Nominal impedance
From Wikipedia, the free encyclopedia
The term nominal impedance in electrical engineering or audio has a number of different meanings. It refers to the approximate designed impedance under certain conditions. The actual impedance may vary from this nominal figure. The term is used when the exact value of impedance is not important to the operation of the circuit or system.
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[edit] Nominal impedance and matching
[edit] Output impedances
Most signal-processing equipment is designed to operate with the internal impedance of a signal source much smaller than the load impedance of the input to which it is connected. This provides the most efficient voltage transfer, and is best in most, but not all, situations. The nominal output impedance is the assumed impedance for the purpose of deciding whether the equipment can drive another piece of equipment with a known input impedance.
If impedances are mismatched (excessive load) between inputs and outputs that are connected together:
- Power is wasted or signal level is reduced
- Frequency response is no longer as linear
- The signal is more subject to crosstalk and electromagnetic interference
- Distortion may result due to excessive loading
- Damage to both units is a possibility
[edit] Professional audio system impedances
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This article's factual accuracy is disputed. Please see the relevant discussion on the talk page. (March 2008) |
The nominal line impedance is 600 Ω. Both of these, and the name "line", come from the standards of the earliest telephone networks. It is unlikely that the cable will have an exact 600 ohm characteristic impedance, but this is generally unimportant for the relatively short runs of cable compared to the highest audio frequencies (20 kHz) in use. The main reason for standardisation is for the source and loads to be matched to each other for maximum power transfer.
This is a common standard for connections between pieces of electronic equipment, but 100 mV at 600 Ω is considerably more level than is common for a low impedance input, and represents a greater signal strength than is common for high impedance connections as well, so connecting a high or low impedance signal into a line input normally requires a pre-amplifier, and going the other way may require a dummy load.
[edit] Loudspeaker impedance
Nominal impedance here refers to the impedance of the speaker at a defined low frequency in free air. The impedance will change with loading and frequency.
In the days of valves or vacuum tubes, most loudspeakers had a nominal impedance of 16 Ω. Valve outputs require an output transformer to match the very high output impedance and voltage of the output valves to this lower impedance. These transformers were commonly tapped to allow matching of the output to a multiple loudspeaker setup. For example, two 16 Ω loudspeakers in parallel will give an impedance of 8 Ω.
Since the advent of solid-state transformerless outputs, these multiple-impedance outputs have become rare, and lower impedance loudspeakers more common. The most common nominal impedance for a single loudspeaker is now 8 Ω. Most solid-state amplifiers are designed to work with loudspeaker combinations of anything from 4 Ω to 8 Ω.
[edit] RF cables
Cables for transmission of high frequency signals usually come in one of two nominal impedances: Either 50 ohm or 75 ohm. This means that the characteristic impedance of the cable is close to one of these figures. The figure may vary slightly with frequency.
In practise, when the signal has a wavelength comparable to or less than the length of the cable, it becomes important that the nominal impedance of the cable be matched to the impedance of the line and load. This is often assured using termination resistors.
Unmatched high frequency cables may become resonators, and interactions between resonance signals may produce interference patterns. This is commonly seen when an inappropriate, damaged, or incorrectly connected cable if sufficient length is used to connect a television to a VCR's composite video jack. In lengths of above 20 feet, visible beat patterns will appear on the television if the cable does not have a nominal impedance of 75 ohms (a cheap audio cable for example). Replacing the composite connection with standard RG-6 fitted with appropriate connectors is usually the cheapest and most effective way to eliminate this problem.
Early computer networks used 50ohm cable, which is somewhat thinner and less durable than RG-6 75ohm cable. These networks were plagued with high frequency effects, often leading installers to have to manually "tune" the cable, by adjusting its length in order to cancel out reflections. These problems often occurred due to damaged or irregularly manufactured cable, poorly crimped connections, or excessive length or population of a particular line. These factors all affect the impedance of the line, increasing the likelihood and amplitude of reflections and interference.
[edit] Comparison with effective impedance
The effective impedance of input or output is the equivalent impedance of all of the output or input circuitry of a device lumped into one (imaginary) component. It should not be confused with nominal impedance. An impedance is a combination of resistance, capacitance, and inductance.
See impedance for a more technical discussion.
