FAQs
Q: What is Characteristic Impedance?
A: Understand first that Resistance is a concept used for DC (direct currents) whereas impedance is the AC (alternating current) equivalent. Electrical impedance is a measurement in (Ohms) of the total resistance that a conductor presents to the electrical current passing through it. It is different for AC vs DC. With a DC circuit the resistance (the magnitude) is the impedance. However, with an AC circuit the impedance takes into account both the resistance (the magnitude) and the phase of the AC. The phase is simply a measurement representing the position at a particular point in time (an instant) on the actual waveform cycle. So charting AC impedance will show highs and lows as the waveform changes. Another way to think of it is that impedance is a more general term for resistance that also includes reactance.
In other words, resistance is the opposition to a steady electric current. Pure resistance does not change with frequency, and typically the only time that just resistance is considered is with a pure DC (not changing) circuit.
Reactance, however, is a measure of the type of opposition to the AC electricity due to capacitance or inductance. And this opposition strongly varies with frequency. At low frequencies the impedance is largely a function of the conductor size, but at high frequencies, conductor size, insulation material and insulation thickness all affect the cable's impedance and ultimately the signal quality. This in addition to inductance and capacitance are critical factors that must be taken into account based on the input signaling.
Quoting the below article, “In order for a cable's characteristic impedance to make any difference in the way the signal passes through it, the cable must be at least a large fraction of a wavelength long for the particular frequency it is carrying. Most wires will have a speed of travel for AC current of 60 to 70 percent of the speed of light, or about 195 million meters per second. An audio frequency of 20,000 Hz has a wavelength of 9,750 meters, so a cable would have to be four or five *kilometers* long before it even began to have an effect on an audio frequency. That's why the characteristic impedance of audio interconnect cables is not something most of us have anything to worry about. Normal video signal rarely exceed 10 MHz. That's about 20 meters for a wavelength. Those frequencies are getting close to being high enough for the characteristic impedance to be a factor. High resolution computer video signals and fast digital signals easily exceed 100 MHz so the proper impedance matching is needed even in short cable runs.” Read more at:
http://www.epanorama.net/documents/wiring/cable_impedance.html
When designing a circuit, if the system is designed to be 100 Ohms, then the components both entering and exiting the circuit should be matched also at 100 Ohms. This is very a crucial element to a good design. If there is any mismatch, error-producing reflections are created at the location of the mismatch and this creates loss. In general, for high voltage, the perfect impedance is 60 ohms. For high power, the perfect impedance is 30 ohms. 50 Ohms is the overall industry standard that was set for most equipment and devices and 75 Ohms is preferable for high quality video.
As a reminder, in a high impedance cable, the capacitance (or ability to hold a charge) will be low. And likewise, in a low impedance cable the capacitance will be high. Why is this? Well in simplest terms, higher frequency => faster rise time => the need to fill up capacitor/cable more quickly => more charges needed => more current => more power.
