Q: What is Capacitance? Why is it so critical?
A: Simply put, capacitance is the ability for something to hold a charge. It is the result of a body coming in contact with an electric charge and a load that results in a closed circuit. The charges carrying current in conductors make capacitance between each other as well as other nearby objects. This effect is called as stray capacitance. In power transmission lines, the stray capacitance could occur between each line as well as between the lines and the earth, supporting structures, etc. Due to the large currents carried by them, these stray effect considerably affects power losses in power transmission lines.
In a cable, the capacitance is usually measured in picofarads per foot (pf/ft). This indicates just how much electrical energy the cable can store. Capacitance can be viewed as an intimate ongoing relationship between the conductor and the ground plane. Along with Direct Current Resistance (DCR) are both crucial, it is one of the primary causes of loss in a circuit.
All materials have some form of capacitance or ability to hold a charge and subsequently dissipate that charge. In a cable, this process all happens very quickly, in a matter of picoseconds. Some materials will hold a charge better than others but everything has some level of capacitance, typically referred to as self-capacitance. In electrical circuits, the term capacitance though is usually a reference to mutual capacitance which is the ability to hold a charge between two adjacent conductors, such as with the two parallel plates in a capacitor separated by a dielectric insulator.
As a voltage is applied to a conductor, the charge gradually builds until the full length of the conductor and cross-section (based on the frequency being delivered) is energized to that potential. This creates a delay in voltage reaching its full potential. This is called propagation delay.
For low speed circuits this delay has very little impact but for a high speed circuit, that uses high frequency pulses, the propagation delay caused by capacitive and resistive affects will cause a loss of signal. Sometimes this is done deliberately in order to filter a circuit but generally capacitance in a cable is not favorable. But because of the complexity of making a low capacitance cable the cost is often higher. So designers must make tradeoffs between cost and performance when it makes good sense to do so.
The higher the frequency, the greater the reactance caused by the capacitance and the greater the signal loss. In the music world, a lower cable capacitance provides a “richer” sound quality. There is more of the natural “brightness,” “presence,” or “bite” from the instrument that can reach the amplifier. The same is true with video. Better signals provide cleaner and richer results with reduced attenuation and an ability to carry that signal significantly longer distances without degradation.
So, high speed circuits favor high impedance cables (or circuit board traces) because the signal quality is cleaner and it is ideal for shorter runs but can be extended to longer “mid-range” reaches without incurring significant loss. Because the cable has high impedance, i.e. low capacitance, it is “reactive” enough to respond quickly to the higher frequency signals with limited degradation.
So too is true of a cable is designed to be high capacitance. It will by nature have low impedance but it will be better for moving large amounts of data where the signal is somewhat less sensitive or critical. It is also better for longer distance applications due to the reduced impedance not restricting the flow. See:
Good conductor materials and dielectrics (that closely match the dielectric value of air) are needed to improve the signaling. Heat also plays a critical role in the capacitance because heat increases the atomic energy of the electrons and protons which results in the potential for more internal collisions of electrons bombarding the protons along the length of the wire. This increases the wires resistance and creates increased propagation delays.
As a wire is charged, the current through the length of the wire varies due to the propagation delay, until it has reached its full voltage potential. But unlike a capacitor there is no maximum charge or off switch state. Instead the wire either stays at the full potential as in a DC circuit or it discharges and charges again with the pulsed waveforms of an AC circuit.
For a cable capacitance can be minimized by either:
- Increasing the insulation wall thickness;
- Decreasing the conductor diameter; or
- Using insulation with a lower dielectric constant or a precisely formed foamed dielectric that leaves voids to mimic an air dielectric.
There are other ways of controlling the cable capacitance as well:
- Using the appropriate damping to resist resonate frequencies from vibrating a cable. Braided Kevlar tightly squeezed between the jacket and shield provides an excellent ability to dampen the overall structure.
- Using materials that are less permeable. Even though all materials leak using materials like PTFE, FEP or PFA can provide for excellent barriers with lower permeability.
- Using the best conductor materials to avoid oxidation on the surface of the conductor. If you think about this, since the current flows primarily along the skin of the conductor at high frequencies, then having any oxidation on this surface will certainly affect the dielectric value. And everything except for Gold below will oxidize. Copper when it oxidizes actually has an insulating effect whereas tin and silver can form whiskers which will create capacitance discontinuities.
The most commonly used materials are:
- Gold - the “Gold Standard” if you can afford it!
• Metallurgy techniques that reduce oxide formation due to crystallization effects.