Smith chart

Smith chart

The Smith Chart, invented by Phillip H. Smith (1905-1987), [Smith, P. H.; Transmission Line Calculator; Electronics, Vol. 12, No. 1, pp 29-31, January 1939] [Smith, P. H.; An Improved Transmission Line Calculator; Electronics, Vol. 17, No. 1, p 130, January 1944] is a graphical aid or nomogram designed for electrical and electronics engineers specializing in radio frequency (RF) engineering to assist in solving problems with transmission lines and matching circuits. [Ramo, Whinnery and Van Duzer (1965); "Fields and Waves in Communications Electronics"; John Wiley & Sons; pp 35-39. ISBN ] Use of the Smith Chart utility has grown steadily over the years and it is still widely used today, not only as a problem solving aid, but as a graphical demonstrator of how many RF parameters behave at one or more frequencies, an alternative to using tabular information. The Smith Chart can be used to represent many parameters including impedances, admittances, reflection coefficients, S_{nn}, scattering parameters, noise figure circles, constant gain contours and regions for unconditional stability. [Pozar, David M. (2005); "Microwave Engineering, Third Edition" (Intl. Ed.); John Wiley & Sons, Inc.; pp 64-71. ISBN 0-471-44878-8.] [Gonzalez, Guillermo (1997); "Microwave Transistor Amplifiers Analysis and Design, Second Edition"; Prentice Hall NJ; pp 93-103. ISBN 0-13-254335-4.] The Smith Chart is most frequently used at or within the unity radius region. However, the remainder is still mathematically relevant, being used, for example, in oscillator design and stability analysis. [Gonzalez, Guillermo (1997) (op. cit);pp 98-101]

Overview

The Smith Chart is plotted on the complex reflection coefficient plane in two dimensions and is scaled in normalised impedance (the most common), normalised admittance or both, using different colours to distinguish between them. These are often known as the Z, Y and YZ Smith Charts respectively. [Gonzalez, Guillermo (1997) (op. cit);p 97] Normalised scaling allows the Smith Chart to be used for problems involving any characteristic impedance or system impedance, although by far the most commonly used is 50 ohms. With relatively simple graphical construction it is straighforward to convert between normalised impedance (or normalised admittance) and the corresponding complex voltage reflection coefficient.

The Smith Chart has circumferential scaling in wavelengths and degrees. The wavelengths scale is used in distributed component problems and represents the distance measured along the transmission line connected between the generator or source and the load to the point under consideration. The degrees scale represents the angle of the voltage reflection coefficient at that point. The Smith Chart may also be used for lumped element matching and analysis problems.

Use of the Smith Chart and the interpretation of the results obtained using it requires a good understanding of AC circuit theory and transmission line theory, both of which are pre-requisites for RF engineers.

As impedances and admittances change with frequency, problems using the Smith Chart can only be solved manually using one frequency at a time, the result being represented by a point. This is often adequate for narrow band applications (typically up to about 5% to 10% bandwidth) but for wider bandwidths it is usually necessary to apply Smith Chart techniques at more than one frequency across the operating frequency band. Provided the frequencies are sufficiently close, the resulting Smith Chart points may be joined by straight lines to create a locus.

A locus of points on a Smith Chart covering a range of frequencies can be used to visually represent:
*how capacitive or how inductive a load is across the frequency range
*how difficult matching is likely to be at various frequencies
*how well matched a particular component is.

The accuracy of the Smith Chart is reduced for problems involving a large spread of impedances or admittances, although the scaling can be magnified for individual areas to accommodate these.

Mathematical basis

Actual and normalised impedance and admittance

A transmission line with a characteristic impedance of Z_0, may be universally considered to have a characteristic admittance of Y_0, where:Y_0 = frac{1}{Z_0},Any impedance, Z_T, expressed in ohms, may be normalised by dividing it by the characteristic impedance, so the normalised impedance using the lower case "z", suffix T is given by :z_T = frac{Z_T}{Z_0},Similarly, for normalised admittance:y_T = frac{Y_T}{Y_0},The SI unit of impedance is the ohm with the symbol of the upper case Greek letter Omega (Ω) and the SI unit for admittance is the siemens with the symbol of an upper case letter S. Normalised impedance and normalised admittance are dimensionless. Actual impedances and admittances must be normalised before using them on a Smith Chart. Once the result is obtained it may be de-normalised to obtain the actual result.

The normalised impedance Smith Chart

Using transmission line theory, if a transmission line is terminated in an impedance (Z_T,) which differs from its characteristic impedance (Z_0,), a standing wave will be formed on the line comprising the resultant of both the forward (V_F,) and the reflected (V_R,) waves. Using complex exponential notation::V_F = A exp(j omega t)exp(-gamma l), and:V_R = B exp(j omega t)exp(gamma l),where:exp(j omega t), is the temporal part of the wave:exp(-gamma l), is the spatial part of the wave and:omega = 2 pi f, where:omega, is the angular frequency in radians per second (rad/s):f, is the frequency in hertz (Hz):t, is the time in seconds (s):A, and B, are constants:l, is the distance measured along the transmission line from the generator in metres (m)
Also:gamma = alpha + jeta, is the propagation constant which has units 1/mwhere:alpha, is the attenuation constant in nepers per metre (Np/m):eta, is the phase constant in radians per metre (rad/m)The Smith Chart is used with one frequency at a time so the temporal part of the phase (exp(omega t),) is fixed. All terms are actually multiplied by this to obtain the instantaneous phase, but it is conventional and understood to omit it. Therefore:V_F = A exp(-gamma l), and:V_R = B exp(gamma l),

The variation of complex reflection coefficient with position along the line

The complex voltage reflection coefficient ho, is defined as the ratio of the reflected wave to the incident (or forward) wave. Therefore: ho = frac{V_R}{V_F} = frac{B exp(gamma l)}{A exp(-gamma l)} =C exp(2 gamma l),where "C" is also a constant.

For a uniform transmission line (in which gamma, is constant), the complex reflection coefficient of a standing wave varies according to the position on the line. If the line is lossy (alpha, is finite) this is represented on the Smith Chart by a spiral path. In most Smith Chart problems however, losses can be assumed negligible (alpha = 0,) and the task of solving them is greatly simplified. For the loss free case therefore, the expression for complex reflection coefficient becomes : ho = ho_0 exp(2j eta l), The phase constant eta, may also be written as :eta = frac{2pi}{lambda},where lambda, is the wavelength "within the transmission line" at the test frequency.

Therefore: ho = ho_0 exp(frac{4 j pi}{lambda}l), This equation shows that, for a standing wave, the complex reflection coefficient and impedance repeats every half wavelength along the transmission line. The complex reflection coefficient is generally simply referred to as reflection coefficient. The outer circumferential scale of the Smith Chart represents the distance from the generator to the load scaled in wavelengths and is therefore scaled from zero to 0.50.

The variation of normalised impedance with position along the line

If V, and I, are the voltage across and the current entering the termination at the end of the transmission line respectively, then:V_F + V_R = V, and:V_F - V_R = Z_0I,.By dividing these equations and substituting for both the voltage reflection coefficient: ho=frac{V_R}{V_F},and the normalised impedance of the termination represented by the lower case "z", subscript T:z_T=frac{V}{Z_0I},gives the result::z_T=frac{1+ ho}{1- ho},.Alternatively, in terms of the reflection coefficient: ho=frac{z_T-1}{z_T+1},These are the equations which are used to construct the Z Smith Chart. Mathematically speaking ho, and z_T, are related via a Möbius transformation.

Both ho, and z_T, are expressed in complex numbers without any units. They both change with frequency so for any particular measurement, the frequency at which it was performed must be stated together with the characteristic impedance.

ho, may be expressed in magnitude and angle on a polar diagram. Any actual reflection coefficient must have a magnitude of less than or equal to unity so, at the test frequency, this may be expressed by a point inside a circle of unity radius. The Smith Chart is actually constructed on such a polar diagram. The Smith chart scaling is designed in such a way that reflection coefficient can be converted to normalised impedance or vice versa. Using the Smith Chart, the normalised impedance may be obtained with appreciable accuracy by plotting the point representing the reflection coefficient "treating the Smith Chart as a polar diagram" and then reading its value directly using the characteristic Smith Chart scaling. This technique is a graphical alternative to substituting the values in the equations.

By substituting the expression for how reflection coefficient changes along an unmatched loss free transmission line: ho = frac{B exp(gamma l)}{A exp(-gamma l)} =frac{B exp(j eta l)}{A exp(-j eta l)},for the loss free case, into the equation for normalised impedance in terms of reflection coefficient:z_T=frac{1+ ho}{1- ho},.and using Euler's identity:exp(j heta) = cos heta + j sin heta,yields the impedance version transmission line equation for the loss free case: [Hayt, William H Jr.; "Engineering Electromagnetics" Fourth Ed;McGraw-Hill International Book Company; pp 428 433. ISBN 0-07-027395-2.] :Z_{IN} = Z_0 frac{Z_L + j Z_0 an (eta l)}{Z_0 + j Z_L an (eta l)},whereZ_{IN}, is the impedance 'seen' at the input of a loss free transmission line of length l, terminated with an impedance Z_L,

Versions of the transmission line equation may be similarly derived for the admittance loss free case and for the impedance and admittance lossy cases.

The Smith Chart graphical equivalent of using the transmission line equation is to normalise Z_L,, to plot the resulting point on a Z Smith Chart and to draw a circle through that point centred at the Smith Chart centre. The path along the arc of the circle represents how the impedance changes whilst moving along the transmission line. In this case the circumferential (wavelength) scaling must be used, remembering that this is the wavelength within the transmission line and may differ from the free space wavelength.

Regions of the Z Smith Chart

If a polar diagram is mapped on to a cartesian coordinate system it is conventional to measure angles relative to the positive x-axis using a counter-clockwise direction for positive angles. The magnitude of a complex number is the length of a straight line drawn from the origin to the point representing it. The Smith Chart uses the same convention, noting that, in the normalised impedance plane, the positive x-axis extends from the center of the Smith Chart at z_T = 1 pm j0, to the point z_T = infty pm jinfty,. The region above the x-axis represents inductive impedances and the region below the x-axis represents capacitive impedances. Inductive impedances have positive imaginary parts and capacitive impedances have negative imaginary parts.

If the termination is perfectly matched, the reflection coefficient will be zero, represented effectively by a circle of zero radius or in fact a point at the centre of the Smith Chart. If the termination was a perfect open circuit or short circuit the magnitude of the reflection coefficient would be unity, all power would be reflected and the point would lie at some point on the unity circumference circle.

Circles of Constant Normalised Resistance and Constant Normalised Reactance

The normalised impedance Smith Chart is composed of two families of circles: circles of constant normalised resistance and circles of constant normalised reactance. In the complex reflection coefficient plane the Smith Chart occupies a circle of unity radius centred at the origin. In cartesian coordinates therefore the circle would pass through the points (1,0) and (-1,0) on the x-axis and the points (0,1) and (0,-1) on the y-axis.

Since both ho and z, are complex numbers, in general they may be expressed by the following generic rectangular complex numbers::z = a + jb,: ho = c + jd,Substituting these into the equation relating normalised impedance and complex reflection coefficient:: ho=frac{z-1}{z+1},gives the following result:: ho = c + jd = frac{a^2+b^2-1}{(a+1)^2+b^2} + j left(frac{2b}{(a+1)^2+b^2} ight),.This is the equation which describes how the complex reflection coefficient changes with the normalised impedance and may be used to construct both families of circles. [Davidson, C. W.;"Transmission Lines for Communications with CAD Programs";Macmillan; pp 80-85. ISBN 0-333-47398-1]

The Y Smith Chart

The Y Smith chart is constructed in a similar way to the Z Smith Chart case but by expressing values of voltage reflection coefficient in terms of normalised admittance instead of normalised impedance. The normalised admittance "y"T is the reciprocal of the normalised impedance "z"T, so:y_T=frac{1}{z_T},Therefore::y_T = frac{1- ho}{1+ ho},and: ho = frac{1-y_T}{1+y_T},The Y Smith Chart appears like the normalised impedance type but with the graphic scaling rotated through 180^circ,, the numeric scaling remaining unchanged.

The region above the x-axis represents capacitive admittances and the region below the x-axis represents inductive admittances. Capacitive admittances have positive imaginary parts and inductive admittances have negative imaginary parts.

Again, if the termination is perfectly matched the reflection coefficient will be zero, represented by a 'circle' of zero radius or in fact a point at the centre of the Smith Chart. If the termination was a perfect open or short circuit the magnitude of the voltage reflection coefficient would be unity, all power would be reflected and the point would lie at some point on the unity circumference circle of the Smith Chart.

Practical examples

A point with a reflection coefficient magnitude 0.63 and angle 60^circ,, represented in polar form as 0.63angle60^circ,, is shown as point P1 on the Smith Chart. To plot this, one may use the circumferential (reflection coefficient) angle scale to find the angle60^circ, graduation and a ruler to draw a line passing through this and the centre of the Smith Chart. The length of the line would then be scaled to P1 assuming the Smith Chart radius to be unity. For example if the actual radius measured from the paper was 100 mm, the length OP1 would be 63 mm.

The following table gives some similar examples of points which are plotted on the Z Smith Chart. For each, the reflection coefficient is given in polar form together with the corresponding normalised impedance in rectangular form. The conversion may be read directly from the Smith Chart or by substitution into the equation.

Using the Smith Chart to solve conjugate matching problems with distributed components

Usually distributed matching is only feasible at microwave frequencies since, for most components operating at these frequencies, appreciable transmission line dimensions are available in terms of wavelengths. Also the electrical behavior of many lumped components becomes rather unpredictable at these frequencies.

For distributed components the effects on reflection coefficient and impedance of moving along the transmission line must be allowed for using the outer circumferential scale of the Smith Chart which is calibrated in wavelengths.

The following example shows how a transmission line, terminated with an arbitrary load, may be matched at one frequency either with a series or parallel reactive component in each case connected at precise positions.

Supposing a loss free air-spaced transmission line of characteristic impedance Z_0 = 50 Omega, operating at a frequency of 800 MHz, is terminated with a circuit comprising a 17.5 Omega resistor in series with a 6.5 nanohenry (6.5 nH) inductor. How may the line be matched?

From the table above, the reactance of the inductor forming part of the termination at 800 MHz is:Z_L = j omega L = j 2 pi f L = j32.7 Omega,so the impedance of the combination (Z_T) is given by:Z_T = 17.5 + j32.7 Omega, and the normalised impedance (z_T)is:z_T = frac{Z_T}{Z_0} = 0.35 + j0.65,This is plotted on the Z Smith Chart at point P20. The line OP20 is extended through to the wavelength scale where it intersects at the point L_1 = 0.098 lambda,. As the transmission line is loss free, a circle centred at the centre of the Smith Chart is drawn through the point P20 to represent the path of the constant magnitude reflection coefficient due to the termination. At point P21 the circle intersects with the unity circle of constant normalised resistance at :z_{P21} = 1.00 + j1.52,.The extension of the line OP21 intersects the wavelength scale at L_2 = 0.177 lambda,, therefore the distance from the termination to this point on the line is given by:L_2 - L_1 = 0.177lambda - 0.098lambda = 0.079lambda,Since the transmission line is air-spaced, the wavelength at 800 MHz in the line is the same as that in free space and is given by:lambda = frac{c}{f},where c, is the velocity of electromagnetic radiation in free space and f, is the frequency in hertz. The result gives lambda = 375 mathrm{mm},, making the position of the matching component 29.6 mm from the load.

The conjugate match for the impedance at P21 (z_{match},) is :z_{match} = - j (1.52),As the Smith Chart is still in the normalised impedance plane, from the table above a series capacitor C_m, is required where:z_{match} = - j 1.52 = frac{-j}{omega C Z_0} = frac{-j}{2 pi f C_m Z_0},Rearranging, we obtain:C_m=frac{-1}{(1.52) omega Z_0} = frac{-1}{(1.52)(2 pi f) Z_0}.Substitution of known values gives:C_m = 2.6 mathrm{pF},To match the termination at 800 MHz, a series capacitor of 2.6 pF must be placed in series with the transmission line at a distance of 29.6 mm from the termination.

An alternative shunt match could be calculated after performing a Smith Chart transformation from normalised impedance to normalised admittance. Point Q20 is the equivalent of P20 but expressed as a normalised admittance. Reading from the Smith Chart scaling, remembering that this is now a normalised admittance gives:y_{Q20} = 0.65 - j1.20,(In fact this value is not actually used). However, the extension of the line OQ20 through to the wavelength scale gives L_3 = 0.152 lambda,. The earliest point at which a shunt conjugate match could be introduced,moving towards the generator, would be at Q21, the same position as the previous P21, but this time representing a normalised admittance given by :y_{Q21} = 1.00 + j1.52,.The distance along the transmission line is in this case :L_2 + L_3 = 0.177lambda + 0.152lambda = 0.329lambda,which converts to 123 mm.

The conjugate matching component is required to have a normalised admittance (y_{match}) of:y_{match} = - j1.52,. From the table it can be seen that a negative admittance would require to be an inductor, connected in parallel with the transmission line. If its value is L_m,, then:-j1.52 = frac{-j}{omega L_m Y_0}= frac{-jZ_0}{2pi f L_m},This gives the result :L_m = 6.5 mathrm{nH},A suitable inductive shunt matching would therefore be a 6.5 nH inductor in parallel with the line positioned at 123 mm from the load.

Using the Smith Chart to analyze lumped element circuits

The analysis of lumped element components assumes that the wavelength at the frequency of operation is much greater than the dimensions of the components themselves. The Smith Chart may be used to analyze such circuits in which case the movements around the chart are generated by the (normalized) impedances and admittances of the components at the frequency of operation. In this case the wavelength scaling on the Smith Chart circumference is not used. The following circuit will be analyzed using a Smith Chart at an operating frequency of 100 MHz. At this frequency the free space wavelength is 3 m. The component dimensions themselves will be in the order of millimetres so the assumption of lumped components will be valid. Despite there being no transmission line as such, a system impedance must still be defined to enable normalization and de-normalization calculations and Z_0 = 50 Omega, is a good choice here as R_1 = 50 Omega,. If there were very different values of resistance present a value closer to these might be a better choice.

The analysis starts with a Z Smith Chart looking into R1 only with no other components present. As R_1 = 50 Omega, is the same as the system impedance, this is represented by a point at the centre of the Smith Chart. The first transformation is OP1 along the line of constant normalized resistance in this case the addition of a normalized reactance of -"j"0.80, corresponding to a series capacitor of 40 pF. Points with suffix P are in the "Z" plane and points with suffix Q are in the "Y" plane. Therefore transformations "P"1 to "Q"1 and "P"3 to "Q"3 are from the Z Smith Chart to the Y Smith Chart and transformation "Q"2 to "P"2 is from the Y Smith Chart to the Z Smith Chart. The following table shows the steps taken to work through the remaining components and transformations, returning eventually back to the centre of the Smith Chart and a perfect 50 ohm match.

References

* P.H.Smith 1969 "Electronic Applications of the Smith Chart." Kay Electric Company

External links

* [http://www.sss-mag.com/smith.html A Collection of Smith Chart Resources] Tutorials, graphics and other info on Smith Chart
* [http://www.jcoppens.com/soft/linsmith/index.en.php linSmith] Smith charting program for Linux.
* [http://www.printfreegraphpaper.com/ Smith Chart] Print free Smith Charts from your computer.
* [http://www.sss-mag.com/pdf/smithchart.pdf Black Magic] Smith Chart - Vector-graphic (infinitely scalable) Smith Chart for practical use.
* [http://www.ate.uni-duisburg.de:8080/hp-renny/download/jsct/ The Java Smith-Chart-Tool] - A free Java-Tool to paint s-parameters in a Smith-Chart.
* [http://www.ocf.berkeley.edu/~joydip/smithchart/ Smith Chart Applet] - a Java applet that simulates operations on a Smith chart
* [http://www.ee.iitb.ac.in/uma/~spalit/old/SmithChart.html Stub Matching Applet] - Java applet that simulates stub matching on a Smith chart
* [http://www.sss-mag.com/txt/SmithChartRevD.xls Smith Excel Graph] plots reflection coefficient data in real and imaginary formats on a customizable Smith Chart (Microsoft Excel Spreadsheet 53K)
* [http://www.nikeserver.net/gian.leonardo.solazzi/smith.html PostScript functions] Functions to plot dots, lines, gamma circle, constant real and imaginary path in PostScript format to make vectorial images.
*An online educational interactive [http://cgi.www.telestrian.co.uk/cgi-bin/www.telestrian.co.uk/smiths.pl Smith chart] . A choice of impedance and admittance charts with a chart marker.
*A MATLAB m file to generate [http://www.mathworks.com/matlabcentral/fileexchange/loadFile.do?objectId=324&objectType=FILE a standard color smith chart] .
* [http://www.nathaniyer.com/qsdw.htm QuickSmith] - a rapid and efficient Smith Chart matching tool
* [http://www.printfreegraphpaper.com/ Printfreegraphpaper.com] Print free Smith Chart Graph Paper.
* [http://bart99.altervista.org/smith/ itSmith Chart demo 1.0] Interactive Smith Chart using Canvas


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