Difference between revisions of "RF Tutorial Lesson 9: Impedance Matching Using Tuning Stubs"

Revision as of 16:42, 27 September 2015

Tutorial Project: Impedance Matching Using Tuning Stubs

Objective: In this project, you will design and test tuning stubs to match inductive and capacitive loads.

Concepts/Features: CubeCAD Visualization Lumped Sources S-Parameters Far Fields Advanced Meshing in EM.Tempo

Minimum Version Required: All versions

' Download Link: [1]

What You Will Learn

In this tutorial you will learn impedance matching using tuning stubs of different types and forms. The goal is to match inductive or capacitive loads to a 50Ω line or source.

Tuning Stubs: An Overview

Impedance matching is one of the most important aspects of RF circuit design. The purpose of impedance matching is to connect an arbitrary complex-valued load impedance to a source with a given resistive internal impedance (usually 50Ω) without causing input reflection and to ensure maximum power transfer from the source to the load. In many cases, the given load needs to be impedance-matched to a transmission line of given characteristic impedance as shown in the figure below:

A transmission line circuit with an unmatched load.

Tuning stubs are segments of open-ended or short-ended transmission lines that are used for distributed impedance matching. Both open and short stubs can be used for impedance matching. From the transmission line theory, we know that open and short stubs have a pure imaginary input impedance. In view of circuit topology, tuning stubs can be connected to the transmission line circuit in series or parallel configurations. In view of circuit complexity, there are single-stub and double-stubs matching network designs.

Single-Stub Design

Single-stub tuning procedure.

In a single-stub design, you insert a T-Line segment of length L between the source and load such that the input impedance looking into the T-Line from the point of view of the source has a real part equal to the source resistance. Then you use an open or short stub either in a series or parallel configuration to cancel the imaginary part of the input impedance.

Single parallel stub. The stub can be either shorted or open.

Single series stub. The stub can be either shorted or open.

Double-Stub Design

Double-stub tuning procedure.

In a double-stub matching network, a tuning stub is placed at the location of the load and another one is placed at a distance of λ_g/8 from the first stub towards the source. The length of the first stub is chosen such that the combination of the load and first stub moves to the 1+jb circle. The combination load is then rotated along the constant VSWR circle by π/2 towards the source until you intersect the unit r = 1 circle. The length of the second stub is chosen to move you to the center of the Smith chart.

Double parallel stubs placed λg/8 apart. The stubs can be either shorted or open.

Double series stubs placed λg/8 apart. The stubs can be either shorted or open.

Single-Stub Impedance Matching Using a Series Open Stub

As a first example, you will design a series open stub to match the inductive load you already encountered at the end of Tutorial Lesson 3. The following is a list of parts needed for this part of the tutorial lesson:

Place and connect the parts as shown in the figure below. You want to design a matching network between the source circuit consisting of the voltage source, source resistor and the 75mm 50Ω T-Line segment and the inductive load of 100 + j80 Ohms at f = 2GHz. When the length of the connecting (spacer) T-Line is chosen to be 18mm and the length of the open stub is chosen to be 59.55mm, the input impedance looking into the combination of the stub and the load to the right becomes 50Ω.

The RF circuit with a single-stub matching network using a series open stub.

To verify this, run a network analysis of the circuit with the tuning stub matching network as a one-port. Use Node 2 (at the input of T-Line segment XTL1) as Port 1.

Plot the s11 data on the Smith chart. Use the same start and stop frequencies as in the previous tutorial lesson, i.e. 1GHz and 3GHz, respectively, with a step size of 100MHz. You can see from the figure below that at f = 2GHz, the plot passes through the center of the Smith chart. It would be informative to plot the return loss, i.e. |s11| as a function of frequency on a Cartesian graph. In the Output tab of the Network Analysis Test Panel, choose "Cartesian (Amplitude)" as the graph type. To obtain a smooth curve, set the frequency step size to 10MHz. The results are depicted in the figure below, showing a vanishing return loss at 2GHz.

Smith chart showing the input reflection coefficient of the transmission line circuit with series open stub.

The return loss of the transmission line circuit with series open stub plotted on a linear scale.

A Summary of Series Stub Matching Theory

The design of the series tuning stub utilized in the previous part is now explained. Let the load admittance be Y_L = 1 / Z_L = G_L + j B_L. The admittance looking into a T-Line segment of length d terminated by the load Y_L is given by:

[math]Y_{in} = Y_0 \frac{ \left(G_L + j B_L \right) + jY_0 t}{Y_0 + j \left( G_L + j B_L \right) t}[/math]

where t = tan (β d). The impedance at this point can then be written as:

[math]Z_{in} = \frac{1}{Y_{in}} = R_{in} + j X_{in} = \frac {G_L ( 1+ t^2)}{G_L^2+(B_L + Y_0 t)^2} + j \frac {G_L^2 t - (Y_0 - B_L t)(B_L + Y_0 t)}{Y_0 \left[ G_L^2+(B_L + Y_0 t)^2 \right]} [/math]

Now we choose the T-Line segment length d such that R_in = Z₀. This results in a quadratic equation for t:

[math] Y_0(G_L - Y_0)t^2 -2B_L Y_0 t + (G_L Y_0 t -G_L^2 -B_L^2) = 0 [/math]

which can be solved for t. In the special case G_L = Y₀, there is one solution t = -B_L / 2Y₀. Otherwise, two distinct roots for t are found. Once t is found, the segment length can be calculated by solving t = tan (2π d / λ_g).

The stub reactance X is chosen such that X = -X_in. From this condition, you can find the length of the stub depending on whether you use a short or open stub. Recall that the impedance of an open-ended or shorted transmission line segment of length L is given by:

[math] Z_{open} = -jZ_0 cot(\beta L) [/math]

and

[math] Z_{short} = jZ_0 tan(\beta L) [/math]

Therefore, the equations for the length L_s of the open stub or short stub are given by:

[math] X_{open} = -Z_0 cot (2\pi L_s / \lambda_g) = -X_{in} [/math]

[math] X_{short} = Z_0 tan (2\pi L_s / \lambda_g) = -X_{in} [/math]

Single-Stub Impedance Matching Using a Shunt Short Stub

Additional parts you need for this part of the tutorial lesson:

• A Generic T-Line (Keyboard Shortcut: T)
• A Generic Short Stub

The following figure on the left shows the original transmission line circuit of RF Tutorial Lesson 2 with a capacitive loading. This time, you will design a matching network between the source circuit and the capacitive load of 60 - j80 Ohms at f = 2GHz. The figure on the right shows a parallel short stub placed at a distance from the load through a segment of a 50Ω T-Line. When the length of the connecting (spacer) T-Line is chosen to be 16.5mm and the length of the short stub is chosen to be 14.25mm, the input impedance looking into the combination of the stub and the load to the right becomes 50Ω.

The original RF circuit with an inductive load.

The RF circuit with a single-stub matching network using a series open stub.

Now run a network analysis of the circuit with the tuning stub matching network. Use Node 2 again as the input port. Plot the s11 data on the Smith chart with a frequency step size of 100MHz and then plot the return loss (|s11|) on a Cartesian graph with a frequency step size of 10MHz. The results are shown in the figures below.

The original RF circuit with an inductive load.

The RF circuit with a single-stub matching network using a series open stub.

A Summary of Shunt Stub Matching Theory

The design of the shunt tuning stub utilized in the previous part is now explained. Let the load impedance be Z_L = 1 / Y_L = R_L + j X_L. The impedance looking into a T-Line segment of length d terminated by the load Z_L is given by:

[math]Z_{in} = Z_0 \frac{ \left(R_L + j X_L \right) + jZ_0 t}{Z_0 + j \left(R_L + j X_L \right) t}[/math]

where t = tan (β d). The admittance at this point can then be written as:

[math]Y_{in} = \frac{1}{Z_{in}} = G_{in} + j B_{in} = \frac {R_L ( 1+ t^2)} {R_L^2+(X_L + Z_0 t)^2} + j \frac {R_L^2 t - (Z_0 - X_L t)(X_L + Z_0 t)} {Z_0 \left[ R_L^2+(X_L + Z_0 t)^2 \right]} [/math]

Now we choose the T-Line segment length d such that G_in = Y₀ = 1 / Z₀. This results in a quadratic equation for t:

[math] Z_0(R_L - Z_0)t^2 -2X_LZ_0 t + (R_L Z_0 -R_L^2 -X_L^2) = 0 [/math]

which can be solved for t. In the special case R_L = Z₀, there is one solution t = -X_L / 2Z₀. Otherwise, two distinct roots for t are found. Once t is found, the segment length can be calculated by solving t = tan (2π d / λ_g).

The stub susceptance B is chosen such that B = -B_in. From this condition, you can find the length of the stub depending on whether you use a short or open stub. Using the expressions for the impedance of an open-ended or shorted transmission line segment of length L given earlier in this tutorial, the equations for the length L_s of the open stub or short stub are found as:

[math] B_{open} = Y_0 tan (2\pi L_s / \lambda_g) = -B_{in} [/math]

[math] B_{short} = -Y_0 cot (2\pi L_s / \lambda_g) = -B_{in} [/math]

Double-Stub Impedance Matching Using Two Open Stubs

Additional parts you need for this part of the tutorial lesson:

• A Generic T-Line (Keyboard Shortcut: T)
• Two Generic Open Stubs

In the last part of this tutorial lesson, you will explore a double-stub matching network. The following figure on the left shows the original transmission line circuit of RF Tutorial Lesson 2 with a capacitive loading. The double-stub matching network will be inserted between the source circuit and the capacitive load of 60 - j80 Ohms at f = 2GHz. The figure on the right shows a shunt open stub connected in parallel with the load and a second shunt open stub placed at a distance of λ_g/8 = 18.75mm from the first stub towards the source through a segment of a 50Ω T-Line. When the lengths of the first and second open stubs are chosen to be 21.9mm and 30.6mm, respectively, the input impedance looking into the combination of the two stubs and the load to the right becomes 50Ω.

The original RF circuit with an inductive load.

The RF circuit with a single-stub matching network using a series open stub.

Similar to the previous case, run a network analysis of the circuit with the tuning stub matching network. Use Node 2 again as the input port. Plot the s11 data on the Smith chart with a frequency step size of 100MHz and then plot the return loss (|s11|) on a Cartesian graph with a frequency step size of 10MHz. As you can see from the results shown in the figures below, this network has a high return loss over most of the frequency range except around f = 2GHz and a prefect match is achieved at 2GHz.

The original RF circuit with an inductive load.

The RF circuit with a single-stub matching network using a series open stub.

A Summary of Double Stub Matching Theory

In this part of the tutorial we scrutinize the design procedure used in the previous part. The first shunt stub is connected in parallel with the load at the load location. Therefore, the total effective load at this point is:

[math]Y_{T} = G_L + j( B_L + B_1) [/math]

where Y_L = 1 / Z_L = G_L + j B_L is the load admittance and B₁ is the susceptance of the first shunt stub. After transforming this admittance through a T-Line segment of length d, the admittance just to the right of the second stub is given by:

[math]Y_{in} = G_{in} + j B_{in} = Y_0 \frac{ G_L + j\left(B_L + B_1 + Y_0 t \right) }{Y_0 + j \left( G_L + j B_L + j B_1 \right) t}[/math]

At this point, to achieve a perfect impedance matching, G_in must equal Y₀ and B_in must equal -B₂, where B₂ is the susceptance of the second shunt stub. This leads to the following equation:

[math] G_L^2 -G_L Y_0 \frac{1+t^2}{t^2} + \frac{ \left( Y_0 -B_L t -B_1 t \right) ^2 } {t^2} = 0 [/math]

Remember that in double stub matching, we fix the distance d between the two shunt stubs. This length is usually chosen to be d = λ_g/8. In this case, t = tan(β d) = tan(2π d/λ_g) = tan(π/4) = 1. In this case, the above equation reduces to:

[math] G_L^2 - 2G_L Y_0 + \left( Y_0 -B_L -B_1 \right) ^2 = 0 [/math]

Solving the above equation, you can find B₁ and the length of the first stub. From the equation B₂ = -B_in, you can determine the length of the second stub. The two stubs can be open, short or mixed.

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