EEE 161 Applied Electromagnetics Laboratory 7 Microstrip Lines and PCB fabrication

Transcription

1 Dr. Milica Markovic Applied Electromagnetics Laboratory page 1 EEE 161 Applied Electromagnetics Laboratory 7 Microstrip Lines and PCB fabrication Part I. Design an impedance matching circuit using actual Microstrip lines (not ideal lines) To solve this problem you will first make an ADS simulation with microstrip transmission lines (MLIN). First you will gather information about the PCB you will be using. Go to the microwave lab, Riverside 5019, and find substrates that we currently have. Select one for your lab group and find εᵣ for that particular substrate on the internet. If a range of dielectric constants are found, take the highest frequency given, or take the average. Follow the steps below. 1. You have to make a transmission-line impedance matching circuit to match impedance given by instructor to 50-Ohms. 2. First use Smith Chart to make an impedance matching circuit. Check with the instructor if the circuit is ok. 3. Open ADS, new schematic. Solve the impedance matching problem using ideal transmission lines (TLIN) first to give you an idea of what the circuit will look like. Write down the values for the line and stub electrical lengths. When you are done with this step, call the instructor to check your circuit. If the circuit is correct, proceed with the next step. In the next step you will be designing and actual microstrip line circuit that can potentially be fabricated on the board. 4. Go to TLines-Microstrip from the pull-down menu. Select MSUB and place it on schematic. 5. Set the substrate parameters in MSub to what your substrate parameters are. 6. Use MLIN for transmission lines and MLOC for open circuited stub. To simulate the effect of Tee junction, we also have to include MTEE where your lines (going to generator and load) and the stub meet. You will see that for Microstrip lines, you have to enter the actual width and length of the lines. Width of the line depends mainly on the transmission line impedance, and length of the line depends mainly on the electrical length and the type of substrate. 7. Make the impedance-matching circuit look the same as the one you have done with ideal lines similar to that shown in figure Now, you have to calculate what the actual width and length of the lines need to be. Use LineCalc for this task. The frequency of operation is 2 GHz. 9. When you are done with LineCalc, write the actual values for the Width and Length of the lines. 10. Simulate the circuit, and make appropriate (minor) changes to the lengths of your lines. DO NOT CHANGE THE WIDTH OF THE LINES.

2 Dr. Milica Markovic Applied Electromagnetics Laboratory page 2 Figure 1 Parallel Matching Circuit Diagram with Transmission Line past MLOC Stub Why making our own circuits? Part II. Fabrication of Microstrip Circuits 1. Necessity. At microwave frequencies, the copper traces on the board are the integral part of the circuit. As you know this kind of circuit is called a microstrip circuit, and the board is called the controlled impedance board. Microstrip circuit cannot be designed without the previous knowledge of the substrate that will be used. 2. Experience. As an engineer you want to know the limitation of different fabrication processes. Unlike at low-frequencies, the board that you will manufacture is a part of the design. (Although if you are not careful at low frequencies, the board MAY become an unintentional part of the circuit design). Learning what you need to know about manufacturing in the design stage of the circuit that will save you time in the manufacturing stage. 3. Speed. You can get your own circuit fabricated in less than an hour. 4. Confidence and satisfaction. You will know in detail how to process a board. 5. Flexibility. You will be able to process the boards of different shapes and sizes, and to add the final touches you want. You can also repeat the fabrication process to fix the problems the next day if necessary.

3 Dr. Milica Markovic Applied Electromagnetics Laboratory page 3 6. Price. It is cheaper to fabricate the board in-house. What are the different ways of PC Board Fabrication at CSUS Applied Electromagnetics Laboratory 1. Mechanical Milling (Router) 2. Etching Manually scribe your patch & matching network outline to one side of the copper clad circuit board near the center, and peel away the copper outside the patch and network. 4. Apply copper tape in the shape of the patch and matching circuit to one side of a 4 x4 plastic sheet obtained from Tap Plastics or Cover the back side with copper tape. Be sure to determine the thickness and type of plastic so you can look up the dielectric constant εᵣ. In any of these methods mount and solder an SMA connector discussed in a later part of this lab. Which companies make PC Boards professionally? For more information on this and other aspects of PC Board manufacturing go to the web site: Part III. Generate the Layout Rectangular Patch antenna design To design a rectangular microstrip patch antenna first find the substrate available in the Microwave Laboratory, Riverside You need to find the electric permittivity εᵣ and height of the substrate. Note which substrate will be used. The design frequency of operation fᵣ will be 2 GHz. Find width (W, non-resonant edge) and length (L, resonant edge) of the patch according to the following equations: L = 0.49λ/ εᵣ (1) Where λ is the wavelength in free space at the frequency of operation f, and εᵣ is the relative dielectric constant of the substrate. The input impedance of the non-resonant-edge fed patch can be calculated as: Zᵢₙ = [90 εᵣ²/ (εᵣ-1)] (L/W)² ohms (2) Example A square microstrip antenna (L = W) has Zᵢₙ=90 εᵣ²/(εᵣ-1). Using the substrates with dielectric constant of 2.35 and substrate thickness of 45mils at ~3 GHz has the width of ~1.2. The input impedance of the patch is Zᵢₙ=~300 ohms.

4 Dr. Milica Markovic Applied Electromagnetics Laboratory page 4 Making an impedance matching circuit Impedance method 1 series matching: Design a square or rectangular patch antenna at 2 GHz using (1) and (2). If the input impedance of your patch is not 50-Ohms, you need to design a matching circuit to match the input impedance of the antenna to 50-Ohms impedance of the network analyzer. There is a simple way to make an impedance matching circuit when the load is purely resistive. Use a single quarter-wave transmission line as shown in figure 2. To find the transmission-line impedance of the quarter-wave transmission line, use the formula found in Ulaby text in example 2-10 p.83: Zₗᵢₙₑ = (Z₀ Zₐₙ) (3) Where Z₀ is the characteristic impedance (50Ω) and Zₐₙ is the antenna impedance from (2). When you calculate the transmission line impedance, you will have to use Line Calc to find the length and width of the actual transmission line on the chosen substrate. In order to open Line Calc you will have to open a new schematic window then select Line Calc from the tools menu. When you are done with Line-calc, add the λ/4 line, of the appropriate transmission line impedance to the patch. λ/4 L Patch Antenna W Matching line. 50 Ω point Figure 2 Patch Antenna with λ/4 series matching line Impedance method 2 parallel matching: If the impedance of your patch antenna is too high, the λ/4 series matching transmission line will be too thin to fabricate. In that case parallel matching is required as described above in part 1.

5 Dr. Milica Markovic Applied Electromagnetics Laboratory page 5 Fabrication When you are done with calculations, make a drawing of the antenna. Include the material type, εᵣ and thickness as reference information on the drawing. Fabrication Method 1: Use a 4 x4 square piece of dual clad printed circuit material from the RF lab Riverside Carefully scribe the outline of your patch antenna and matching network into one side. Leave the copper intact on the other side. Place the patch feed point near the center of the material. You will need to remove the excess copper on the patch antenna side with a heat gun and X- ACTO knife. When the copper is hot it is possible with some difficulty to peel it off. Fabrication Method 2: Apply copper tape in the shape of your patch and matching circuit to one side of a 4 x4 plastic sheet obtained from Tap Plastics, or in Riverside Cover the back side with copper tape. Determine the type of plastic so you can look up the dielectric constant εᵣ. After fabrication using method 1 or 2, you will attach an SMA panel mount connector to the circuit using two #2 bolts, lock washers, and nuts found in the microwave lab Drill 3 holes shown in figure 3 through the substrate so that the center conductor is at the beginning (50 Ω point) of your matching network. Using a thin wire, solder the center conductor connector, to your circuit. 3/32 R thru 2 pl. 3/16 R thru dimensions in inches ± Figure 3 SMA Panel Mount Connector & Mounting Pattern A typical patch antenna is shown in figure 4. Your patch antenna is ready for testing in Lab 8 upon completion of Lab 7.

6 Dr. Milica Markovic Applied Electromagnetics Laboratory page 6 Figure 4 Patch antenna made with copper tape on bare substrate