spins.
This checklist is built from the specification gaps we see most often across roughly 200 custom ultrasound cable assembly programs we've quoted or built. Use it as a template when preparing your next RFQ, or hand it to your cable supplier and ask them to fill in what they'd recommend for each parameter. Either approach gets you to a complete, unambiguous specification faster than the typical iterative email chain.
Step 1: Electrical Core Parameters
Start with the four numbers that define the cable's electrical identity: impedance, tolerance, test frequency, and channel count.
Nominal impedance is almost always 50Ω for ultrasound probe cables . The rare exceptions are some legacy 75Ω designs and certain specialty probes with non-standard matching networks. If you're not sure, it's 50Ω.
Impedance tolerance is where precision matters. State it as an absolute value (±1.5Ω) rather than a percentage (±3%), because absolute tolerance is what TDR test equipment measures directly. And—this is the part that trips people up—specify the test frequency. A cable that measures 50Ω ±1Ω at 1 MHz might be 48-52Ω at 10 MHz. If your transducer operates at 7.5 MHz, write "50Ω ±1.5Ω at 7.5 MHz" on the spec. No ambiguity.
Channel count should include not just signal channels but also any dedicated ground conductors, power lines, and digital control channels. A "128-channel" probe cable might actually be a 128+8+4 = 140-conductor assembly when you count the ground bus and ASIC power lines. Specifying the total conductor count and a channel map prevents surprises during routing and connector layout.
Capacitance and Attenuation
Specify capacitance in pF/m at the cable level (not per assembly—assembly capacitance depends on length, which is a separate parameter). For 42 AWG, 50Ω cables with solid PTFE dielectric, typical capacitance is 83-87 pF/m. ePTFE dielectric drops this to 55-70 pF/m. If you have a specific maximum capacitance requirement, state it—otherwise the manufacturer will optimize for impedance tolerance, which usually takes care of capacitance by default.
Attenuation should be specified at the operating frequency per meter. We often receive specs that state maximum attenuation at 1 GHz, which is a standard RF cable parameter but largely irrelevant for ultrasound cables operating at 2-20 MHz. Stating "max 3.0 dB/m at 10 MHz" is far more useful for your application than "max 15 dB/m at 1 GHz."
Step 2: Phase Matching and Signal Integrity
Phase matching tolerance defines how tightly the propagation delay must be controlled across all channels in the cable bundle. This is critical for beamforming performance and directly affects image quality.
Before specifying a tight phase matching tolerance, ask your beamformer design team: what's the minimum phase variation that produces a detectable difference in image quality on our system? We've seen RFQs specifying ±0.5% phase matching when the system's ADC timing jitter alone is larger than the cable phase variation at ±1.5%. Over-specifying phase matching is the single most expensive specification mistake on ultrasound cable RFQs—going from ±1.5% to ±0.5% can add 30-50% to cable cost due to element pre-sorting, tighter process controls, and lower manufacturing yield.
For standard 2D imaging at 128 channels, ±1.5-2% is almost always sufficient. For advanced 3D/4D matrix arrays or high-element-count phased arrays, ±1% is typical. Below ±0.5%, you're in territory that only a handful of manufacturers can reliably deliver.
State phase matching as either a percentage of cable propagation delay (±1%) or as an absolute time (±150 ps at 2.0m length)—both are unambiguous.
Step 3: Conductor and Dielectric Selection
AWG selection is primarily a trade-off between signal attenuation and cable OD. The decision tree is straightforward:
| Parameter | 42 AWG | 44 AWG | 46 AWG |
|---|---|---|---|
| Element OD (typical) | 0.47 mm | 0.36 mm | 0.28 mm |
| 128-ch bundle OD | 6.5–7.5 mm | 5.2–6.0 mm | 4.0–4.8 mm |
| Attenuation @ 10 MHz | 2.8 dB/m | 3.4 dB/m | 4.2 dB/m |
| Termination difficulty | Standard | Moderate | High (laser strip required) |
| Best fit | Cart-based systems, >10 MHz probes | Portable, standard frequency | Catheter-based, extreme miniaturization |
Conductor material is almost always silver-plated copper (SPC) stranded 7×50 AWG for 42 AWG (or equivalent for other gauges). Bare copper saves a fraction of a cent per meter but gives measurably higher skin-effect loss at ultrasound frequencies. Unless cost pressure is extreme, specify SPC.
For dielectric selection, refer to our ePTFE vs FEP vs PFA comparison . The short version: solid PTFE is the safe default. ePTFE if you need lower capacitance or lower loss and the budget allows. FEP if you need ease of processing and the temperature/sterilization requirements permit it.
Step 4: Shielding Specification
Specify shielding at two levels: individual element shield and overall bundle shield.
Individual element shields determine inter-channel crosstalk. For ultrasound probes, specify braided shields with ≥90% optical coverage. State the shield material—silver-plated copper wire is standard, though some specifications call for tinned copper for cost or tin-plated copper for specific corrosion environments. Include the target crosstalk isolation between adjacent channels: ≤ -55 dB at the operating frequency is a good baseline for medical imaging.
Overall bundle shield determines EMI rejection. Specify ≥95% for cart-based probes used in hospital environments with multiple RF sources. Portable/POCUS probes used in less EMI-dense environments can sometimes use ≥90%, but we generally recommend ≥95% as the default—it adds negligible cost and provides margin against worst-case EMI environments.
Step 5: Mechanical and Environmental Requirements
This is the section most often incomplete on RFQs. The mechanical spec defines how the cable will survive in clinical use—miss it and you'll discover the gaps during field trials, not on the bench.
Cable length: State as nominal ±tolerance (e.g., 2000mm ±20mm). Include whether the length is measured to the cable jacket end or to the connector mating face—this difference can be 15-25mm per end.
Minimum bend radius: Specify both dynamic (repeated flexing) and static (installed bend). Dynamic minimum bend is typically 5-8× overall cable OD. If the probe will be used in portable Applications with tight coiling, specify the coil diameter requirement separately.
Flex life: State the number of cycles, the bend radius, and the bend angle. "100,000 cycles" alone is ambiguous—100,000 cycles at 50mm radius is a completely different test than 100,000 cycles at 25mm radius. Also specify post-flex acceptance criteria: maximum impedance shift per channel, maximum attenuation increase, and continuity pass/fail.
Pull force: Specify the axial pull force the terminated assembly must withstand at each connector without conductor pullout or strain relief failure. Typical: 15-25N sustained for 60 seconds.
Sterilization compatibility: If the probe is reusable, state the sterilization method (autoclave, EtO, plasma, high-level disinfection) and the number of sterilization cycles the cable must survive without degradation. This single parameter can eliminate entire material families from consideration—autoclave kills TPU and foamed PE, for example.
Step 6: Connector Interface
Connector specification is where most engineering time gets spent during the quoting process. For each cable end, provide:
Connector model number if using standard connectors (e.g., I-PEX 20453-040T-01, Hirose DF81-30P-LCH(27), Hirose DF81 series ). If using a custom or OEM-proprietary connector, provide a drawing or a physical sample.
Pinout map: Which cable channel connects to which connector pin. Include ground connections, shield termination scheme (individual shield grounds, common ground bus, or both), and any pins reserved for power or digital signals.
Termination method: Solder (reflow or hand), crimp, or press-fit. For micro coaxial connectors, solder is most common. Specify the solder alloy if regulated (lead-free per RoHS, or leaded for specific exemptions under EU medical device regulations).
Open end handling: If one end of the cable is unterminated for customer board-level assembly, specify the strip length, stagger pattern (do all channels strip to the same length, or staggered for fan-out?), and any individual channel identification requirements (color coding, numbering).
Step 7: Quality and Compliance
State the required certifications for the cable manufacturer: ISO 13485 for medical devices, ISO 9001 as a baseline. If the probe is destined for a specific regulatory market, note the applicable device regulations (FDA 21 CFR 820, EU MDR, etc.) that the cable must support.
Specify the incoming inspection tests you'll perform and the acceptance criteria. FRS Technology provides full TDR impedance profiles, insertion loss data, and phase matching reports on every production assembly—but not every manufacturer does. If you require 100% testing (versus statistical sampling), state it in the spec. If you need specific test equipment certifications (NIST-traceable calibration, for example), include that too.
For traceability, specify whether you need lot-level or serial-number-level traceability on cable materials (conductor, dielectric, shield). Medical device regulations increasingly require component-level traceability, and the earlier this is built into the cable specification, the smoother the regulatory documentation process.
What to Send with the RFQ
Beyond the specification itself, three supporting documents dramatically accelerate the quoting process. A system block diagram showing where the cable fits in the probe assembly—this helps the cable manufacturer understand the upstream and downstream interfaces. A mechanical drawing or 3D model of the probe handle showing the cable entry point, available space for strain relief, and any routing constraints. And if you're replacing an existing cable (second-source qualification or aftermarket program), send a sample of the current cable—we can reverse-engineer and match it faster than we can interpret an incomplete spec.
If you're preparing an RFQ for a custom ultrasound cable assembly and want to verify your specification is complete before sending it out, send us a draft spec for a free DFM review —we'll flag any gaps, ambiguities, or parameters that might cause problems in manufacturing, and suggest values for anything that's missing. The review typically takes 2-3 business days and has saved multiple customers from prototype re-spins.
One thing that doesn't belong on the cable spec but should be discussed early: your volume ramp plan. A cable manufacturer's approach to tooling investment, material procurement, and production cell setup changes significantly depending on whether you need 50 prototypes or 50,000 production units. Annual volume projections don't need to be in the technical specification, but sharing them during the quoting process helps the manufacturer optimize their proposal for your actual needs rather than guessing. It's the kind of detail that experienced procurement teams share upfront—and that less experienced teams discover matters only when pricing comes back higher than expected.
Related Products
Our production covers 4 to 512 cores in 36-50 AWG, with ePTFE dielectric and 100% electrical inspection on every channel. Related products:
Not sure which construction fits? Talk to our engineers - share your channel count, frequency and space constraints.
