Characteristic impedance A transmission line drawn as two black wires. At a distance x into the line, there is current phasor I(x) traveling through each wire, and there is a voltage difference phasor V(x) between the wires (bottom voltage minus top voltage). If $Z_{0}}$ is the characteristic impedance of the line, then $V(x)/I(x)=Z_{0}}$ for a wave moving rightward, or $V(x)/I(x)=-Z_{0}}$ for a wave moving leftward. Schematic representation of a circuit where a source is coupled to a load with a transmission line having characteristic impedance $Z_{0}}$ .

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. Alternatively and equivalently it can be defined as the input impedance of a transmission line when its length is infinite. 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. Energy supplied by a source at one end of such a line is transmitted through the line without being dissipated in the line itself. A transmission line of finite length (lossless or lossy) that is terminated at one end with an impedance equal to the characteristic impedance appears to the source like an infinitely long transmission line and produces no reflections.

Transmission line model

The characteristic impedance $Z(\omega )}$ of an infinite transmission line at a given angular frequency $\omega }$ is the ratio of the voltage and current of a pure sinusoidal wave of the same frequency travelling along the line. This definition extends to DC by letting $\omega }$ tend to 0, and subsists for finite transmission lines until the wave reaches the end of the line. In this case, there will be in general a reflected wave which travels back along the line in the opposite direction. When this wave reaches the source, it adds to the transmitted wave and the ratio of the voltage and current at the input to the line will no longer be the characteristic impedance. This new ratio is called the input impedance. The input impedance of an infinite line is equal to the characteristic impedance since the transmitted wave is never reflected back from the end. It can be shown that an equivalent definition is: the characteristic impedance of a line is that impedance which, when terminating an arbitrary length of line at its output, produces an input impedance of equal value. This is so because there is no reflection on a line terminated in its own characteristic impedance.

Applying the transmission line model based on the telegrapher's equations as derived below, the general expression for the characteristic impedance of a transmission line is:

$Z_{0}={\sqrt {\frac {R+j\omega L}{G+j\omega C}}}}$ where

$R}$ is the resistance per unit length, considering the two conductors to be in series,
$L}$ is the inductance per unit length,
$G}$ is the conductance of the dielectric per unit length,
$C}$ is the capacitance per unit length,
$j}$ is the imaginary unit, and
$\omega }$ is the angular frequency.

Although an infinite line is assumed, since all quantities are per unit length, the characteristic impedance is independent of the length of the transmission line.

The voltage and current phasors on the line are related by the characteristic impedance as:

${\frac {V^{+}}{I^{+}}}=Z_{0}=-{\frac {V^{-}}{I^{-}}}}$ where the superscripts $+}$ and $-}$ represent forward- and backward-traveling waves, respectively. A surge of energy on a finite transmission line will see an impedance of Z0 prior to any reflections arriving, hence surge impedance is an alternative name for characteristic impedance.

Derivation

Using telegrapher's equation Consider one section of the transmission line for the derivation of the characteristic impedance. The voltage on the left would be V and on the right side would be V+dV. This figure is to be used for both the derivation methods.

The differential equations describing the dependence of the voltage and current on time and space are linear, so that a linear combination of solutions is again a solution. This means that we can consider solutions with a time dependence ejωt, and the time dependence will factor out, leaving an ordinary differential equation for the coefficients, which will be phasors depending on space only. Moreover, the parameters can be generalized to be frequency-dependent.

Let

$V(x,t)=V(x)e^{j\omega t}}$ and

$I(x,t)=I(x)e^{j\omega t}}$ The positive directions of V and I are in a loop of clockwise direction.

We find that

$dV=-(R+j\omega L)Idx=-ZIdx}$ and

$dI=-(G+j\omega C)Vdx=-YVdx}$ or

${\frac {dV}{dx}}=-ZI}$ and

${\frac {dI}{dx}}=-YV}$ These first-order equations are easily uncoupled by a second differentiation, with the results:

${\frac {d^{2}V}{dz^{2}}}-ZYV=0}$ and ${\frac {d^{2}I}{dz^{2}}}-ZYI=0}$ Both V and I satisfy the same equation. Since ZY is independent of z and t, it can be represented by a constant -k2. The minus sign is included so that k will appear as ±jkz in the exponential solutions of the equation. In fact,

$V=V^{+}e^{-\gamma kz}+V^{-}e^{\gamma kz}}$ where V+ and V- are the constant of integration, The above equation will be the wave solution for V, and

$I=(jk/Z)(V^{-}e^{-\gamma kz}-V^{+}e^{\gamma kz})}$ from the first-order equation.

If lumped circuit analysis has to be valid at all frequencies, the length of the sub section must tend to Zero.

$\lim _{\Delta x\to 0}{\frac {\Delta V}{\Delta x}}={\frac {dV}{dx}}=-(R+j\omega L)I}$ $\lim _{\Delta x\to 0}{\frac {\Delta I}{\Delta x}}={\frac {dI}{dx}}=-(G+j\omega C)V}$ Substituting the value of V in the above equation, we get.

${\frac {d}{dx}}{V^{+}e^{-\gamma x}+V^{-}e^{+\gamma x}}=-(R+j\omega L){I^{+}e^{-\gamma x}+I^{-}e^{+\gamma x}}}$ $-\gamma V^{+}e^{-\gamma x}+\gamma V^{-}e^{+\gamma x}=-(R+j\omega L){I^{+}e^{-\gamma x}+I^{-}e^{+\gamma x}}}$ Co-efficient of $e^{-\gamma x}}$ : $-\gamma V^{+}=-(R+j\omega L)I^{+}}$ Co-efficient of $e^{\gamma x}}$ : $\gamma V^{-}=-(R+j\omega L)I^{-}}$ Since $\gamma ={\sqrt {(R+j\omega L)(G+j\omega C)}}}$ ${\frac {V^{+}}{I^{+}}}={\frac {R+j\omega L}{\gamma }}={\sqrt {\frac {R+j\omega L}{G+j\omega C}}}}$ ${\frac {V^{-}}{I^{-}}}=-{\frac {R+j\omega L}{\gamma }}={\sqrt {\frac {R+j\omega L}{G+j\omega C}}}}$ It can be seen that, the above equations has the dimensions of Impedance (Ratio of Voltage to Current) and is a function of primary constants of the line and operating frequency. It is therefore called the “Characteristic Impedance” of the transmission line , often denoted by $Z_{o}}$ .

$Z_{o}={\sqrt {\frac {R+j\omega L}{G+j\omega C}}}}$ Alternative approach

We follow an approach posted by Tim Healy. The line is modeled by a series of differential segments with differential series $(Rdx,Ldx)}$ and shunt $(Cdx,Gdx)}$ elements (as shown in the figure above). The characteristic impedance is defined as the ratio of the input voltage to the input current of a semi-infinite length of line. We call this impedance $Z_{o}}$ . That is, the impedance looking into the line on the left is $Z_{o}}$ . But, of course, if we go down the line one differential length dx, the impedance into the line is still $Z_{o}}$ . Hence we can say that the impedance looking into the line on the far left is equal to $Z_{o}}$ in parallel with $Cdx}$ and $Gdx}$ , all of which is in series with $Rdx}$ and $Ldx}$ . Hence:

$Z_{o}=(R+j\omega L)dx+{\frac {1}{(G+j\omega C)dx+{\frac {1}{Z_{o}}}}}}$ $Z_{o}=(R+j\omega L)dx+{\frac {Z_{o}}{Z_{o}(G+j\omega C)dx+1}}}$ $Z_{o}+Z_{o}^{2}(G+j\omega C)dx=(R+j\omega L)dx+Z_{o}(G+j\omega C)dx(R+j\omega L)dx+Z_{o}}$ The term above containing two factors of $dx}$ may be discarded, since it is infinitesimal in comparison to the other terms, leading to:

$Z_{o}+Z_{o}^{2}(G+j\omega C)dx=(R+j\omega L)dx+Z_{o}}$ and hence to

$Z_{o}=\pm {\sqrt {\frac {R+j\omega L}{G+j\omega C}}}}$ Reversing the sign of the square root may be regarded as changing the direction of the current.

Lossless line

The analysis of lossless lines provides an accurate approximation for real transmission lines that simplifies the mathematics considered in modeling transmission lines. A lossless line is defined as a transmission line that has no line resistance and no dielectric loss. This would imply that the conductors act like perfect conductors and the dielectric acts like a perfect dielectric. For a lossless line, R and G are both zero, so the equation for characteristic impedance derived above reduces to:

$Z_{0}={\sqrt {\frac {L}{C}}}.}$ In particular, $Z_{0}}$ does not depend any more upon the frequency. The above expression is wholly real, since the imaginary term j has canceled out, implying that Z0 is purely resistive. For a lossless line terminated in Z0, there is no loss of current across the line, and so the voltage remains the same along the line. The lossless line model is a useful approximation for many practical cases, such as low-loss transmission lines and transmission lines with high frequency. For both of these cases, R and G are much smaller than ωL and ωC, respectively, and can thus be ignored.

The solutions to the long line transmission equations include incident and reflected portions of the voltage and current:

In electric power transmission, the characteristic impedance of a transmission line is expressed in terms of the surge impedance loading (SIL), or natural loading, being the power loading at which reactive power is neither produced nor absorbed:

${\mathit {SIL}}={\frac {{V_{\mathrm {LL} }}^{2}}{Z_{0}}}}$ in which $V_{\mathrm {LL} }}$ is the line-to-line voltage in volts.

Loaded below its SIL, a line supplies reactive power to the system, tending to raise system voltages. Above it, the line absorbs reactive power, tending to depress the voltage. The Ferranti effect describes the voltage gain towards the remote end of a very lightly loaded (or open ended) transmission line. Underground cables normally have a very low characteristic impedance, resulting in an SIL that is typically in excess of the thermal limit of the cable. Hence a cable is almost always a source of reactive power.

Practical examples

Standard Impedance (Ω) Tolerance
Ethernet Cat.5 100 ±5 Ω
USB 90 ±15%
HDMI 95 ±15%
IEEE 1394 108 +3
−2
%
VGA 75 ±5%
DisplayPort 100 ±20%
DVI 95 ±15%
PCIe 85 ±15%

Coaxial cable

The characteristic impedance of coaxial cables (coax) is commonly chosen to be 50 Ω for RF and microwave applications. Coax for video applications is usually 75 Ω for its lower loss.

References

1. ^ a b c "The Telegrapher's Equation". mysite.du.edu. Retrieved 2018-09-09.
2. ^ a b c "Derivation of Characteristic Impedance of Transmission line". GATE ECE 2018. 2016-04-16. Retrieved 2018-09-09.
3. ^ "Characteristic Impedance". www.ee.scu.edu. Retrieved 2018-09-09.
4. ^ "SuperCat OUTDOOR CAT 5e U/UTP" (PDF). Archived from the original (PDF) on 2012-03-16.
5. ^ "USB in a NutShell—Chapter 2—Hardware". Beyond Logic.org. Retrieved 2007-08-25.
6. ^ a b c d https://www.nxp.com/documents/application_note/AN10798.pdf (PDF) modified 2011-07-04
7. ^ http://materias.fi.uba.ar/6644/info/reflectometria/avanzado/ieee1394-evalwith-tdr.pdf (PDF), modified 2005-06-11 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".