Balun simulation and design

Prepared by Dr. Aaron Scher
[email protected]
Oregon Institute of Technology

Back to Aaron's home page.

Introduction

In this assignment you will use NI AWR Design Environment to design and simulate a balun for an Atmel AT86RF230 low-power 2.4 GHz radio transceiver. The RF port (pins RFP and RFN on the AT86RF230) constitute a differential port with a 100 Ohm differential impedance. We want to connect this differential RF port to a single ended (unbalanced) antenna with an impedance of 50 Ohms. The microstrip antenna in Assignment 3 is an example of an antenna with an unbalanced feed.

A useful application note on the design of a lumped element balun for the AT86RF230 and user manual are given in the links below.

  1. Application note: LC-Balun for AT86RF230
  2. User Manual for AT86RF230
Figure 1. Location of balun in block diagram of AT86RF230.

For this assignment, you will create a PowerPoint presentation (or use a similar presentation program), save it as a PDF, and submit it on-line according to the instructions given in class. Please make sure your name is on Slide 1.

Lumped-element balun design and simulation

  1. Read sections 1 through 3 of the application note. This note presents a pretty good introduction to baluns in general.
  2. What is the purpose/function of capacitors Ck in Figure 1-1 in the application note? Present your answer in Slide 1.
  3. What is the center (operating) frequency of the balun that is being designed in the application note? Present your answer in Slide 1.
  4. Equation 2-1 in the application note presents the design equations and component values of the L's and C's that comprise the lumped element balun. Confirm these values using an online balun calculator. Below are links to different calculators (choose one). Take a screenshot of your calculator with the correct component values and present this screenshot in Slide 2.
  5. Perform a time-domain simulation in Microwave Office to illustrate the balun principle, similar to the analysis presented in Figure 3-1 in the application note. Specifically, set up the AC voltage source at the unbalanced port, and confirm that the voltages are 180 degrees out of the phase (with respect to each other) at the differential ports, as shown in Figure 2 below. Present a plot of the voltage at the balanced and unbalanced ports in Slide 3.

    Figure 2. Simulate the balun principle.

  6. Set up a frequency domain simulation (i.e. S-parameter measurement) using ports to confirm that all ports are matched for the balun. Figure 3 below shows the circuit setup in Microwave Office. At the center frequency confirm that the return losses S11 and S22 indicate a match and the insertion loss S21 is 0 dB. Present a plot of your S-parameters in Slide 4. Note that Figure 3 shows a "trick" for setting up a differential port by using an ordinary port and a center tapped transformer. For this trick to work, make sure the impedance of Port 2 (the differential port) is set to be the differential impedance (100 Ohms). More about simulating differential signals in Microwave Office is presented in video 10 in the AWR NI Design Environment video tutorial page

    Figure 3. Confirm the balun is matched at all ports.

  7. Section 6 of the application note discusses the common mode rejection ratio (CMRR). The CMRR is a measure of how balanced a balun actually is. Ideally, a signal at the unbalanced port excites two signals which are 180 degrees out of phase at the differential ports. This means that, in the ideal case, the two currents at the differential ports will be equal in magnitude but opposite in direction, and thus no current will flow through ground. However, if these two signal are not 180 degrees out of phase, than there will be a "common mode signal" that is generated, which results in some ground current.

    Figure 4 shows the simulation setup for simulating CMRR. Make sure the impedance of Port 1 (unbalanced port) is 50 Ohms, the impedance of Port 2 (differential port) is 100 Ohms, and the impedance of Port 3 (common mode port) is 25 Ohms. Ideally, a signal generated at port 1 should not result in a signal at port 3 (i.e. the common mode port) in Figure 4. CMRR is given as |S31|/|S21|, where ports 1, 2, and 3 are defined in Figure 4. Present a plot of CMRR versus frequency in Slide 5. To obtain this plot in Microwave Office, you will need to define CMRR as a variable and set up an equation (Right click on "Output Equations" in the project tree, select "New Output Equations", Go to Draw -> "Add Output Equations", enter a variable name, etc.) Now change ONE of the inductors from 4.9 nH to 6 nH and plot the CMRR. What happens to the CMRR?

    Figure 4. Measure common mode rejection ratio (CMRR).

  8. What is the purpose of the 3 GHz-Chebychev filter in Figure 5-4 in the applicaiton note? is this a low pass or a high pass filter? Present your response in Slide 6
  9. There are numerous types of baluns. Figure 5 shows the layout of a particular transmission line balun. In Slide 7 explain the operation of this transmission line balun using the principles you learned in class. Design a 3 GHz microstrip balun based on this design. The substrate parameters are given in Assignment 3. Use the Microstrip element models in AWR Microwave Office for your simulations. To prove the operation of your balun, present the following information.

    Figure 5. Transmission line balun design.

  10. Bonus (not very difficult, but it might take you some time to figure out). This bonus is worth +15%. Connect the unbalanced port of your balun to the microstrip line antenna in Assignment 3. Drive the antenna with a differential signal and demonstrate an impedance match at 3 GHz. One way to get the antenna data into your schematic is by importing the EM structure (see help menu) and then incorporate the microstrip antenna data into your schematic as a subcircuit. Present your results in bonus slides at the end of your presentation.