Similar to nearly every technology out there, coaxial cable has its things it is great for and others that it is severely lacking in. Generally, coax has lower error rates due to the inner conductor being in a Faraday shield. Because of this, noise immunity is improved and, therefore, slightly better performance than twisted pair. Coax cable also outperforms twisted pair in that is supports high bandwidth signal transmission. And these are only a couple of the numerous advantages—but don't get it too twisted (pun intended), coaxial cable can also be a real pain as it is normally a thicker cable and can restrict where it can be used. But in this post, we'll outline the benefits of using coaxial cables with low capacitance, specifically, in the medical world.
Why having low capacitance (low cap) in coaxial cables for medical applications is ideal?
- Higher-definition diagnostic images (Ultrasound diagnostic and endoscope cables)
- Softer, more supple and flexible cable for best user feel and friendliness
- Least obtrusive when handling the device during a procedure.
- The softness and flexibility are a byproduct of the dielectric materials used for low cap – explained below.
- Smaller diameter
- Can be achieved for the same capacitance or impedance. More coaxes can be stuffed into the same ID catheter, for instance.
- The limiting factor in how small a coax can be is the maximum dc resistance of the center conductor that the system can tolerate.
Low Cap offers the following advantages for DIGITAL systems as well
- Higher Data Transmission Rate. Due to capacitance, the leading and trailing edges of the square waves (blue) get rounded off (red). The square waves stay more intact, thus allowing a higher clock rate. This reduces the signal transmission speed by reducing time spent above the trigger voltage (for sake of illustration, assume 0.4 volts as the trigger voltage here). The trigger voltage is where the binary signal switches from a 0 to a 1 and vice versa. The time it takes the leading edge to rise to a certain voltage is called rise time. A transmission line’s bandwidth is directly related to it’s rise time. We want high bandwidth for many applications.
- Higher Velocity of Propagation (Vp) can also be achieved. The material property that lowers the capacitance also increases the Vp. There are even more advantages in the digital realm. I will go into more detail in a subsequent writing.
How Do We Get Low Cap Coax?
For a given geometry, it all comes down to using a dielectric material that has a low dielectric constant (Ref. equation for Capacitance below). The dielectric constant (Dk) - also called the relative permittivity (εr), indicates how easily a material can become polarized by the imposition of an electric field on an insulator. Relative permittivity is the ratio of "the permittivity of a substance to the permittivity of space or vacuum." Dk is a unitless number, with its standard being air at 1.0 - the lowest Dk possible. For the lowest capacitance, we start with low Dk material... then add air. Here are some ways to do it:
- Foamed dielectric (Polyethylene, propylene, FEP, PFA, to name a few) – Add foaming agents or injecting nitrogen gas into the molten dielectric during extrusion. This creates bubbles in the material. Not practical for very small coaxes. Although Hitachi claims to do it – I have not been able to obtain a sample.
- Filaments – The voids between the filaments provide the air, while the filaments provide support. Again, not the most practical approach for small coaxes because the filaments are so small. This is a technique used by Tempflex, and now, apparently, Hitachi as well.
- Exotic Shapes – Extruded “wagon-wheel” or other shapes that provide structure, but have air pockets. This concept does not work for small coaxes.
- Expanded PTFE tape - PTFE (Polytetrafluoroethylene) is more commonly known by DuPont’s tradename as “Teflon”. It is unique in that it's not melt-processable like the polymers mentioned above. Air can be induced by stretching the heated PTFE tape rapidly, giving us expanded PTFE (ePTFE). An example of ePTFE is Gore-Tex. This ePTFE tape is then wrapped around the center conductor. ePTFE can found be as thin as 0.001”, which allows the making of small coax.