Twinaxial cables feature a two-conductor twisted balanced wire line and a specific impedance, plus a shielding braid around both wires. They’re similar to coaxial cables, but, with two inner conductors instead of one.
This construction matters for a couple reasons:
- It lowers cable losses.
- It provides protection against ground loops and capacitive fields.
- By using more copper wire in a more tightly weaved braid, it improves coverage to 90 percent (triaxial cables also benefit from this design).
- It protects against low-frequency magnetic noise that passes through the copper braid.
- By twisting the two balanced-signal-carrying wires, it makes it possible to cancel any random induced noise voltage pickup.
- With two dielectric fillers under the braid, it allows the signal pair to be separated away from the braid, lowering the leakage capacitance to ground.
As a result, twinax cables are a cost-efficient, effective option for short-range, high-speed differential signaling applications. However, the cables begin to lose their effectiveness around 15 MHz, after which transmission losses begin to increase. Therefore, they’re best suited for low-frequency digital and video distribution systems like connecting MIL-STD-1553 bus and stub devices.
Triaxial cables are essentially coaxial cables with an added outer copper braid (basically, a shield), which is insulated from the signal-carrying conductors. The shield is grounded and passes both ground loop and capacitive field noise currents away from the signal-carrying coax.
Triaxial cabling can also be used in “driven shield” applications, where the inner conductor and the first braid are driven in parallel at the transmitting end, and work against the outer braid that’s insulated above ground. The inner braid is left floating at the receiving end, providing a sort of “Faraday shield” (which protects the internal conductors from electrical noise) between the inner conductor and outer braid.
This design is full of benefits, including:
- Bandwidth and rejection of interference are increased.
- The extra outer copper braid acts as a shield, protecting the enclosed coax conductors.
- The grounded “shield” significantly improves signal-to-noise ratio (compared to, say, a standard coax cable).
- The “Faraday” shield significantly reduced the cable’s distributed capacitance.
- Cable losses and loading are also reduced.
Triax cables are effective for a broad range of applications, including high-frequency transducer data systems and precision low-current measurements. Also, the outer braids are useful as a low-impedance transmission line.