Nominal impedance
Encyclopedia
Nominal impedance in electrical engineering
and audio engineering
refers to the approximate designed impedance
of an electrical circuit or device. The term is applied in a number of different fields, most often being encountered in respect of:
The actual impedance may vary quite considerably from the nominal figure with changes in frequency. In the case of cables, there is also variation along the length of the cable. It is usual practice to speak of nominal impedance as if it were a constant resistance, that is, it is invariant with frequency and has a zero reactive component, despite this often being far from the case. Depending on the field of application, nominal impedance is implicitly referring to a specific point on the frequency response of the circuit under consideration. This may be at low-frequency, mid-band or some other point and specific applications are discussed in the sections below.
In most applications, there are a number of values of nominal impedance that are recognised as being standard. The nominal impedance of components and circuits are often assigned one of these standard values, regardless of whether the measured impedance exactly corresponds to it. The item is assigned the nearest standard value.
equipment impedance to the characteristic impedance of the cable reduces reflections
(and they are eliminated altogether if the match is perfect) and power transfer is maximised. To this end, all cables and equipment started to be specified to a standard nominal impedance. The earliest, and still the most widespread, standard is 600 Ω, originally used for telephony
. It has to be said that the choice of this figure had more to do with the way telephones were interfaced into the local exchange
than any characteristic of the local telephone cable. Telephones (old style
analogue telephones) connect to the exchange through twisted pair cabling. Each leg of the pair is connected to a relay
coil which detect the signalling
on the line (dialling, handset off-hook
etc). The other end of one coil is connected to volts and the second coil is connected to ground. A telephone exchange relay coil is around 300 Ω so the two of them together are terminating the line in 600 Ω.
The wiring to the subscriber in telephone networks is generally done in twisted pair cable. This format at audio frequencies, and especially at the more restricted telephone band frequencies, is far from constant. It is possible to manufacture this kind of cable to have a 600 Ω characteristic impedance but it will only be this value at one specific frequency. This might be quoted as a nominal 600 Ω impedance at 800 Hz or 1 kHz. Below this frequency the characteristic impedance rapidly rises and become more and more dominated by the ohmic resistance of the cable as the frequency falls. At the bottom of the audio band the impedance can be several tens of kilohms. On the other hand, at high frequency in the MHz region, the characteristic impedance flattens out to something almost constant. The reason for this response is explained at primary line constants.
Local area network
s (LANs) commonly use a similar kind of twisted pair cable, but screened and manufactured to tighter tolerances than is necessary for telephony. Even though it has a very similar impedance to telephone cable, the nominal impedance is rated at 100 Ω. This is because the LAN data is in a higher frequency band where the characteristic impedance is substantially flat and mostly resistive.
Standardisation of line nominal impedance led to two-port networks such as filters being designed to a matching nominal impedance. The nominal impedance of low-pass symmetrical T- or Pi-filter sections (or more generally, image filter sections) is defined as the limit of the filter image impedance as the frequency approaches zero and is given by,
where L and C are as defined in constant k filter
. As can be seen from the expression, this impedance is purley resistive. This filter transformed to a band-pass filter
will have an impedance equal to the nominal impedance at resonance rather than low frequency. This nominal impedance of filters will generally be the same as the nominal impedance of the circuit or cable that the filter is working into.
While 600 Ω is an almost universal standard in telephony for local presentation at customer's premises from the exchange, for long distance transmission on trunk lines between exchanges other standard nominal impedances are used and are usually lower, such as 150 Ω.
(RF) and microwave
engineering, by far and away the most common transmission line standard is 50 Ω coaxial cable
(coax), which is an unbalanced line
. 50 Ω first arose as a nominal impedance during world war two work on radar
and is a compromise between two requirements. This standard was the work of the wartime US joint Army-Navy RF Cable Coordinating Committee. The first requirement is for minimum loss. The loss of coaxial cable is given by,
neper
s/metre
where R is the loop resistance per metre and Z0 is the characteristic impedance. Making the diameter of the inner conductor larger will decrease R and decreasing R decreases the loss. On the other hand, Z0 depends on the ratio of the diameters of outer and inner conductors (Dr) and will decrease with increasing inner conductor diameter thus increasing the loss. There is a specific value of Dr for which the loss is a minimum and this turns out to be 3.6. For an air dielectric
coax (wartime coax was rigid air insulated pipe and this remained the case for some time afterwards) this corresponds to a characteristic impedance of 77 Ω. The second requirement is for maximum power handling and was an important requirement for radar. This is not the same condition as minimum loss because power handling is usually limited by the breakdown voltage
of the dielectric. However, there is a similar compromise in terms of the ratio of conductor diameters. Making the inner conductor too large results in a thin insulator which breaks down at a lower voltage. On the other hand, making the inner conductor too small results in higher electric field strength near the inner conductor (because the field lines are closer together on the smaller circumference) and again reduces the breakdown voltage. The ideal ratio, Dr, for maximum power handling turns out to be 1.65 and corresponds to a characteristic impedance of 30 Ω in air. 50 Ω was arrived at by taking the geometric mean of these two figures;
and then rounding to a convenient whole number.
Wartime production of coax, and for a period afterwards, tended to use standard plumbing pipe sizes for the outer conductor and standard AWG
sizes for the inner conductor. This resulted in coax that was nearly, but not quite, 50 Ω. Matching is a much more critical requirement at RF than it is at voice frequencies, so when cable started to become available that was truly 50 Ω a need arose for matching circuits to interface between the new cables and legacy equipment, such as the rather strange 51.5 Ω to 50 Ω matching network.
While 30 Ω cable is highly desirable for its power handling capabilities, it has never been in commercial production because the large size of inner conductor makes it difficult to manufacture. This is not the case with 77 Ω cable. 75 Ω nominal impedance cable has been in use from an early period in telecommunications for its low loss characteristic. According to Stephen Lampen of Belden
Wire & Cable 75 Ω was chosen as the nominal impedance rather than 77 Ω because it corresponded to a standard AWG wire size for the inner conductor. 75 Ω is now the near universal standard nominal impedance for coaxial video interfaces and transmission lines.
is, in fact, a myth. It is true, however, that several common antennae are easily matched to these cables. A quarter wavelength monopole
has an impedance of 36.5 Ω, and a half wavelength dipole
has an impedance of 72 Ω. A half-wavelength folded dipole, commonly seen on television antennae, on the other hand, has an impedance four times that of a dipole, that is 288 Ω. The 0.5λ dipole and the 0.5λ folded dipole are commonly taken as having nominal impedances of 75 Ω and 300 Ω respectively.
). Unfortunately, there is no easy, non-destructive method of directly measuring impedance along a cables' length. It can, however, be indicated indirectly by measuring reflections, that is, return loss
. Return loss by itself does not reveal much, since the cable design will have some intrinsic return loss anyway due to not having a purely resistive characteristic impedance. The technique used is to carefully adjust the cable termination to obtain as close a match as possible and then to measure the variation of return loss with frequency. The minimum return loss so measured is called the structural return loss (SRL). SRL is a measure of a cables' adherence to its nominal impedance but it is not a direct correspondence, errors further from the generator have less effect on SRL than those close to it. The measurement must also be carried out at all in-band frequencies to be significant. The reason for this is that equally spaced errors introduced by the manufacturing process will cancel and be invisible, or at least much reduced, at certain frequencies due to quarter wave impedance transformer
action.
, considerations of matching and reflections dictate that a telecommunications standard is used, which would normally mean using 600 Ω nominal impedance (although other standards are sometimes encountered, such as sending at 75 Ω and receiving at 600 Ω which has bandwidth advantages). The nominal impedance of the transmission line and of the amplifiers and equalisers in the transmission chain will all be the same value.
Nominal impedance is used, however, to characterise the transducer
s of an audio system, such as its microphones and loudspeakers. It is important that these are connected to a circuit capable of dealing with impedances in the appropriate range and assigning a nominal impedance is a convenient way of quickly determining likely incompatibilities. Loudspeakers and microphones are dealt with in separate sections below.
impedances are kept relatively low compared with other audio components so that the required audio power can be transmitted without using inconveniently (and dangerously) high voltages. The most common nominal impedance for loudspeakers is 8 Ω. Also used are 4 Ω and 16 Ω. The once common 16 Ω is now mostly reserved for high frequency compression driver
s since the high frequency end of the audio spectrum does not usually require so much power to reproduce.
The impedance of a loudspeaker is not constant across all frequencies. In a typical loudspeaker the impedance will rise with increasing frequency from its dc value (see diagram) until it reaches a point of mechanical resonance. Following resonance, the impedance falls to a minimum and then begins to rise again. Speakers are usually designed to operate at frequencies above their resonance, and for this reason it is the usual practice to define nominal impedance at this minimum and then round to the nearest standard value. The ratio of the peak resonant frequency to the nominal impedance can be as much as 4:1. It is, however, still perfectly possible for the low frequency impedance to actually be lower than the nominal impedance. A given audio amplifier may not be capable of driving this low frequency impedance even though it is capable of driving the nominal impedance, a problem that can be solved either with the use of crossover filter
s or underrating the amplifier supplied.
In the days of valves or vacuum tube
s, 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.
and there are correspondingly large differences in impedance between them. They range from the very low impedance of ribbon microphone
s (can be less than one ohm) to the very large impedance of piezoelectric microphones which are measured in megohms. The Electronic Industries Alliance
(EIA) has defined a number of standard microphone nominal impedances to aid categorisation of microphones.
The International Electrotechnical Commission
defines a similar set of nominal impedances, but also has a coarser classification of low, medium and high impedance with medium being 600 Ω to 10 kΩ.
inputs are usually high impedance so that they only minimally affect the circuit being measured when connected. However, the input impedance is made a specific nominal value, rather than arbitrarily high, because of the common use of X10 probes. A common value for oscilloscope nominal impedance is 1 MΩ resistance and 20 pF capacitance. With a known input impedance to the oscilloscope, the probe designer can ensure that the probe input impedance is exactly ten times this figure (actually oscilloscope plus probe cable impedance). Since the impedance included the input capacitance and the probe is an impedance divider circuit, the result is that the waveform being measured is not distorted by the RC circuit formed by the probe resistance and the capacitance of the input (or the cable capacitance which is generally higher).
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
and audio engineering
Audio engineering
An audio engineer, also called audio technician, audio technologist or sound technician, is a specialist in a skilled trade that deals with the use of machinery and equipment for the recording, mixing and reproduction of sounds. The field draws on many artistic and vocational areas, including...
refers to the approximate designed impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
of an electrical circuit or device. The term is applied in a number of different fields, most often being encountered in respect of:
- The nominal value of the characteristic impedanceCharacteristic impedanceThe characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
of a cable of other form of transmission lineTransmission lineIn communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
. - The nominal value of the inputInput impedanceThe input impedance of an electrical network is the equivalent impedance "seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law...
, outputOutput impedanceThe output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
or image impedanceImage impedanceImage impedance is a concept used in electronic network design and analysis and most especially in filter design. The term image impedance applies to the impedance seen looking in to the ports of a network. Usually a two-port network is implied but the concept is capable of being extended to...
of a port of a networkTwo-port networkA two-port network is an electrical circuit or device with two pairs of terminals connected together internally by an electrical network...
, especially a network intended for use with a transmission line, such as filtersElectronic filterElectronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
, equalisers and amplifierAmplifierGenerally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s. - The nominal value of the input impedance of a radio frequency antennaAntenna (radio)An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
The actual impedance may vary quite considerably from the nominal figure with changes in frequency. In the case of cables, there is also variation along the length of the cable. It is usual practice to speak of nominal impedance as if it were a constant resistance, that is, it is invariant with frequency and has a zero reactive component, despite this often being far from the case. Depending on the field of application, nominal impedance is implicitly referring to a specific point on the frequency response of the circuit under consideration. This may be at low-frequency, mid-band or some other point and specific applications are discussed in the sections below.
In most applications, there are a number of values of nominal impedance that are recognised as being standard. The nominal impedance of components and circuits are often assigned one of these standard values, regardless of whether the measured impedance exactly corresponds to it. The item is assigned the nearest standard value.
600 Ω
Nominal impedance first started to be specified in the early days of telecommunications. At first amplifiers were not available and when they did become available they were expensive. It was consequently necessary to squeeze every last drop of transmitted power from the cable at the receiving end in order to maximise the lengths of cables that could be installed. It also became apparent that reflections on the transmission line would severely limit the bandwidth that could be used or the distance that it was practicable to transmit. MatchingImpedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
equipment impedance to the characteristic impedance of the cable reduces reflections
Signal reflection
Signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber, some of the signal power may be reflected back to its origin rather than being carried all the way along the cable to the far end. This happens because imperfections in the...
(and they are eliminated altogether if the match is perfect) and power transfer is maximised. To this end, all cables and equipment started to be specified to a standard nominal impedance. The earliest, and still the most widespread, standard is 600 Ω, originally used for telephony
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
. It has to be said that the choice of this figure had more to do with the way telephones were interfaced into the local exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
than any characteristic of the local telephone cable. Telephones (old style
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
analogue telephones) connect to the exchange through twisted pair cabling. Each leg of the pair is connected to a relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
coil which detect the signalling
Line signaling
Line signaling is a class of telecommunications signaling protocols. Line signaling is responsible for off-hook, ringing signal, answer, ground start, on-hook unidirectional supervision messaging in each direction from calling party to called party and vice versa...
on the line (dialling, handset off-hook
Off-hook
In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...
etc). The other end of one coil is connected to volts and the second coil is connected to ground. A telephone exchange relay coil is around 300 Ω so the two of them together are terminating the line in 600 Ω.
The wiring to the subscriber in telephone networks is generally done in twisted pair cable. This format at audio frequencies, and especially at the more restricted telephone band frequencies, is far from constant. It is possible to manufacture this kind of cable to have a 600 Ω characteristic impedance but it will only be this value at one specific frequency. This might be quoted as a nominal 600 Ω impedance at 800 Hz or 1 kHz. Below this frequency the characteristic impedance rapidly rises and become more and more dominated by the ohmic resistance of the cable as the frequency falls. At the bottom of the audio band the impedance can be several tens of kilohms. On the other hand, at high frequency in the MHz region, the characteristic impedance flattens out to something almost constant. The reason for this response is explained at primary line constants.
Local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
s (LANs) commonly use a similar kind of twisted pair cable, but screened and manufactured to tighter tolerances than is necessary for telephony. Even though it has a very similar impedance to telephone cable, the nominal impedance is rated at 100 Ω. This is because the LAN data is in a higher frequency band where the characteristic impedance is substantially flat and mostly resistive.
Standardisation of line nominal impedance led to two-port networks such as filters being designed to a matching nominal impedance. The nominal impedance of low-pass symmetrical T- or Pi-filter sections (or more generally, image filter sections) is defined as the limit of the filter image impedance as the frequency approaches zero and is given by,
where L and C are as defined in constant k filter
Constant k filter
Constant k filters, also k-type filters, are a type of electronic filter designed using the image method. They are the original and simplest filters produced by this methodology and consist of a ladder network of identical sections of passive components...
. As can be seen from the expression, this impedance is purley resistive. This filter transformed to a band-pass filter
Band-pass filter
A band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range.Optical band-pass filters are of common usage....
will have an impedance equal to the nominal impedance at resonance rather than low frequency. This nominal impedance of filters will generally be the same as the nominal impedance of the circuit or cable that the filter is working into.
While 600 Ω is an almost universal standard in telephony for local presentation at customer's premises from the exchange, for long distance transmission on trunk lines between exchanges other standard nominal impedances are used and are usually lower, such as 150 Ω.
50 Ω and 75 Ω
In the field of radio frequencyRadio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) and microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
engineering, by far and away the most common transmission line standard is 50 Ω coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
(coax), which is an unbalanced line
Unbalanced line
In Electrical engineering, an unbalanced line is a transmission line, usually coaxial cable, whose conductors have unequal impedances with respect to ground; as opposed to a balanced line.Microstrip and single-wire lines are also unbalanced lines....
. 50 Ω first arose as a nominal impedance during world war two work on radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
and is a compromise between two requirements. This standard was the work of the wartime US joint Army-Navy RF Cable Coordinating Committee. The first requirement is for minimum loss. The loss of coaxial cable is given by,
neperNeper
The neper is a logarithmic unit for ratios of measurements of physical field and power quantities, such as gain and loss of electronic signals. It has the unit symbol Np. The unit's name is derived from the name of John Napier, the inventor of logarithms...
s/metre
where R is the loop resistance per metre and Z0 is the characteristic impedance. Making the diameter of the inner conductor larger will decrease R and decreasing R decreases the loss. On the other hand, Z0 depends on the ratio of the diameters of outer and inner conductors (Dr) and will decrease with increasing inner conductor diameter thus increasing the loss. There is a specific value of Dr for which the loss is a minimum and this turns out to be 3.6. For an air dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
coax (wartime coax was rigid air insulated pipe and this remained the case for some time afterwards) this corresponds to a characteristic impedance of 77 Ω. The second requirement is for maximum power handling and was an important requirement for radar. This is not the same condition as minimum loss because power handling is usually limited by the breakdown voltage
Breakdown voltage
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse...
of the dielectric. However, there is a similar compromise in terms of the ratio of conductor diameters. Making the inner conductor too large results in a thin insulator which breaks down at a lower voltage. On the other hand, making the inner conductor too small results in higher electric field strength near the inner conductor (because the field lines are closer together on the smaller circumference) and again reduces the breakdown voltage. The ideal ratio, Dr, for maximum power handling turns out to be 1.65 and corresponds to a characteristic impedance of 30 Ω in air. 50 Ω was arrived at by taking the geometric mean of these two figures;
and then rounding to a convenient whole number.
Wartime production of coax, and for a period afterwards, tended to use standard plumbing pipe sizes for the outer conductor and standard AWG
American wire gauge
American wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire...
sizes for the inner conductor. This resulted in coax that was nearly, but not quite, 50 Ω. Matching is a much more critical requirement at RF than it is at voice frequencies, so when cable started to become available that was truly 50 Ω a need arose for matching circuits to interface between the new cables and legacy equipment, such as the rather strange 51.5 Ω to 50 Ω matching network.
While 30 Ω cable is highly desirable for its power handling capabilities, it has never been in commercial production because the large size of inner conductor makes it difficult to manufacture. This is not the case with 77 Ω cable. 75 Ω nominal impedance cable has been in use from an early period in telecommunications for its low loss characteristic. According to Stephen Lampen of Belden
Belden Inc.
Belden Inc.Belden Inc. designs, manufactures, and markets highly engineered signal transmission products for demanding applications. These cable, connectivity and networking products serve the industrial automation, enterprise, security, transportation, infrastructure, residential and consumer...
Wire & Cable 75 Ω was chosen as the nominal impedance rather than 77 Ω because it corresponded to a standard AWG wire size for the inner conductor. 75 Ω is now the near universal standard nominal impedance for coaxial video interfaces and transmission lines.
Radio antennae
The widespread idea that 50 Ω and 75 Ω nominal impedances are connected with the input impedance of various antennaeAntenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
is, in fact, a myth. It is true, however, that several common antennae are easily matched to these cables. A quarter wavelength monopole
Monopole antenna
A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output voltage is taken,...
has an impedance of 36.5 Ω, and a half wavelength dipole
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
has an impedance of 72 Ω. A half-wavelength folded dipole, commonly seen on television antennae, on the other hand, has an impedance four times that of a dipole, that is 288 Ω. The 0.5λ dipole and the 0.5λ folded dipole are commonly taken as having nominal impedances of 75 Ω and 300 Ω respectively.
Cable quality
One measure of cable manufacturing and installation quality is how closely the characteristic impedance adheres to the nominal impedance along its length. Impedance changes can be caused by variations in geometry along the cable length. In turn, these can be caused by a faulty manufacturing process or by faulty installation (such as not observing limits on bend radiiBend radius
Bend radius, which is measured to the inside curvature, is the minimum radius one can bend a pipe, tube, sheet, cable or hose without kinking it, damaging it, or shortening its life. The smaller the bend radius, the greater is the material flexibility...
). Unfortunately, there is no easy, non-destructive method of directly measuring impedance along a cables' length. It can, however, be indicated indirectly by measuring reflections, that is, return loss
Return loss
In telecommunications, return loss or reflection loss is the loss of signal power resulting from the reflection caused at a discontinuity in a transmission line or optical fiber. This discontinuity can be a mismatch with the terminating load or with a device inserted in the line...
. Return loss by itself does not reveal much, since the cable design will have some intrinsic return loss anyway due to not having a purely resistive characteristic impedance. The technique used is to carefully adjust the cable termination to obtain as close a match as possible and then to measure the variation of return loss with frequency. The minimum return loss so measured is called the structural return loss (SRL). SRL is a measure of a cables' adherence to its nominal impedance but it is not a direct correspondence, errors further from the generator have less effect on SRL than those close to it. The measurement must also be carried out at all in-band frequencies to be significant. The reason for this is that equally spaced errors introduced by the manufacturing process will cancel and be invisible, or at least much reduced, at certain frequencies due to quarter wave impedance transformer
Quarter wave impedance transformer
A quarter-wave impedance transformer, often written as λ/4 impedance transformer, is a component used in electrical engineering consisting of a length of transmission line or waveguide exactly one-quarter of a wavelength long and terminated in some known impedance. The device presents at its...
action.
Audio systems
For the most part, audio systems both professional and domestic, have their components interconnected with low impedance outputs connected to high impedance inputs. These impedances are poorly defined and nominal impedances are not usually assigned for this kind of connection. The exact impedances make little difference to performance as long as the latter is many times larger than the former. This is a common interconnection scheme, not just for audio, but for electronic units in general which form part of a larger equipment or are only connected over a short distance. Where audio needs to be transmitted over large distances, which is often the case in broadcast engineeringBroadcast engineering
Broadcast engineering is the field of electrical engineering, and now to some extent computer engineering and information technology, which deals with radio and television broadcasting...
, considerations of matching and reflections dictate that a telecommunications standard is used, which would normally mean using 600 Ω nominal impedance (although other standards are sometimes encountered, such as sending at 75 Ω and receiving at 600 Ω which has bandwidth advantages). The nominal impedance of the transmission line and of the amplifiers and equalisers in the transmission chain will all be the same value.
Nominal impedance is used, however, to characterise the transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
s of an audio system, such as its microphones and loudspeakers. It is important that these are connected to a circuit capable of dealing with impedances in the appropriate range and assigning a nominal impedance is a convenient way of quickly determining likely incompatibilities. Loudspeakers and microphones are dealt with in separate sections below.
Loudspeakers
LoudspeakerLoudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
impedances are kept relatively low compared with other audio components so that the required audio power can be transmitted without using inconveniently (and dangerously) high voltages. The most common nominal impedance for loudspeakers is 8 Ω. Also used are 4 Ω and 16 Ω. The once common 16 Ω is now mostly reserved for high frequency compression driver
Compression driver
A compression driver is a type of loudspeaker driver which uses the technique of "compression" to achieve high efficiencies. In this context compression refers to the fact that the area of the loudspeaker diaphragm is significantly larger than the aperture through which the sound is radiated....
s since the high frequency end of the audio spectrum does not usually require so much power to reproduce.
The impedance of a loudspeaker is not constant across all frequencies. In a typical loudspeaker the impedance will rise with increasing frequency from its dc value (see diagram) until it reaches a point of mechanical resonance. Following resonance, the impedance falls to a minimum and then begins to rise again. Speakers are usually designed to operate at frequencies above their resonance, and for this reason it is the usual practice to define nominal impedance at this minimum and then round to the nearest standard value. The ratio of the peak resonant frequency to the nominal impedance can be as much as 4:1. It is, however, still perfectly possible for the low frequency impedance to actually be lower than the nominal impedance. A given audio amplifier may not be capable of driving this low frequency impedance even though it is capable of driving the nominal impedance, a problem that can be solved either with the use of crossover filter
Audio crossover
Audio crossovers are a class of electronic filter used in audio applications. Most individual loudspeaker drivers are incapable of covering the entire audio spectrum from low frequencies to high frequencies with acceptable relative volume and lack of distortion so most hi-fi speaker systems use a...
s or underrating the amplifier supplied.
In the days of valves or vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s, 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.
Microphones
There are a large number of different types of microphoneMicrophone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
and there are correspondingly large differences in impedance between them. They range from the very low impedance of ribbon microphone
Ribbon microphone
A ribbon microphone is a type of dynamic microphone that uses a thin aluminum, duraluminum or nanofilm ribbon placed between the poles of a magnet to generate voltages by electromagnetic induction...
s (can be less than one ohm) to the very large impedance of piezoelectric microphones which are measured in megohms. The Electronic Industries Alliance
Electronic Industries Alliance
The Electronic Industries Alliance was a standards and trade organization composed as an alliance of trade associations for electronics manufacturers in the United States. They developed standards to ensure the equipment of different manufacturers was compatible and interchangeable...
(EIA) has defined a number of standard microphone nominal impedances to aid categorisation of microphones.
| Range (Ω) | EIA nominal impedance (Ω) |
|---|---|
| 20-80 | 38 |
| 80-300 | 150 |
| 300-1250 | 600 |
| 1250-4500 | 2400 |
| 4500-20,000 | 9600 |
| 20,000-70,000 | 40,000 |
The International Electrotechnical Commission
International Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
defines a similar set of nominal impedances, but also has a coarser classification of low, medium and high impedance with medium being 600 Ω to 10 kΩ.
Oscilloscopes
OscilloscopeOscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
inputs are usually high impedance so that they only minimally affect the circuit being measured when connected. However, the input impedance is made a specific nominal value, rather than arbitrarily high, because of the common use of X10 probes. A common value for oscilloscope nominal impedance is 1 MΩ resistance and 20 pF capacitance. With a known input impedance to the oscilloscope, the probe designer can ensure that the probe input impedance is exactly ten times this figure (actually oscilloscope plus probe cable impedance). Since the impedance included the input capacitance and the probe is an impedance divider circuit, the result is that the waveform being measured is not distorted by the RC circuit formed by the probe resistance and the capacitance of the input (or the cable capacitance which is generally higher).