Nominal impedance
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 nominal value of the characteristic impedance of a cable or other form of transmission line.
- The nominal value of the input, output or image impedance of a port of a network, especially a network intended for use with a transmission line, such as filters, equalisers and amplifiers.
- 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 lines, 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,[1] 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.[2]
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 [edit]
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 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
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
Local area networks (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
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. This impedance is purely resistive. This filter, when 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.[5]
While 600
50 Ω and 75 Ω [edit]
In the field of radio frequency (RF) and microwave engineering, by far and away the most common transmission line standard is 50
- nepers/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, which turns out to be 3.6. For an air dielectric coax, this corresponds to a characteristic impedance of 77
and then rounding to a convenient whole number.[7][8]
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
While 30
Radio antennae[edit]
The widespread idea that 50
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.[14][15]
Cable quality[edit]
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. 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.[16][17]
Audio systems[edit]
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.[18] 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, considerations of matching and reflections dictate that a telecommunications standard is used, which would normally mean using 600
Nominal impedance is used, however, to characterise the transducers 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[edit]
Loudspeaker 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
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.[22] 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.[23][24] The ratio of the peak resonant frequency to the nominal impedance can be as much as 4:1.[25] It is, however, still perfectly possible for the low frequency impedance to actually be lower than the nominal impedance.[19] 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.[26]
In the days of valves (vacuum tubes), most loudspeakers had a nominal impedance of 16
Microphones[edit]
There are a large number of different types of microphone 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 (EIA) has defined[28] a number of standard microphone nominal impedances to aid categorisation of microphones.[29]
Range ( |
EIA nominal impedance ( |
---|---|
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 defines a similar set of nominal impedances, but also has a coarser classification of low (less than 600
Oscilloscopes[edit]
Oscilloscope 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 M
References[edit]
- ^ Maslin, p.78
- ^ Graf, p.506.
- ^ Schmitt, pp.301–302.
- ^ a b Schmitt, p.301.
- ^ Bird, pp.564, 569.
- ^ a b Whitaker, p.115.
- ^ a b Golio, p.6-41.
- ^ a b c Breed, pp.6–7.
- ^ Harmon Banning (W. L. Gore & Associates, Inc.), "The History of 50
Ω ", RF Cafe - ^ Steve Lampen, "Coax History" (mailing list), Contesting.com. Lampen is Technology Development Manager at Belden Wire & Cable Co. and is the author of Wire, Cable and Fiber Optics.
- ^ Chen, pp.574–575.
- ^ Gulati, p.424.
- ^ Gulati, p.426.
- ^ Heys (1989), pp. 3–4
- ^ Straw (2003)
- ^ Rymaszewski et al, p.407.
- ^ Ciciora, p.435.
- ^ Eargle & Foreman, p.83.
- ^ a b Davis&Jones, p.205.
- ^ Ballou, p.523.
- ^ Vasey, pp.34–35.
- ^ Davis&Jones, p.206.
- ^ Davis&Jones, p.233.
- ^ Stark, p.200.
- ^ Davis&Jones, p.91.
- ^ Ballou, pp.523, 1178.
- ^ van der Veen, p.27.
- ^ Electronic Industries Standard SE-105, August 1949.
- ^ Ballou, p.419.
- ^ International standard IEC 60268-4 Sound system equipment – Part 4: Microphones.
- ^ pp.97–98.
- ^ Hickman, pp.33–37.
- ^ O'Dell, pp.72–79.
Bibliography[edit]
- Glen Ballou, Handbook for Sound Engineers, Gulf Professional Publishing, 2005 ISBN 0-240-80758-8.
- John Bird, Electrical Circuit Theory and Technology, Elsevier, 2007 ISBN 0-7506-8139-X.
- 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 ISBN 0-12-170960-4.
- Walter S. Ciciora, Modern Cable Television Technology: Video, Voice, and Data Communications, Morgan Kaufmann, 2004 ISBN 1-55860-828-1.
- Gary Davis, Ralph Jones, The Sound Reinforcement Handbook, Hal Leonard Corporation, 1989 ISBN 0-88188-900-8.
- John M. Eargle, Chris Foreman, Audio engineering for Sound Reinforcement, Hal Leonard Corporation, 2002, ISBN 0-634-04355-2.
- John Michael Golio, The RF and Microwave Handbook, CRC Press, 2001 ISBN 0-8493-8592-X.
- Rudolf F. Graf, Modern Dictionary of Electronics, Newnes, 1999 ISBN 0-7506-9866-7.
- R.R. Gulati, Modern Television Practice Principles, Technology and Servicing, New Age International, ISBN 81-224-1360-9.
- John D. Heys, Practical Wire Antennas, Radio Society of Great Britain, 1989 ISBN 0-900612-87-8.
- Ian Hickman, Oscilloscopes: How to Use Them, How They Work, Newnes, 2001 ISBN 0-7506-4757-4.
- Stephen Lampen, Wire, Cable and Fiber Optics for Video and Audio Engineers, McGraw-Hill 1997 ISBN 0-07-038134-8.
- A.K.Maini, Electronic Projects For Beginners, Pustak Mahal, 1997 ISBN 81-223-0152-5.
- Nicholas M. Maslin, HF Communications: a Systems Approach, CRC Press, 1987 ISBN 0-273-02675-5.
- Thomas Henry O'Dell, Circuits for Electronic Instrumentation, Cambridge University Press, 1991 ISBN 0-521-40428-2.
- R. Tummala, E. J. Rymaszewski (ed), Alan G. Klopfenstein, Microelectronics Packaging Handbook, Volume 3, Springer, 1997 ISBN 0-412-08451-1.
- Ron Schmitt, Electromagnetics Explained: a Handbook for Wireless/RF, EMC, and High-speed Electronics, Newnes, 2002 ISBN 0-7506-7403-2.
- Scott Hunter Stark, Live Sound Reinforcement: a Comprehensive Guide to P.A. and Music Reinforcement Systems and Technology, Hal Leonard Corporation, 1996 ISBN 0-918371-07-4.
- John Vasey, Concert Sound and Lighting Systems, Focal Press, 1999 ISBN 0-240-80364-7.
- Menno van der Veen, Modern High-end Valve Amplifiers: Based on Toroidal Output Transformers, Elektor International Media, 1999 ISBN 0-905705-63-7.
- Jerry C. Whitaker, Television Receivers, McGraw-Hill Professional, 2001 ISBN 0-07-138042-6.