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Branchline Coupler

Branchline Coupler

The Branchline coupler is a signal divider that can separate an incoming signal into two equal power, but 90 degree phase shifted signals. It consists of 4 ports: Input (P1), Ouput 1 (P2) and Ouput 2 (P3). The fourth port (P4) is terminated at 50 ohms for the signal splitter and is isolated from the input.

The branchline coupler is used to generate quadrature signals for our QAM radio.

The final PCB from the kit is 2 in x 2in, your branchline must fit on this size PCB!

Design and Simulation

In this part of the workshop you will design a 0.95GHz 3dB Branchline coupler circuit. The branchline coupler has the following parameters:

Zh: Characteristic impedance of the horizontal t-lines

Zv: Characteristic impedance of the vertical t-lines

EL: electrical length of transmission line

Z: impedance of the 4 ports

F0: operating frequency

Ideal Transmission Line Implementation

Create a new schematic design and call it “BranchLine ideal”. Add four TLIN elements and four ports and connect them as shown below. TLIN is an ideal transmission line specified by electrical length and impedance – it does not have a physical representation. Set Zh=35.36Ohm , Zv=50Ohm, EL=90deg, Z=50Ohm, F0=0.95GHz.

Components can be added as follows:

  • PORT and GND can be added directly from the tool bar at the top of the software (labeled “PORT” and “GND”)
  • All other elements can be added by Ctrl+L then type the name of the element, e.g. RES or CAP, and you will find it in the list that appears.
  • Special components: MVIA1P (special via to ground) must be copied from the Design for Components Schematic in the template provided.

Setup the simulation by going to Options->Project Options->Frequencies and setting the range from 0 to 4 GHz in 0.01Ghz steps. Hit Apply and make sure replace is selected.

Add a Rectangular graph, Project->Add Graph. Then Right-Mouse Click->Add Measurement.

Add a measurement of magnitude of S11 (port 1 to port 1), S21 (port 1 to port 2), S31 (port 1 to port 3), and S41 (port 1 to port 4) in dB scale. Add marker on S11 and S31 curves at 0.95GHz. You should see the image below with similar magnitude values.

Real Transmission Line Implementation

Create a new schematic and call it “BranchLine real”. Add four MLIN elements, four ports. MLIN are transmission lines with a microstrip transmission line realization – this model incorporates the non-idealities of a microstrip transmission line and depends on the specific stackup of your substrate. The MSUB from the Global Definitions has the correct information for our workshop. Do not add an MSUB block!

Determine the Width and Length of the transmission lines using TXLINE and bulid the real Brachline coupler schematic. TXLINE can be found under Tools-TXLINE. It is a calculator to help you convert electrical parameters such as electrical length and impedance to a physical implementation. The following screen shot show the calculator interface.

The picture shows a transmission line with a 50 Ohm impedance and an electrical length of 90 degrees at 950 MHz. The calculator works bidirectional. To find a transmission line from electrical length, insert all of the the parameters on the left. Also set Dielectric Constant=4.5, Conductor=Copper, Height=1.53 mm, Thickness=18um. These are obtained from the PCB stackup we are using in the workshop. Note that the Height and Thickness are set on the right side, even though we are calculating from left to right in this example. Click the right arrow – it will calculate the width and length to the line. Likewise, if you change the Length or Width on the right side and click the left arrow, it will calculate the new Electrical Length and Impedance.

Note: length and width should only have two significant digits, i.e. 2.35 not 2.357.

Your schematic should look similar to the one below.

Set simulation frequency range from 0 to 4GHz with steps of 0.01GHz. Plot the magnitude of S11 (port 1 to port 1), S21 (port 1 to port 2), S31 (port 1 to port 3), and S41 (port 1 to port 4) in dB of the real Btanchline coupler model in Rectangular graph. Add marker on S11 and S31 curves at 0.95GHz. (1 pts). The result should look like the figure below.

Right click on the “BranchLine real” under the Circuit Schemtics in project browser and select View Layout. The layout window will open and below is an example of how it may look like.

This means orientation of the 4 transmission lines with respect to each other is not known even though you placed them perpendicular in the schematic. The red dashed lines indicate where an electrical connection is made. You can select one of the lines and move it around with your mouse and you will see that the red lines still show the correct electrical connection.

Try to re-arrange the lines, and rotate them to the correct orientation until all the red lines disappear. You can rotate elements by Ctrl-R (or access through th Edit menu). If you are confused which component in the layout correspond to which transmission line in the schematic, you can have both schematic and layout views side by side, and see that every time you select an element in one view, its corresponding element in the other view is highlighted. If you placed everything correctly your final layout should look like this.

As you see lines overlapp which each other in the corners resulting in the effective length of lines to be shorter than what you specified. Also there are discountinuities where lines meet, and there is no clear point where input and output connectors (represeted by ports in the schemtaic) should be attached on the PCB.

In order to fix these problems we will add a few new components to the schematic.

MTEE$ – A T-junction with three edges widths W1, W2, and W3. The width of W1, W2, and W3 is set automatically to the same width of tline which T-junction is connected to.

Also lets add short pieces of additional microstrip line MLIN between each port and T-junction. Ue TXLine to find a 50 Ohm width. If you keep characteristic impedacne of these transmission lines to 50Ohm, they will just act as extention to the 50ohm ports and will not affect performance of the coupler. Schematic of the modified real Branchline coupler is shown below.

Try to view the layout of the Branchline coupler now. As shown in the figure below, lines are at correct orientations but still not properly arranged. Select all the components (Ctrl+A) and under edit menu select Snap Together. This will re-arrange the lines to form the final layout of your coupler.

You can also see the 3D view of the layout by clicking the following icon (seen when the layout window is active)

.

The 3D layout is below.

EM Transmission Line Implementation

Note that generating and correcting the layout does not affect the simulation results of the coupler, since AWR uses the symbols in the schematic view and the layout view. It is however important to have the correct layout of the coupler for EM simulations.

For those who have used AWR before, the substrate definition is hidden in the Projects->Global Definitions and is called SUB1. You should not add any additional substrate symbols to the schematic.

To perform an Electromagnetic simulation of the coupler, go to Scripts->EM->Create Stackup this will setup the schematic for EM simulation. Your schematic should have the Stackup and Extract components in it:

Now select all of the tline components (use Shift and mouse button to select several at one time). Then Right-mouse Click->Model Opetions and enable EM Extract.

When you click on the Extract element, everything that is being extracted should turn red. Your circuit should look like the one below. This figure has used Window->Tile to show both the layout and schematic. The schematic and layout are linked, so both turn red.

NOTE: The ports are blue and are not extracted (there is not an EM model for them).

You can now rerun the simulation and an EM extraction of the exact layout will be done and the simultion results updated with the new results.

IMPORTANT: Your simulation will take a very long time if you do not perform the following:

In the Extract component text (just double click directly on schematic), set the X_cell_size=0.3mm  and the Y_cell_size=.3mm. Then right mouse click on the Extract block, and go the the frequencies tab, set the EM simulated frequencies to .55 to 1.55 by .1 GHz steps. Make sure the Use Project Defaults is unchecked. This allows you to run your EM simulation on fewer points for faster simulation.

You can now re-run the simulation and an EM extraction of the exact layout will be done and the simulation results updated with the new results. When using MLIN and straight layouts, very little difference will be seen. This means the MLIN component is already modeling the EM effects accurately. The difference will be more significant when using meandered transmission lines, lines that are close to other structures and non-transmission line conductors, eg. Ground plane.

The extract simulation results should looks similar to those below.

Note that we have added the phase out of each port. They should be 90 degrees apart.

Testing

The branchline is tested by connecting 2 of the three ports to the VNA and measuring S11 and S21 magnitude and phase.

Below are the two configurations for testing, note the 50 Ohm termination on the port without the cable. The left upper port connects to the Tx port of the VNA and the right cable (top or bottom) connects tot eh Rx port of the VNA.

You can measure S11 to see the input impedance on the left port. You should then measure the phase on the right port bottom, then right port top. The difference in phase is the phase output, it should be close to 90 degrees. Below are the two measurements for the measurement of the bottom and top branchline ports.

The phase difference is 217.5-130.3=87.2 degrees, which is a good quadrature separation. The loss however, is imbalanced, with one path having -4.88 dB and the other -3.56 dB. Ideal would be – 3dB in each leg.