Nominal impedance in
electrical engineering
Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems that use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
and
audio engineering
Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to:
Sound
*Audio signal, an electrical representation of sound
* Audio frequency, a frequency in the audio spectrum
*Digital audio, representation of soun ...
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
nominal value
In economics, nominal value refers to value measured in terms of absolute money amounts, whereas real value is considered and measured against the actual goods or services for which it can be exchanged at a given time. Real value takes into acc ...
of the
characteristic impedance
The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a wave travelling in one direction along the line in the absence of reflections in th ...
of a cable or other form of
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
.
* The nominal value of the
input,
output
Output may refer to:
* The information produced by a computer, see Input/output
* An output state of a system, see state (computer science)
* Output (economics), the amount of goods and services produced
** Gross output in economics, the valu ...
or
image impedance of a
port
A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Hamburg, Manch ...
of a network, especially a network intended for use with a transmission line, such as
filters
Filtration is a physical process that separates solid matter and fluid from a mixture.
Filter, filtering, filters or filtration may also refer to:
Science and technology
Computing
* Filter (higher-order function), in functional programming
* Fil ...
,
equalisers and
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power su ...
s.
* The nominal value of the input impedance of a
radio frequency antenna
The actual impedance may vary quite considerably from the nominal figure with changes in frequency. In the case of cables and other
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
s, there is also variation along the length of the cable, if it is not properly terminated.
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 a component or circuit is 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
Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...
. At first, amplifiers were not available and, when they did become available, they were expensive. It was consequently necessary to achieve maximum power transfer from the cable at the receiving end in order to maximize 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.
Matching 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
Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunications services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is ...
. The choice of this figure had more to do with the way telephones were interfaced into the local
exchange
Exchange or exchanged may refer to:
Arts, entertainment and media Film and television
* Exchange (film), or ''Deep Trap'', 2015 South Korean psychological thriller
* Exchanged (film), 2019 Peruvian fantasy comedy
* Exchange (TV program), 2021 Sou ...
than any characteristic of the local telephone cable. Telephones (
old style
Old Style (O.S.) and New Style (N.S.) indicate dating systems before and after a calendar change, respectively. Usually, they refer to the change from the Julian calendar to the Gregorian calendar as enacted in various European countries betwe ...
analogue telephones) connect to the exchange through twisted pair cabling. Each leg of the pair is connected to a
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switc ...
coil which detects the
signalling
A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology.
In ...
on the line (
dialling, handset
off-hook
In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing ...
etc.). The other end of one coil is connected to a supply voltage 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. Its impedance 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 becomes 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 lies in
primary line constants.
[Schmitt, p.301.]
Local area network
A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, campus, or building, and has its network equipment and interconnects locally managed. LANs facilitate the distribution of da ...
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 network
In electronics, a two-port network (a kind of four-terminal network or quadripole) is an electrical network (i.e. a circuit) or device with two ''pairs'' of Terminal (electronics), terminals to connect to external circuits. Two terminals consti ...
s such as filters being designed to a matching nominal impedance. The nominal impedance of low-pass
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
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 filters, also k-type filters, are a type of electronic filter designed using the Image impedance, image method. They are the original and simplest filters produced by this methodology and consist of a Electronic filter topology#Ladder ...
. This impedance is purely resistive. This filter, when transformed to a band-pass filter
A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects ( attenuates) frequencies outside that range.
It is the inverse of a '' band-stop filter''.
Description
In electronics and s ...
, 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.[Whitaker, p.115.]
50 Ω and 75 Ω
In the field of radio frequency
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
(RF) and microwave
Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
engineering, by far and away the most common transmission line standard is 50 Ω coaxial cable
Coaxial cable, or coax (pronounced ), is a type of electrical cable consisting of an inner Electrical conductor, conductor surrounded by a concentric conducting Electromagnetic shielding, shield, with the two separated by a dielectric (Insulat ...
(coax), which is an unbalanced line. 50 Ω first arose as a nominal impedance during World War II
World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
work on radar
Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
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
The neper (symbol: Np) is a logarithmic unit for ratios of measurements of physical field and power quantities, such as gain and loss of electronic signals. 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 ''Z''0 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, ''Z''0 depends on the ratio of the diameters of outer and inner conductors (''D''r) and will decrease with increasing inner conductor diameter thus increasing the loss. There is a specific value of ''D''r for which the loss is a minimum, which turns out to be 3.6. For an air dielectric
In electromagnetism, a dielectric (or dielectric medium) is an Insulator (electricity), electrical insulator that can be Polarisability, polarised by an applied electric field. When a dielectric material is placed in an electric field, electric ...
coax, this corresponds to a characteristic impedance of 77 Ω. The coax produced during the WW2 was rigid air-insulated pipe, and this remained the case for some time afterwards. 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
The breakdown voltage of an insulator (electrical), insulator is the minimum voltage that causes a portion of an insulator to experience electrical breakdown and become electrically Conductor (material), conductive.
For diodes, the breakdown vo ...
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 same field energy is accumulated around smaller conductor surface) and again reduces the breakdown voltage. The ideal ratio, ''D''r, for maximum power handling, is 1.65 and corresponds to a characteristic impedance of 30 Ω in air. The 50 Ω impedance is the geometric mean of these two figures;
:
and then rounding to a convenient whole number.[Golio, p.6-41.][Breed, pp.6–7.]
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 radio frequencies than it is at voice frequencies, so when cable started to become available, that was truly 50 Ω and 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. Cable with 75 Ω nominal impedance 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. For coax video cables and interfaces, 75 Ω is now the near universal standard nominal impedance.][
]
Radio antennae
The widespread idea that 50 Ω and 75 Ω cable nominal impedances arose in connection with the input impedance of various antennae is a myth. Several common antennae are easily matched to cables with these nominal impedances.[ A quarter wavelength monopole in free space has an impedance of 36.5 Ω, and a half wavelength ]dipole
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
* An electric dipole moment, electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple ...
in free space has an impedance of 72 Ω. A half-wavelength folded dipole, commonly seen on television antennae, on the other hand, has a 288 Ω impedance—four times that of a straight-line dipole. The λ dipole and the λ folded dipole are commonly taken as having nominal impedances of 75 Ω and 300 Ω, respectively.
An installed antenna's feed-point impedance varies above and below the quoted value, depending on its installation height above the ground and the electrical properties of the surrounding earth.
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 radii. Unfortunately, there is no easy, non-destructive method of directly measuring impedance along a cable's length. It can, however, be indicated indirectly by measuring reflections, that is, return loss
In telecommunications, return loss is a measure in relative terms of the power of the signal reflected by a discontinuity in a transmission line or optical fiber. This discontinuity can be caused by a mismatch between the termination or load co ...
. 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, as 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.
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 engineering
Broadcast engineering or radio engineering is the field of electrical engineering, and now to some extent computer engineering and information technology, which deals with radio and television broadcasting. Audio engineering and RF engineering a ...
, 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
A transducer is a device that Energy transformation, converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.
Transducers are often employed at the boundaries of automation, M ...
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
Loudspeaker
A loudspeaker (commonly referred to as a speaker or, more fully, a speaker system) is a combination of one or more speaker drivers, an enclosure, and electrical connections (possibly including a crossover network). The speaker driver is an ...
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
A compression driver is a small specialized diaphragm loudspeaker which generates the sound in a horn loudspeaker. It is attached to an horn (acoustic), acoustic horn, a widening duct which serves to radiate the sound efficiently into the air. ...
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, as shown in the diagram, until it reaches a point of its 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 filters or underrating the amplifier supplied.
In the days of valves (vacuum tube
A vacuum tube, electron tube, thermionic valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. It ...
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 amplifiers whose outputs require no transformer, the once-common 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 microphone
A microphone, colloquially called a mic (), or mike, is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publi ...
and there are correspondingly large differences in impedance between them. They range from the very low impedance of ribbon microphones (can be less than one ohm) to the very large impedance of piezoelectric microphones which are measured in megohms. The Electronic Industries Alliance
The Electronic Industries Alliance (EIA; until 1997 Electronic Industries Association) was an American standards organization, standards and trade organization composed as an alliance of trade associations for electronics manufacturers in the ...
(EIA) has defined a number of standard microphone nominal impedances to aid categorisation of microphones.
The International Electrotechnical Commission
The International Electrotechnical Commission (IEC; ) is an international standards organization that prepares and publishes international standards for all electrical, electronics, electronic and related technologies. IEC standards cover a va ...
defines a similar set of nominal impedances, but also has a coarser classification of low (less than 600 Ω), medium (600 Ω to 10 kΩ) and high (more than 10 kΩ) impedances.
Oscilloscopes
Oscilloscope
An oscilloscope (formerly known as an oscillograph, informally scope or O-scope) is a type of electronic test instrument that graphically displays varying voltages of one or more signals as a function of time. Their main purpose is capturing i ...
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).[O'Dell, pp.72–79.]
References
Bibliography
*Glen Ballou, ''Handbook for Sound Engineers'', Gulf Professional Publishing, 2005 .
*John Bird, ''Electrical Circuit Theory and Technology'', Elsevier, 2007 .
*Gary Breed
"There's nothing magic about 50 ohms"
''High Frequency Electronics'', pp. 6–7, June 2007, Summit Technical Media LLC
archived
26 June 2015.
*Wai-Kai Chen, ''The Electrical Engineering Handbook'', Academic Press, 2005 .
*Walter S. Ciciora, ''Modern Cable Television Technology: Video, Voice, and Data Communications'', Morgan Kaufmann, 2004 .
*Gary Davis, Ralph Jones, ''The Sound Reinforcement Handbook'', Hal Leonard Corporation, 1989 .
* John M. Eargle, Chris Foreman, ''Audio engineering for Sound Reinforcement'', Hal Leonard Corporation, 2002, .
*John Michael Golio, ''The RF and Microwave Handbook'', CRC Press, 2001 .
*Rudolf F. Graf, ''Modern Dictionary of Electronics'', Newnes, 1999 .
*R.R. Gulati, ''Modern Television Practice Principles, Technology and Servicing'', New Age International, .
*John D. Heys, ''Practical Wire Antennas'', Radio Society of Great Britain, 1989 .
*Ian Hickman, ''Oscilloscopes: How to Use Them, How They Work'', Newnes, 2001 .
*Stephen Lampen, ''Wire, Cable and Fiber Optics for Video and Audio Engineers'', McGraw-Hill 1997 .
*A.K.Maini, ''Electronic Projects For Beginners'', Pustak Mahal, 1997 .
*Nicholas M. Maslin, ''HF Communications: a Systems Approach'', CRC Press, 1987 .
*Thomas Henry O'Dell, ''Circuits for Electronic Instrumentation'', Cambridge University Press, 1991 .
*R. Tummala, E. J. Rymaszewski (ed), Alan G. Klopfenstein, ''Microelectronics Packaging Handbook'', Volume 3, Springer, 1997 .
*Ron Schmitt, ''Electromagnetics Explained: a Handbook for Wireless/RF, EMC, and High-speed Electronics'', Newnes, 2002 .
*Scott Hunter Stark, ''Live Sound Reinforcement: a Comprehensive Guide to P.A. and Music Reinforcement Systems and Technology'', Hal Leonard Corporation, 1996 .
*John Vasey, ''Concert Sound and Lighting Systems'', Focal Press, 1999 .
*Menno van der Veen, ''Modern High-end Valve Amplifiers: Based on Toroidal Output Transformers'', Elektor International Media, 1999 .
*Jerry C. Whitaker, ''Television Receivers'', McGraw-Hill Professional, 2001 {{ISBN, 0-07-138042-6.
Electrical parameters
Audio amplifier specifications