What is impedance?
Impedance is very technical measurement that is hard to explain without equations and some scientific jargon. When you want to understand impedance, first think about electrical resistance (represented by R), measured in Ohms. In basic battery circuit battery generates a voltage which tries to force a current around the circuit connected between the battery’s two terminals – the higher the value of the resistor, the lower the current will be. This was resistance for Direct Current (DC) circuits.
In AC circuits the situation becomes slightly more complex, because, in addition to the resistance, there are two other fundamental components which affect the current flowing around an AC circuit: capacitance and inductance. In addition to the simple resistance we have already discussed, there is also capacitance and inductance to consider – and how those affect the current flow depends on the AC signal frequency.
To make life slightly easier for ourselves, we often consider the total ‘resistance’ of a complex circuit involving resistors, capacitors and inductors as a composite lump, and that’s what we call the impedance. Impedance has the symbol Z and is still measured in Ohms. However, the actual value depends to some degree on the frequency of the signal voltages involved. Any device which generates a voltage has what is called an output impedance and any device that takes in signal had input impedance. The output voltage from the source is developed across the input impedance of the destination (often called the load impedance).
Characteristic impedance
Besides inputs and outputs interconnection cables from source to destination have impedance. So, when we say that the input impedance of your TV’s composite video jack is 75 ohms, that’s what we mean. It doesn’t mean is that the resistance of the cable will be 75 ohms. When we say that the characteristic impedance of a cable is 75 ohms–or 50, 110, 300, or what-have-you–what we mean is that if we attach a load of the specified impedance to the other end of the cable, it will look like a load of that impedance regardless of the length of the cable between. The characteristic impedance of a transmission line is the ratio of the voltage and current of a wave travelling along the line.
Wikipedia definition: 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 single wave propagating along the line; that is, a wave travelling in one direction in the absence of reflections in the other direction. Characteristic impedance is determined by the geometry and materials of the transmission line and, for a uniform line, is not dependent on its length. The SI unit of characteristic impedance is the ohm. The characteristic impedance of a lossless transmission line is purely real, with no reactive component.
The formula for characteristic impedance is Zo = sqrt(L/C) – this is the impedance for a lossless line.
To achieve this characteristic impedance we need two things, firstly the inductance in the cable, and secondly the capacitance between the cable and ground. These each present a complex impedance of opposite polarity and thus come together to form a real impedance. The impedance of the cable is typically determined by the selection of cable geometry and the used insulation materials. Try a Coaxial calculator if you want to see how different materials and dimenstions affect the impedance.
When an RF voltage and current are transmitted along a wire, the impedance of the cable itself becomes significant, and for any distance that is “significant” – which is to say any distance greater than about 0.1 of the signal’s wavelength – matching is necessary. The RF systems are typically built as matched systems where the signal source, cable and load impedance have the same value (or at least “close enough” for the application).
Most typical impedance seen on cables are 50 Ohm single ended impedance (coaxial cables) and 100Ohm differential impedance (usually twisted pair or twinaxial). If you play with coax, short for coaxial cable, you probably know this it is available in a number of different impedances. Another common is 75 ohm, like video cable or antenna cable. There are also special coaxila products range from 32 ohms up to 124 ohms.
In a PCB layout you have different technologies to get the impedance of traces right, but the predominant one is called “Microstrip”. In PCB / PWB boards we are following 50 ohm impedance for single ended lines and 100 ohm impedance for Differential lines. To get ideas on impedances on transmission lines on circuit boards try Transmission line calculator.
Typical impedance values
4 ohms: 4 ohms is a common speaker impedance. Typical speakers have impedance ratings of 4 ohms, 8 ohms or 16 ohms.
8 ohms: Most common nominal speaker impedance. The impedance of most speaker systems is not constant with frequency. A speaker that is rated at 8 ohms may be exactly 8 ohms at only a few frequencies. The rest of the time it may wander above and below this value several times. Typical speakers have impedance ratings of 4 ohms, 8 ohms or 16 ohms. Speaker systems are rated for a nominal impedance, which will be typically 8 or 4 Ohms for hi-fi systems. Many of these claimed impedances are very misleading, since the actual impedance varies widely with frequency. At the very worst, a nominal 8 Ohm speaker system may have an impedance at some frequencies (usually at or near the crossover frequencies) of perhaps 4 Ohms – some may be lower than this.
15 ohms: This is the lowest impedance coaxial cable inductance I could find available….
16 ohms: Typical speakers have impedance ratings of 4 ohms, 8 ohms or 16 ohms. The physical differences between an 8-ohm and a 16-ohm speaker of the same type generally come down to voice-coil wire size and the number of voice-coil wire turns in the magnetic gap. The 16 ohm impedance is a British thing. There are some guitar speakers with different parameters for the 8ohm vs. 16ohm model.
25 ohms: Advanced users often need non-standard coaxial cable types and 25 ohms seem to be one that pop ups somewhat often. 25 ohm miniature RF cable is extensively used in magnetic core broadband transformers. Typical field of use for these coaxial cables is in various impedance transformers, internal wiring, combiners, couplers, splitters etc.
30 ohms: For high power, the perfect impedance for coaxial cable is 30 ohms.
32 ohms: Common headphone impedance (Typical designs provide impedances in the 8-32 ohms region)
50 ohms: 50 ohms is the most commonly used impedance for RF cables for general use and transmitters. In most commercial RF applications 50 ohms coax cable has been taken as the standard for very many years. Experimentation in the early 20th century determined that the best POWER HANDLING capability could be achieved by using 30 Ohm Coaxial Cable, whereas the lowest signal ATTENUATION (LOSS) could be achieved by using 77 Ohm Coaxial Cable. However, there are few dielectric materials suitable for use in a coaxial cable to support 30 Ohm impedance. Thus, 50 Ohm Coaxial Cable was selected as the ideal compromise; offering high power handling AND low attenuation characteristics. According to Wikipedia the arithmetic mean between 30 Ω and 77 Ω is 53.5 Ω; the geometric mean is 48 Ω. The selection of 50 Ω as a compromise between power-handling capability and attenuation is in general cited as the reason for the number. With 50 Ohm Coaxial Cables being the best compromise solution, practically any application that demands high power handling capacity. 50 ohm cables are intended to be used to carry power and voltage, like the output of a transmitter. Also several common antennae are easily matched to 50 ohms cable impedance. 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.
60 ohms: For high voltage, the perfect impedance is 60 ohms. 60 ohms cable was used in some early CATV systems (later replaced with 75 ohms cable).
75 ohms: 75 ohm coax cable is used almost exclusively for domestic TV and VHF FM receiving applications – The approximate impedance required to match a centre-fed dipole antenna in free space (i.e., a dipole without ground reflections) is 73 Ω, so 75 Ω coax was commonly used for connecting shortwave antennas to receivers. In addition 75 ohms coaxial cable is widely used for all kinds of video applications and in telecomunications systems. If you have a small signal, like video, or receive antenna signals, the graph above shows that the lowest loss or attenuation is 75 ohms. These typically involve such low levels of RF power that power-handling and high-voltage breakdown characteristics are unimportant when compared to attenuation. 75 Ohm Coax is very good widely available coaxial cable offering not only low signal attenuation (loss), but also relatively low capacitance. This combination of low attenuation and capacitance effectively make 75 Ohm Coaxial Cable the cable of choice for practically all types of digital audio, digital video and data signals. So if you are, for instance, connecting a 75 Ohm video camera connection to a studio monitor, the coaxial cable must also be 75 Ohm AND the connectors on the coaxial cable (i.e. BNC connectors) must be 75 Ohm in Impedance. All common home analog video standards use 75 ohm cable, as do coaxial digital audio (S/PDIF) connections.
90 ohms: The USB (1.0 and 2.0) low speed/full speed bus has a characteristic impedance of 90 ohms +/- 15%. High speed USB must have both ends of the line terminated with 90 Ohm differential load (e.g. 45 Ohm resistors to the ground on RX side and in series on TX side.
93 ohms: RG-62 is a 93 Ω coaxial cable originally used in mainframe computer networks in the 1970s and early 1980s (it was the cable used to connect IBM 3270 terminals to IBM 3274/3174 terminal cluster controllers). Later, some manufacturers of LAN equipment, such as Datapoint for ARCNET, adopted RG-62 as their coaxial cable standard. The cable has the lowest capacitance per unit-length when compared to other coaxial cables of similar size. Technically 93 Ohm Coaxial Cable has the lowest capacitance of any type, but 93 Ohm Coax is rare and expensive.
100 ohms: Local area networks (LANs) commonly use a 100 ohms impedance twisted pair cable, manufactured to tighter tolerances than is necessary for telephony. 100 ohms is the differential impedance of CAT5/6/7 UTP cables (EIA/TIA-568).100 ohms UTP cable is also the most common cable used in computer networking (modern Ethernet). UTP cables are found in many Ethernet networks and telephone systems. UTP cables are typically made with copper wires measured at 22 or 24 American Wire Gauge (AWG), with the colored insulation typically made from an insulator such as polyethylene or FEP and the total package covered in a polyethylene jacket. Hard disk interface SATA uses fully shielded twin-ax conductors and termination resistor is 100 Ohms [+/- 5 Ohms] differential.
110 ohms: If you have balanced AES/EBU type digital audio lines, you’ll want 110 ohm AES/EBU cable because the system is matched to this impedance.
120 ohms: 120 ohms is the typical nominal impedance of many telecom cables (differential impedance) at the frequencies used by communications systems like ISDN, ADSL, E1 and T1 (cable impedance is somewhat higher at voice frequency range is considerably higher, typically around 600 ohms). 120 ohms is also the impedance level used on many RS-485 systems and suitable for CAN bus. For RS-485 networks twisted pair wire with a characteristic impedance of 120 ohms is recommended with 120 ohm termination at each end of the communications line
150 ohms: An early example of shielded twisted-pair is IBM STP-A, which was a two-pair 150 ohm foil shielded cable defined in 1985 by the IBM Cabling System specifications, and used with token ring or FDDI networks.
300 ohms: 300 ohms is commonly used twin-lead antenna cable impedance. Twin-lead is supplied in several different sizes, with values of 600, 450, 300, and 75 ohms characteristic impedance. The most common, 300 ohm twin-lead, was once widely used to connect television sets and FM radios to their receiving antennas (today it has been largely replaced with 75 ohm coaxial cable feedlines). Twin-lead is still used in amateur radio stations as a transmission line for balanced transmission of radio frequency signals. Half-wavelength folded dipole, commonly seen on television antennae, has a 288 Ω impedance.
600 ohms: 600 ohms is telephone line nominal impedance and old balanced audio standards Matched impedance was once used in professional audio systems, but nowadays it is rare to find any 600 ohms matched-impedance audio equipment outside venerable broadcast institutions.The istory of 600 ohms impedance comes from telephone companies – Telegraph companies used a vast network with a huge installed base of open wire pair transmission lines strung along wooden poles. Typical lines used AWG 6 wire at 12 inch spacing and the characteristic impedance was about 600 ohms. Early telephone companies started to use those, therefore 600 ohms became the standard impedance for these balanced duplex (bi-directional) wire pairs and subsequently most telephone equipment in general. 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. 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 higher frequencies impedance drops due larger effect of capacitance and inductance (at hundreds of kHz to MHz range it typically is in 100-120 ohms range).
What is impedance matching?
In electronics, impedance matching is the practice of designing the input impedance of an electrical load or the output impedance of its corresponding signal source to maximize the power transfer or minimize signal reflection from the load.
Power transfer
Whenever a source of power with a fixed output impedance such as an electric signal source, a radio transmitter or a mechanical sound (e.g., a loudspeaker) operates into a load, the maximum possible power is delivered to the load when the impedance of the load (load impedance or input impedance) is equal to the complex conjugate of the impedance of the source (that is, its internal impedance or output impedance). For two impedances to be complex conjugates their resistances must be equal, and their reactances must be equal in magnitude but of opposite signs.
Reflection-less matching
Impedance matching to minimize reflections is achieved by making the load impedance equal to the source impedance. If the source impedance, load impedance and transmission line characteristic impedance are purely resistive, then reflection-less matching is the same as maximum power transfer matching.
In electrical systems involving transmission lines (such as radio and fiber optics)—where the length of the line is long compared to the wavelength of the signal (the signal changes rapidly compared to the time it takes to travel from source to load)— the impedances at each end of the line must be matched to the transmission line’s characteristic impedance (
) to prevent reflections of the signal at the ends of the line.
When impedance matching is needed?
In cases where it is necessary to transfer the maximum power from a source to a destination (power being proportional to both voltage and current), the output impedance of the source and the input impedance of the destination must be equalload).In a matched system like this we have the ideal power transfer arrangement, but the output voltage from the source device is shared equally across both the output and input impedances (assuming negligible cable effects) – this is taken into account in the design of equipment for matched systems.
If you are into RF (radio frequency) design, or perhaps telecommunications, impedance matching becomes an important topic. In systems where high frequencies and/or long cables are used, the input and output impedance should match the impedance of the cable used to connect them (avoids signal losses and reflections at the cable ends). Typical application fields where source impedance, cable impedance and load impedance are matched are radio frequency systems, video systems, high speed digital signal transfer, digital audio connections, industrial automation bus systems, telecommunications circuits and computer networks. Video interfaces normally operate with 75(omega) matched-impedance connections. RF systems operate at 50 ohms and 75 ohms impedance systems depending on application. At modern high frequency electronics sometimes impedance matching needs to be considered on the circuit board signal traces level.
When an RF voltage and current are transmitted along a wire, the impedance of the cable itself becomes significant, and for any distance that is “significant” – which is to say any distance greater than about 0.1 of the signal’s wavelength – matching is necessary. The wavelength is calculated from the speed of light (3 x 10 ^ 8 m/s, or 300,000km/s) multiplied by the “velocity factor” of the cable. Impedance mismatch in RF connections causes power to be reflected back to the source from the boundary between the high and the low impedance. The reflection creates a standing wave if there is reflection at both ends of the transmission line. Tried and true, the Smith chart is still the basic tool for determining transmission-line impedances.
https://en.wikipedia.org/wiki/Coaxial_cable
All of the components of a coaxial system should have the same impedance to avoid internal reflections at connections between components. Such reflections may cause signal attenuation and ghosting TV picture display; multiple reflections may cause the original signal to be followed by more than one echo. In analog video or TV systems, this causes ghosting in the image.
In the early part of the 20th century, it was important to match impedance in telecommunications networks. Bell Laboratories found that to achieve maximum power transfer in long distance telephone circuits, the impedances of different devices should be matched. For this application 600 ohms output feeding a 600 ohms input is perfectly matched. Typical open wire lines used AWG 6 wire at 12 inch spacing and the characteristic impedance was about 600 ohms. Early telephone companies started to use those, therefore 600 ohms became the standard impedance for these balanced duplex (bi-directional) wire pairs and subsequently most telephone equipment in general.
Impedance matching is a requirement in telecommunications networks even today, but in many cases not for any of the reasons you might think. It is actually rare for an analogue phone line to run more than about 5km from the exchange (Central Office in the US) to the user’s location. Impedance matching is required to enable the hybrid – a circuit that allows simultaneous transmit and receive on a single pair without interference – to function properly. If the impedances are not properly matched, you will hear too much of your own voice when you speak, the far end speech will be too soft, and both parties will very likely get lots of echo on the line. The nominal impedance used in normal analogue telephone (PSTN) system is 600 ohms.
Matched impedance (6oo ohms) was once used in professional audio systems, but nowadays it is rare to find any 600 ohms matched-impedance audio equipment outside venerable broadcast institutions.
Audio circuits and impedance matching
There are also systems where things are built intentionally so that impedance is not matched. Examples of such systems are found especially in the audio systems. When it comes to low level audio signals, we’re not concerned about transferring lots of energy — we just want to get the signal voltage from equipment A to equipment B with as little loss as possible. The goal of the signal transmission system is to deliver maximum voltage, not maximum power. Sometimes called “voltage matching.”
Audio cables are NOT transmission lines. Audio systems the signal distances are so small compared to highest signal frequency (20 kHz typically) that we don’t have to worry about impedance matching. In theory, we could send an audio signal 12km without having to worry about impedance matching, although at extreme line lengths matching can reduce high frequency signal losses.
For hi-fi and professional audio, impedance matched system is a meaningless concept and will actually cause an increase in noise. When you use impedance matching, the output will be loaded with the same impedance as it’s output impedance. If the output is loaded, then the available voltage from the source drops, and that in turn means that more amplification is needed to obtain the final voltage needed. The output signal level in impedance matched system is reduced by 6dB, this means that an additional 6dB of gain is required to compensate – therefore the circuit will have 6dB more noise.
For audio applications generally, it is desirable that the output impedance is low, and the destination impedance high, and this is the case with the majority of modern audio equipment. Preamps usually have an output impedance of less than 1k Ohm, and power amps will have an input impedance of at least 10k Ohms, but more commonly 22k or 47k. Line outputs usually present a source impedance of from 100 to 600 ohms. The voltage can reach 2 volts peak-to-peak with levels referenced to −10 dBV (300 mV) at 10 kΩ.
In professional audio devices need low differential (signal) output impedances and high differential (signal) input impedances. This practice is the subject of a 1978 IEC standard requiring output impedances to be 50 ohms or less and input impedances to be 10 k or more.
It has often been claimed that a 600 Ohm microphone should be matched to a 600 Ohm input for best performance – but this is simply wrong. The ideal for a microphone is to use a high impedance input, but this creates other problems, so a compromise is needed. Typically, a good mic preamp (for microphones of up to 600 Ohms) will have an input impedance of between 1.2k and 3k Ohms – does not cause any problems for the microphone. There are three general classifications for microphone impedance: Low Impedance (less than 600Ω), Medium Impedance (600Ω – 10,000Ω) and High Impedance (greater than 10,000Ω). Low impedance microphones are usually the preferred choice.
There are some cases where you need to think impedance levels in audio connections (sometimes this is called impedance matching although not any accurate matches are made). Perhaps the most common mismatch is when someone plugs a guitar or other instrument level device direct into a mixer. Mixers typically only have LoZ and line level inputs, so plugging in the HiZ guitar output is a bad recipe. The Direct box or DI box as it is sometimes called changes the unbalanced HiZ signal to a balanced LoZ signal.
Loudspeakers (4-8 ohms) and headphones (8-32 ohms) presents a known impedance load to the amplifier that drives to them, but the output impedance of driving amplifier is not matched to load (power amplifier driving speaker typically has very low output impedance). An amplifier will always have a defined (and measurable) output impedance (typically one ohms or less for power amplifiers that drive speakers) – measuring it can be hard and it can vary depending on frequency. The speakers also have varying impedance. At the very worst, a nominal 8 Ohm speaker system may have an impedance at some frequencies (usually at or near the crossover frequencies) of perhaps 4 Ohms – some may be lower than this. When matching amplifiers to loudspeakers, the rated load impedance of the amplifier match that of the loudspeakers – the actual output impedance of the power amplifier output does not typically match the speaker impedance (on modern solid state hifi amplifiers output impedance is typically considerably less than one ohm, it can be several ohms on some tube amplifiers). Audio signals are carried by voltage, not power high-level amplifier output impedances can be made small compared to the speakers they drive. Maximum power theorem is for matching loads to sources, not matching sources to loads.
Different make and model headphones have widely varying impedances, from a common low of 20-32 Ω to a few hundred ohms. Most headphones with low impedance (less than 25 ohms, approximately) require little power to deliver high audio levels. For example, low impedance headphones will work well with equipment with weak amplification like portable music players, phones, and other portable devices. Headphones with higher impedance (25 ohms and over, approximately) demand more power to deliver high audio levels.
Impedance matching is not used in modern audio electronics:
A mic output might be around 600 Ω, while mic preamp inputs are 1 kΩ or more.
A line output will be something like 100 Ω, while a line input is more like 10 kΩ.
A loudspeaker amplifier will be less than 0.5 Ω, while loudspeakers are more like 4 Ω.
A guitar output might be 100 kΩ, while a guitar amp input is at least 1 MΩ.
For more details on audio systems inter-connections read Lines and Interconnections study material.
Measuring impedance
Because Impedance is an AC property it cannot be easily measured like resistance. Connecting an Ohm meter across the input or output of an amplifier only indicates the DC resistance. It is quite possible however to measure input and output impedance at any frequency using a signal generator, an oscilloscope (or AC voltmeter) and a decade resistance box or a variable resistor.
The input or output impedance can be determined by measuring the small signal AC currents and voltages. The input or output impedance of a two terminal network can be determined by measuring the small signal AC currents and voltages. The classical measurement for the the input is the following: the voltage is measured across the input terminals and the current measured by inserting the meter in series with the signal generator.
Output impedance of signal generating equipment may also be determined using this simple technique: A load resistor is used to load the signal source. The output voltage is measured first with full load (V1), then without the load (V2). If the load resistance is exactly the same a output resistance, the signal voltage voltage V2 is twice the voltage V1. Other resistance values can be also used. I have made an Output Impedance Measurement Calculator to help to make impedance measurements using this method. NOTE: Keep the load impedance within the allowed load impedance range!
Then talking on the cable characteristic impedance, you need some more tools to measure it. Why not use TDR and measure it. Time Domain Reflectometer (TDR) is is an electronic instrument that uses time-domain reflectometry to characterize and locate faults in metallic cables (for example, twisted pair wire or coaxial cable). A TDR measures reflections along a conductor. Simple way to do it is with a scope and a fast pulse. The TDR waveform shows the effect of all the reflections created by all of the impedance discontinuities. You can use TDR and adjustable resistor to determine the impedance of a cable. Usually when talking on cable impedance best is to do first the calculation and then rely on actual measurement in the end.
Main information sources:
https://www.soundonsound.com/sos/jan03/articles/impedanceworkshop.asp
http://sound.westhost.com/impedanc.htm
http://sound.westhost.com/articles/balanced-2.htm
https://en.wikipedia.org/wiki/Line_level
http://www.mediacollege.com/audio/microphones/impedance.html
http://shure.custhelp.com/app/answers/detail/a_id/224/~/should-i-match-impedances-of-my-microphone-to-my-mixer%3F
https://en.wikipedia.org/wiki/Line_level
http://electronics.stackexchange.com/questions/6846/how-important-is-impedance-matching-in-audio-applications
Understanding Impedance
https://www.soundonsound.com/sos/jan03/articles/impedanceworkshop.asp
The Importance Of Impedance
http://www.soundonsound.com/sos/1994_articles/oct94/impedance.html
http://artsites.ucsc.edu/EMS/music/tech_background/te-15/teces_15.html
What Is Headphone Impedance?
http://www.turntablelab.com/pages/headphone-buying-guide-what-is-headphone-impedance
http://tweakheadz.com/impedance-for-musicians/
http://electronics.stackexchange.com/questions/6846/how-important-is-impedance-matching-in-audio-applications
How important is impedance matching in audio applications?
http://electronics.stackexchange.com/questions/6846/how-important-is-impedance-matching-in-audio-applications
http://www.learnabout-electronics.org/ac_theory/impedance73.php
https://en.wikipedia.org/wiki/Impedance_matching
https://www.maximintegrated.com/en/app-notes/index.mvp/id/742
http://www.zen22142.zen.co.uk/Theory/inzoz.htm
https://en.wikipedia.org/wiki/Characteristic_impedance
http://www.epanorama.net/documents/wiring/cable_impedance.html
http://www.bluejeanscable.com/articles/impedance.htm
http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm
http://electronics.stackexchange.com/questions/194826/why-we-must-to-follow-50ohm-and-100ohm-impedance-in-pcb-boards-is-it-a-standard?lq=1
http://www.highfrequencyelectronics.com/Jun07/HFE0607_Editorial.pdf
https://en.wikipedia.org/wiki/Nominal_impedance
https://en.wikipedia.org/wiki/Coaxial_cable
http://cablesondemandblog.com/wordpress1/2014/03/06/whats-the-difference-between-50-ohm-and-75-ohm-coaxial-cable/
https://en.wikipedia.org/wiki/Twisted_pair
http://www.interfacebus.com/Design_Connector_Serial_ATA.html
https://en.wikipedia.org/wiki/Serial_ATA
http://electronics.stackexchange.com/questions/180811/why-do-cables-have-multiple-grounds
http://www.pcs-electronics.com/semirigid-25ohm-other-special-coaxial-cable-p-1275.html
http://www.prestonelectronics.com/audio/Impedance.htm
http://www.eeweb.com/electronics-forum/characteristic-impedance-considerations
74 Comments
Tomi Engdahl says:
Cable impedance
http://www.epanorama.net/documents/wiring/cable_impedance.html
This document tries to clear out some details of transmission lines and cable inductance. This document is only a brief introduction to those topics. If you expect to work much with transmission lines, coaxial or otherwise, then it will be worth your while to get a book on that subject. The ideal book depends on your background in phsics or electrical engineering, and in mathematics.
Read more at: http://www.epanorama.net/documents/wiring/cable_impedance.html
Tomi Engdahl says:
Analog Fundamentals: Instrumentation for impedance measurement
http://www.edn.com/electronics-blogs/analog-fundamentals/4423720/Analog-Fundamentals–Instrumentation-for-impedance-measurement-
When creating designs, engineers must meet or exceed their objectives for instrumentation and measurement equipment. Having accurate and correct instruments for test and measurement methods and solutions is crucial for the design process. To better equip design engineers, we will discuss the challenges of precision data acquisition specifically in a complex impedance measurement system.
Tomi Engdahl says:
What is the aspect ratio for 50 ohm microstrip?: Rule of Thumb #27
http://www.edn.com/electronics-blogs/all-aboard-/4441967/What-is-the-aspect-ratio-for-50–microstrip–Rule-of-
Thumb–27?
_mc=NL_EDN_EDT_pcbdesigncenter_20160509&cid=NL_EDN_EDT_pcbdesigncenter_20160509&elqTrackId=53fabc7608ef4494a6ff28
90b78094b9&elq=50c556de8ef441f4b56995ab8558ae20&elqaid=32169&elqat=1&elqCampaignId=28082
All 50 ohm microstrip lines in FR4 have the same aspect ratio.
Spoiler summary: The ratio of the line width to the dielectric thickness of a 50 ohm microstrip in FR4 is about
2:1. This is a simple to remember, easy to use consistency check for your designs.
Most single-ended controlled-impedance interconnects are designed for a characteristic impedance of 50 ohm. The
historical reason for the use of 50 ohm is related to the minimum attenuation in a coax cable and was driven by
the early days of radar applications.
Tomi Engdahl says:
RF impedance matching for rocket scientists
http://www.edn.com/electronics-blogs/out-of-this-world-design/4442051/RF-impedance-matching-for-rocket-scientists?_mc=NL_EDN_EDT_EDN_weekly_20160519&cid=NL_EDN_EDT_EDN_weekly_20160519&elqTrackId=0545e568c8524bf6a3961fe79d647893&elq=5923aebb34184fbeb2e9900dfb074a58&elqaid=32329&elqat=1&elqCampaignId=28237
To deliver the next generation of satellite services, operators are increasingly exploiting the benefits of wideband digital payloads. Broadband ADCs/DACs now directly digitise/re-construct IF/RF carriers and impedance matching is required to optimise the integrity of key interfaces and the overall signal path. Once a key skill of RF analog engineers, digital and mixed-signal members of the avionics design team are now having to grapple with the concept of impedance matching for high-throughput applications.
Impedance matching is used to make a load or line impedance equal that of a driving source for maximum power transfer. If these are not matched, less energy will be delivered, standing waves will develop, and the load will not absorb all of the power sent down the line. Consequently, some of the energy reflects back to the source with the potential to cause damage. For complex impedances, maximum transfer occurs when the load equals the conjugate of the source, i.e. the reactances have the same magnitude but opposite sign.
Within a spacecraft, impedance matching networks have traditionally been used between the receive antenna and the LNA, to connect internal amplifier stages, and also between the power amplifier and the downlink transmitter to maximise power transfer.
Lumped, tuning circuits such as L, Π or T-networks can be used to transform a load impedance into one which is conjugate matched to a source for maximum power transfer. These are narrowband and multiple sections can be cascaded for wider bandwidths.
This post has briefly introduced RF impedance matching for space applications, and there are limits on how good a match can be achieved over a specified bandwidth and the complexity of the tuning network as defined by Bode-Fano.
Tomi Engdahl says:
What’s Special about Fifty Ohms?
http://hackaday.com/2016/07/11/whats-special-about-fifty-ohms/
If you’ve worked with radios or other high-frequency circuits, you’ve probably noticed the prevalence of 50 ohm coax. Sure, you sometimes see 75 ohm coax, but overwhelmingly, RF circuits work at 50 ohms.
[Microwaves 101] has an interesting article about how this became the ubiquitous match. Apparently in the 1930s, radio transmitters were pushing towards higher power levels. You generally think that thicker wires have less loss. For coax cable carrying RF though, it’s a bit more complicated.
http://www.microwaves101.com/encyclopedias/why-fifty-ohms
electrician bognor regis says:
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Tomi Engdahl says:
Impedance Matching 101
https://www.youtube.com/watch?v=QLUrwoHRLJM
Impedance Matching 101 presentation by Ward Silver, N0AX at Pacificon 2012. A great introduction on methodology and techniques to achieve optimum energy transfer between a electronic circuit source and load. Knowing these fundamentals allows for best system performance.
Tomi Engdahl says:
#138: How to Measure Output Impedance
https://www.youtube.com/watch?v=ieAhBejHe2M
This video shows a few methods for measuring the output impedance or resistance of a function generator, amplifier, or other circuits. These methods generally apply to relatively low frequency use cases, and won’t necessarily apply to all circuit types. The methods shown are all variations on measuring the response of the circuit to different load impedances, and calculating the device or circuit’s output impedance from those readings. A low frequency sinusoid (sinewave) is used as a test signal to prevent problems from reflections on misterminated transmission lines, etc. Notes from the video can be found here:
http://www.qsl.net/w/w2aew//youtube/How_to_measure_output_impedance.pdf
Tomi Engdahl says:
#112: Use an Oscilloscope and Signal Generator help tune an HF Antenna, measure complex impedance
https://www.youtube.com/watch?v=eYN7dhdt1Dw
Tomi Engdahl says:
Impedance matching
https://en.wikipedia.org/wiki/Impedance_matching
Reflection-less matching
Impedance matching to minimize reflections is achieved by making the load impedance equal to the source impedance. If the source impedance, load impedance and transmission line characteristic impedance are purely resistive, then reflection-less matching is the same as maximum power transfer matching.
Maximum power transfer matching
Complex conjugate matching is used when maximum power transfer is required
This differs from reflection-less matching only when the source or load have a reactive component.
If the source has a reactive component, but the load is purely resistive, then matching can be achieved by adding a reactance of the same magnitude but opposite sign to the load. This simple matching network, consisting of a single element, will usually only achieve a perfect match at a single frequency. This is because the added element will either be a capacitor or an inductor, whose impedance in both cases is frequency dependent, and will not, in general, follow the frequency dependence of the source impedance.
Whenever a source of power with a fixed output impedance such as an electric signal source, a radio transmitter or a mechanical sound (e.g., a loudspeaker) operates into a load, the maximum possible power is delivered to the load when the impedance of the load (load impedance or input impedance) is equal to the complex conjugate of the impedance of the source (that is, its internal impedance or output impedance). For two impedances to be complex conjugates their resistances must be equal, and their reactances must be equal in magnitude but of opposite signs. In low-frequency or DC systems (or systems with purely resistive sources and loads) the reactances are zero, or small enough to be ignored. In this case, maximum power transfer occurs when the resistance of the load is equal to the resistance of the source
Impedance matching is not always necessary.
Transformers are sometimes used to match the impedances of circuits.
Resistive impedance matches are easiest to design and can be achieved with a simple L pad consisting of two resistors. Power loss is an unavoidable consequence of using resistive networks
Most lumped-element devices can match a specific range of load impedances.
Filters are frequently used to achieve impedance matching in telecommunications and radio engineering.
A simple electrical impedance-matching network requires one capacitor and one inductor.
In electrical systems involving transmission lines (such as radio and fiber optics)—where the length of the line is long compared to the wavelength of the signal (the signal changes rapidly compared to the time it takes to travel from source to load)— the impedances at each end of the line must be matched to the transmission line’s characteristic impedance ( Z c {\displaystyle Z_{c}} Z_c) to prevent reflections of the signal at the ends of the line.
Tomi Engdahl says:
The complete simulation test bench for op amps, Part 1: Output impedance
https://www.edn.com/design/analog/4460430/The-complete-simulation-test-bench-for-op-amps–Part-1–Output-impedance
In a world of tight timelines and ever-increasing performance requirements, it’s critical to create circuit designs right the first time; thus, engineers in the analog and mixed-signal industry often turn to simulation to improve their chances of success. A circuit simulation is only as good as the models that it contains, however. For crucial designs, it’s important to verify that your models match the specs promised by their data sheets.
In this series, I’ll provide a complete simulation test bench for operational amplifiers (op amps), covering every key op amp specification, how they impact application performance and the approach behind the test circuit designs.
Open loop output impedance – Zo
One of the most critical (and yet often overlooked) characteristics of an op amp – especially when performing small-signal stability analysis and dealing with small-signal output load transients such as driving an analog-to-digital converter (ADC) – is the open loop small-signal AC output impedance. Before getting into the details of output impedance, let’s first define some terms.
Throughout this series, I’ll use the term Zo to indicate the open loop small-signal AC output impedance of an op amp and the term Zout to indicate the closed loop small-signal AC output impedance of an op amp. It’s important to distinguish between the two, for reasons that will become clear later. Unfortunately, there does not seem to be a standard for these terms in the analog semiconductor industry, with data sheets from different manufacturers using Zo, Zout, Ro, and Rout somewhat inconsistently.
Zo is an impedance in the op amp’s small-signal path that occurs between the open loop gain stage (Aol) and the output pin (Vout). This impedance interacts with Aol across frequency to create the op amp’s overall AC response.
Zo is a characteristic of the output stage of the op amp. In the past, when bipolar amplifiers with simpler designs dominated the industry, the open loop output impedance of most devices was resistive, or constant, over frequency. Now, Zo can be a highly complex characteristic with capacitive, inductive, and resistive regions that roll off sharply over frequency.
Tomi Engdahl says:
Don’t Fall Into This Output Impedance Trap!
https://www.youtube.com/watch?v=e446nQ9cXdc
Function Gen says 1V, Scope says 2V. Which one’s right?
Tomi Engdahl says:
Characteristic impedance, or not
https://www.edn.com/electronics-blogs/living-analog/4461288/Characteristic-impedance-not–?utm_source=newsletter&utm_campaign=link&utm_medium=EDNFunFriday-20181123
I was asked to certify that a particular twisted-pair cable would exhibit a particular characteristic impedance. There were two wires inside of a braided shield and in examining the cable, I could feel the two wires through the braid. The wires were not dimensionally consistent along the cable’s length. Instead they were kinky, gnarled, and whatever other adjectives you might care to apply.
I explained that to be able to assign a characteristic impedance to any particular cable, for whatever cable structure we might be discussing of which the above are just a few examples, the physical dimensions of the conductors and their inter-conductor spacings had to be known and controlled, which those of his twisted-pair cable sample were not.
We ended our meeting without coming to a consulting agreement and the next thing I learned was that someone else had taken the job, declared that the twisted-pair cable’s characteristic impedance was 90 ohms and collected a fee.
I didn’t get the consulting assignment, but in hindsight, that was a good thing because in my view, an engineering fraud had been committed and I was not a part of it. I wouldn’t want to have to defend that engineering report in a court of law.
Tomi Engdahl says:
Power transfer and phase basics
https://www.edn.com/electronics-blogs/fun-with-fundamentals/4461194/Power-transfer-and-phase-basics?utm_source=Aspencore&utm_medium=EDN&utm_campaign=social
Jacobi’s Law
Most engineers are familiar with the Maximum Power Transfer Theorem (also known as Jacobi’s Law).
This principle can be stated as: “Maximum power is transferred when the internal resistance of the source equals the resistance of the load, when the external resistance can be varied, and the internal resistance is constant,”
Complex impedance
Now consider the AC case where the impedances are complex,
It’s all about the phase
Power engineers use the concepts of True Power and Apparent Power to quantify the effect that phase has on power (Ref. 3). True Power represents the actual power transferred, which includes the effect of the phase between v and i, measured in units of Watts. Apparent Power is a more simplistic concept of just raw current times voltage, measured in units of Volt-Amps or VA to distinguish it from True Power.
Power engineers also use the concept of Power Factor (PF)
Power factor is a straightforward way to quantify how much of the apparent power is being translated into useful (true) power.
I intentionally ignored any discussion of transmission lines but these power transfer concepts have a lot in common with the usual transmission line concepts (standing wave ratio, return loss, reflection coefficient).
Tomi Engdahl says:
Cascade ABCD two-port networks
https://www.edn.com/5G/4359961/Cascade-ABCD-two-port-networks?utm_source=Aspencore&utm_medium=EDN&utm_campaign=social
Tomi Engdahl says:
Voltage transfer: Give me low output impedance
https://www.edn.com/electronics-blogs/fun-with-fundamentals/4461241/Voltage-transfer–Give-me-low-output-impedance?utm_source=Aspencore&utm_medium=EDN&utm_campaign=social
Tomi Engdahl says:
#138: How to Measure Output Impedance
https://www.youtube.com/watch?v=ieAhBejHe2M
This video shows a few methods for measuring the output impedance or resistance of a function generator, amplifier, or other circuits. These methods generally apply to relatively low frequency use cases, and won’t necessarily apply to all circuit types. The methods shown are all variations on measuring the response of the circuit to different load impedances, and calculating the device or circuit’s output impedance from those readings. A low frequency sinusoid (sinewave) is used as a test signal to prevent problems from reflections on misterminated transmission lines, etc.
Tomi Engdahl says:
[US$84.86 12% OFF]ALL SUN EM480B Audio Impedance Tester Portable Insulation CATIII Test Ranges 20/200/2000 Resistance Meter 1KHz Timer Function Data Hold Measurement & Analysis Instruments from Tools, Industrial & Scientific on banggood.com
https://banggood.app.link/DQyos5DK64
Tomi Engdahl says:
Back to Basics: Impedance Matching (Part 1)
https://www.electronicdesign.com/technologies/communications/article/21796367/back-to-basics-impedance-matching-part-1
Tomi Engdahl says:
Audio impedance meter (schematic)
https://www.freestompboxes.org/viewtopic.php?t=21829
Tomi Engdahl says:
Loudspeaker Impedance Meter
http://www.redcircuits.com/Page166.htm
Tomi Engdahl says:
http://www.sengpielaudio.com/calculator-InputOutputImpedance.htm#:~:text=The%20measurement%20of%20input%20impedance,small%20and%20difficult%20to%20measure.
Tomi Engdahl says:
Spring Reverb Tanks Explained and Compared
https://www.tubesandmore.com/tech-corner/spring-reverb-tanks-explained-and-compared
Tomi Engdahl says:
Measuring the input impedance, bandwidth and efficiency of transformers.
http://www.crystal-radio.eu/entrafometing.htm
To determine the input impedance of a transformer, we must load the output of the transformer with a load resistor RL.
The input impedance depends on the load resistor.
Tomi Engdahl says:
https://www.tvtechnology.com/news/audio-cable-impedance-use-digital-cable-for-digital-applications
Everyone working in video knows that video cables have a characteristic impedance of 75 Ohms, and most probably realize that pinching a video cable (such as using an overly tight cable tie to hold it in place) can change the impedance at that point. And most know that impedance changes lead to signal reflections, loss of signal and distortion.
Just as it is with video cables, audio cables have a characteristic impedance and they are also subject to the same impedance changes when poorly installed and handled. In the case of AES/EBU digital audio, the standard calls for 110-Ohm cable, although it looks much like audio cable that’s been used for decades to carry analog audio. The typical broadcast-quality analog audio cable has a characteristic impedance of 45 Ohms.
Tomi Engdahl says:
https://sites.google.com/view/quipa/faq-hub/output-impedance
Tomi Engdahl says:
Measuring Impedance
https://learnabout-electronics.org/ac_theory/impedance73.php
Tomi Engdahl says:
Measuring Input and Output Impedance
http://www.zen22142.zen.co.uk/Theory/inzoz.htm
Tomi Engdahl says:
Vector Impedance Meter
https://www.electrical4u.com/vector-impedance-meter/
The Vector Impedance Meter is employed for measuring both the amplitude and phase angle of impedance (Z).
Normally, in other measuring techniques of impedance, the individual values of resistance and reactance are obtained in rectangular form. That is
But here, the impedance can be obtained in polar form. That is |Z| and phase angle (θ) of impedance can be acquired by this meter.
Tomi Engdahl says:
http://www.rcl-meter.strobbe.org/
Tomi Engdahl says:
Loudspeaker Impedance Meter
Also suitable for Headphones
Operates in conjunction with a DVM
http://www.redcircuits.com/Page166.htm
Tomi Engdahl says:
Electronic Design LabEE-344I
MPEDANCE METER USING AUDIO I/O
https://www.ee.iitb.ac.in/course/~rohitrothe/imp_meter.pdf
Tomi Engdahl says:
Vector Impedance Meter(Direct Reading):
https://www.eeeguide.com/vector-impedance-meter/
Tomi Engdahl says:
Baluns: Choosing the Correct Balun
https://www.dxengineering.com/techarticles/balunsandfeedlinechokes/baluns-choosing-the-correct-balun
Balun Ratio
The balun’s ratio is normally stated from balanced to unbalanced (just as the words appear in the acronym). A 4:1 balun has four times the balanced impedance as unbalanced impedance.
Tomi Engdahl says:
Effects of impedance matching between 50 and 75 Ohm coaxial cables for 10 Mbit/s, Manchester-coded signals (20 MHz)
https://electronics.stackexchange.com/questions/130655/effects-of-impedance-matching-between-50-and-75-ohm-coaxial-cables-for-10-mbit-s
Tomi Engdahl says:
Balun for 50/75 ohm coax
https://www.linkradio.it/index.php?option=com_content&view=article&id=238:balun-for-5075-ohm-coax&catid=7&Itemid=149
Tomi Engdahl says:
What’s effective return loss, anyway? (Part 1)
https://www.edn.com/whats-effective-return-loss-anyway-part-1/
Think of it like this: Impedance mismatches at the pins of the transmitter and receiver plus connectors, vias, and other discontinuities between them cause reflections. If the distance between the transmitter and receiver is an inch, then the group delay between them is about 8.5 UI for a 56 Gbaud signal. The reflection of a symbol at the receiver travels 8.5 UI back to the transmitter, experiences a secondary reflection and travels another 8.5 UI to the receiver. Since the round trip takes 17 UI, that reflection degrades the symbol that was transmitted 17 UI after the original symbol. A 17-tap decision feedback equalization (DFE) is perfectly suited to tidy up these reflections before the symbol enters the decoder/slicer.
Due to insertion loss and the fact that none of the reflections are perfect, subsequent reflections have ever smaller amplitudes.
Enter effective return loss (ERL), a quantity introduced in 802.3cd (50/100/200/400 Gigabit Ethernet) by Rich Mellitz, a Distinguished Engineer at Samtec. ERL incorporates return loss with the effects of equalization—especially DFE—as well as transmitter noise and receiver frequency response into a signal-to-noise-like figure of merit in a way that is similar to channel operating margin (COM).
February 28, 2019
by Ransom Stephens
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Remember ringing? The oscillations that infect signals and are caused by reflections? Back when you could assume that signals traveled from chip-to-chip instantaneously, we either fixed the problem by matching impedances, not such a big deal at MHz, or waited for them to settle down (Figure 1 ).
Reflections produce ringing in rising edges.
Figure 1. Reflections produce ringing in rising edges.
I don’t remember those days, either. The speed of light is decidedly finite. The difference between then and now is that the time it takes a signal to travel from transmitter to receiver, reflect from the receiver back to the transmitter, and then reflect a third time from transmitter to receiver is much less than a symbol unit interval (UI)—whether it’s a non-return-to-zero (NRZ) or a four-level pulse amplitude modulation (PAM4) signal.
In gEEk terms, the path from transmitter to receiver is a transmission line, as fundamental a network element as a capacitor, inductor, or resistor. The UI of a 56 Gbaud signal is less than 20 ps, which spans about 3 mm in typical PCB; any channel longer than several a couple of centimeters would qualify as a transmission line for this signal.
Think of it like this: Impedance mismatches at the pins of the transmitter and receiver plus connectors, vias, and other discontinuities between them cause reflections. If the distance between the transmitter and receiver is an inch, then the group delay between them is about 8.5 UI for a 56 Gbaud signal. The reflection of a symbol at the receiver travels 8.5 UI back to the transmitter, experiences a secondary reflection and travels another 8.5 UI to the receiver. Since the round trip takes 17 UI, that reflection degrades the symbol that was transmitted 17 UI after the original symbol. A 17-tap decision feedback equalization (DFE) is perfectly suited to tidy up these reflections before the symbol enters the decoder/slicer.
Due to insertion loss and the fact that none of the reflections are perfect, subsequent reflections have ever smaller amplitudes.
RL(f) (return loss as a function of frequency) and IL(f) (insertion loss) are given by the differential scattering parameter Sdd22 and Sdd21, respectively. Sdd22 measures the total reflected signal energy. S-parameter masks have been used for years to specify the maximum allowed RL(f) and IL(f), but they don’t account for the effects of equalization (Figure 2 ).
Masks for a typical PAM4 28 Gbaud application
Figure 2. (a) RL(f) and (b) IL(f) masks for a typical PAM4 28 Gbaud application. Courtesy of Ransom’s Notes.
Enter effective return loss (ERL), a quantity introduced in 802.3cd (50/100/200/400 Gigabit Ethernet) by Rich Mellitz, a Distinguished Engineer at Samtec. ERL incorporates return loss with the effects of equalization—especially DFE—as well as transmitter noise and receiver frequency response into a signal-to-noise-like figure of merit in a way that is similar to channel operating margin (COM).
Like COM, ERL does two things: (1) it provides a flexible design parameter space in which engineers can optimize their designs for the system as a whole, allowing for different elements of the design to accommodate different signal impairments while assuring that compliant components will be able to interoperate. And, (2) it takes what had been a simple, easy-to-understand measurement with a cut-and-dry requirement and turns it into a figure of merit that’s so complicated that you have to read part 2 of this article to understand what it is.
What’s effective return loss, anyway? (Part 2)
https://www.edn.com/whats-effective-return-loss-anyway-part-2/
The ultimate performance question must be defined with respect to the error ratio which, for PAM4 systems, is symbol error ratio (SER). Effective return loss (ERL) was introduced by Rich Mellitz, a Distinguished Engineer at Samtec, to combine return loss and equalization into a figure of merit that is traceable to SER.
Like channel operating margin (COM), ERL is derived from the pulse response of a channel under specific assumptions about the quality of the transmitted signal. With ERL, we now consider the pulse response’s reflection .
Similar to how COM is calculated, ERL is given by the ratio of the signal amplitude to the amount of eye closure caused by reflections, defined with respect to a prescribed SER . The prescribed SER is called the detector error ratio (DER0 ). Typically DER0 = 1E-6
The advantage of a requirement like ERL >3 dB rather than S-parameter mask requirements is that ERL incorporates the effects of equalization and S-parameter masks do not. (Hopefully) ERL provides a figure of merit for channels, including transmitter and receiver packages and every potential impedance mismatch in the signal path, that more closely reflects (not punny!) the channel performance of equalized systems.
https://blog.samtec.com/wp-content/uploads/2020/11/11_05_2020_what_is_ERL.pdf
Tomi Engdahl says:
Understanding Impedance
How To Avoid Lifeless Guitar Sounds, Digital Glitches, And Fried Amps!By Hugh RobjohnsPublished January 2003
No home studio is immune from issues of impedance, yet the subject can seem very confusing. In this workshop we explain what the recording musician needs to know about impedance, and show you how to avoid lifeless guitar sounds, digital glitches, and fried amps!
https://www.soundonsound.com/techniques/understanding-impedance
Tomi Engdahl says:
Dealing with Cable Impedance Mismatch
https://www.showmecables.com/blog/post/cable-impedance-mismatching
Tomi Engdahl says:
RF SIGNAL LOSS IN CABLE RUNS – IMPEDANCE OR CABLE LENGTH?
https://service.shure.com/s/article/rf-signal-loss-in-cable-runs-impedance-or-cable-length?language=en_US
RF signal loss due to the length of the cable run is far more significant than loss due to an impedance mismatch. Using a 100 foot run of 75 ohm antenna cable with low loss is better than using a 100 foot run of 50 ohm antenna cable with high loss. There are different grades of 75 ohm cable and of 50 ohm cable.
Professional wireless microphone antennas are 50 ohms, and therefore 50 ohm cable is preferred. If 75 ohm cable is used, add 2 dB of loss for this impedance mismatch, no matter the length of the cable run. For example, a 100 foot run of 75 ohm RG6U has a loss of 6 dB (at 500 MHz) when used with a 75 ohm antenna. When used with a 50 ohm antenna, the loss is 8 dB [6 dB + 2 dB for the impedance mismatch.]
Tomi Engdahl says:
Baluns: Choosing the Correct Balun
https://www.dxengineering.com/techarticles/balunsandfeedlinechokes/baluns-choosing-the-correct-balun
Tomi Engdahl says:
I have heard of “high impedance air-gaps” talking about a cable unplugged
Tomi Engdahl says:
Understanding Impedance
How To Avoid Lifeless Guitar Sounds, Digital Glitches, And Fried Amps!
By Hugh Robjohns
https://www.soundonsound.com/techniques/understanding-impedance?amp
No home studio is immune from issues of impedance, yet the subject can seem very confusing. In this workshop we explain what the recording musician needs to know about impedance, and show you how to avoid lifeless guitar sounds, digital glitches, and fried amps!
Tomi Engdahl says:
Basic impedance equation: Zo = sqrt[ (R+jwL) / (G + jwC) ]
where RLCG are resistance, inductance, capacitance, conductance; j = sgrt(-1), w = freq (radians/s)
G is usually neglected. When R is small the impedance is just sqrt(L/C); R is the lossy part of the line.
Tomi Engdahl says:
Is there Really a True 75 Ohm RCA Plug?
Characteristic Impedance and Video Connectors
http://www.bluejeanscable.com/articles/75ohmrca.htm
Maintaining 75 ohm impedance on mini-DIN and HD-15 plugs is a lost cause; but is there such a thing as a true 75 ohm RCA plug? Not really; Canare’s RCAP-series plugs, which we feel are the best RCA plugs available for video, are often referred to as “true 75 ohm” plugs, but that’s not quite accurate. At the same time, a look at the construction of these plugs shows that they are easily the best plug on the market for a good impedance match with 75 ohm cable.
Characteristic impedance of a coaxial cable, or of a connector with a coaxial (that is, two conductors sharing a common axis) construction, is determined by the size of the conductors, the distance between them, and the type of dielectric that separates them. Unfortunately, where RCA plugs are concerned, this just doesn’t work out. The plug’s dimensions don’t present a 75 ohm characteristic impedance, and obviously these dimensions are pretty well fixed by the standard sizing of the jacks the plugs must fit.
How’d it get this way? Well, the RCA plug didn’t originate as a video connector at all. Its original function was simply as an analog audio connector, used to connect phonographs to other equipment (hence the term “RCA phono plug”), and its design predates broadcast video. We used to own a beautiful 1940′s Hallicrafters shortwave receiver, which among its features offered an input for a phonograph–and the RCA jack on the back of the Hallicrafters looked in every way like the jacks one sees on home video equipment today. Until the VCR came along, consumers had little use for baseband video hookups, and the RCA plug continued to play its traditional role as an audio connector; along came the VCR, and for whatever reason, the RCA plug quickly became the standard for a composite video connection on consumer gear–a bad decision, when 75 ohm BNCs were readily available, but there it is.
So, the dimensions of the RCA plug and jack are all wrong for 75 ohm characteristic impedance. What’s to be done? The best answer to the problem is to limit the damage. The shorter the distance over which an impedance mismatch occurs, the less likelihood there is that it will result in perceptible image degradation. This is where the Canare design–though it can’t remedy the basic faults of the RCA plug dimensions–really shines.
Tomi Engdahl says:
XLR, RCA, and the Magic of Matching Impedance
https://audiophilereview.com/cables/xlr-rca-and-the-magic-of-matching-impedance/
IMHO, the bulk of the so-called “High-End” cables out there seem to have been designed by someone just like that guy up in the peanut gallery: With no true understanding of how things actually work, their “designers” just seem to have thrown together everything they’ve ever read in somebody else’s “White Paper”; seen in a high school physics text; or picked-up on the internet or in audiophile conversation, and combined it on the assumption that it’s all good, and it can’t possibly hurt – just like that suggested enema. Whether it actually works, though, or is just speculation or wish-fulfilling fantasy seems to be beyond either their ability or desire to determine.
One of the things that sounds technical and that finds a willing audience among both the manufacturers and buyers of cables is the need to very precisely impedance match the connectors and the cables that they are fitted to. All kinds of commentary has been written on that subject, much of it pointing to the “need” to use impedance matching RCA connectors for digital or video applications.
apparently out-of-the-air – people had later come to decide that it was actually a 50 Ohm connector, and that, because of that, it was unsuitable for 75 Ohm digital or video application. I also wrote, in that same article, about how a company called Canare was building RCA connectors that claim – at least when used with Canare’s own 75 Ohm cable – to be a true 75 Ohms. That’s all true, but how important is it? Does it really matter? Or is it just marketing or more amateur orthodoxy? You know what I’m talking about – like the amateur cook very precisely measuring the ingredients for a recipe he’s never made before, it’s the kind of obsessive attention to detail that people who have no solid background in what they’re doing use to give themselves comfort and to hide from others the fact that they don’t really know what they’re doing.
With RCA connectors, the fact that they have only two conductors makes it most likely that they will be used in (shielded or unshielded) unbalanced-line circuits, which probably, except for certain specialized applications like digital and video, will be “loaded” (usually with many times the output impedance of the source component) instead of impedance-matched. In fact, in most unbalanced circuits, impedance matching is simply not possible – the output impedance of the source and the input impedance of the load are wildly different, and there’s simply nothing there to match.
Even so, the range of applications for XLR-type connectors (including the Cannon “X”, and later the “XL” and the “XLR”) has been very broad: Their original application was for 600 Ohm signal transmission. Then, when they were applied to microphones as the standard low-impedance connector, the mics they were built into had impedances ranging all the way from 50 Ohms to 250 Ohms. It was the same thing for balanced line interconnects at all of the same impedances as the microphones.
And, when the AES (Audio Engineering Society) and the EBU (European Broadcasting Union) got together to establish the “AES3” standard (in 1985, as revised in1992 and 2003), for transmitting two (stereo) channels of PCM audio between professional devices, the XLR was the connector they chose for 110 Ohm balanced line application (IEC 60958 Type I).
the XLR connector either really has an impedance of 110 Ohms or that its actual impedance doesn’t matter. There really aren’t any other options. And, given that it has also been used and accepted by the professional audio institutional establishment at other impedances (600, 50, 150, and 250 Ohms) I can only take it as a tacit statement by that institutional establishment that impedance matching of cables and connectors, at least in applications that are of concern to audiophiles, is not a major concern.
There are all kinds of other weird, wonderful, and even outright silly things about cables and their connectors.
Tomi Engdahl says:
https://www.epanorama.net/index.php?index=calc_impedance
Tomi Engdahl says:
typical unbalanced audio cables have capacitance of around 70 to 250 pF per meter between signal wire and shield.
Tomi Engdahl says:
http://www.sengpielaudio.com/calculator-InputOutputImpedance.htm
Tomi Engdahl says:
Impedance Matching Basics: Smith Charts
Sept. 7, 2021
This article offers an introduction to the Smith chart and how it’s used to make transmission-line calculations and fundamental impedance-matching circuits.
https://www.mwrf.com/technologies/systems/article/21174601/electronic-design-impedance-matching-basics-smith-charts?utm_source=RF%20MWRF%20Today&utm_medium=email&utm_campaign=CPS210910077&o_eid=7211D2691390C9R&rdx.ident%5Bpull%5D=omeda%7C7211D2691390C9R&oly_enc_id=7211D2691390C9R
What you’ll learn:
Plot complex impedances on a Smith chart.
Determine SWR from the Smith chart.
Determine the impedance of a load at the end of a transmission line.
Identify impedance-matching component values from the Smith chart.