NuHertz is another tool available with the Electronic desktop from Ansys. It's a powerful filter design tool. The interface is simple and easy to learn, and there are tens of designs you can choose from.
The NuHertz tool's interface has a lot of options. In order to use this tool, the user needs some background on filters. NuHertz won't tell the user what to pick.
Figure 1: FilterQuick interface
At the top left corner, there 4 band options: lowpass, highpass, bandpass, and bandstop.
Figure 2: Filter Band Classes
In today's example, we chose bandpass.
The pattern: Bessel, Butterworth, Chebyshev with ripples in the passband, or Chebyshev with ripples in the stopband, and finally Elliptical. The user needs to know about these types, their advantages, and which one is right for your application. It doesn't matter what you gain by using any of them, you will lose something else.
Figure 3: Filter Types
NuHertz can design lumped filters, distributed filters, active filters, switched capacitors, and finally digital filters with NuHetz. For example, if one chooses distributed, NuHertz will fill up the box with many topologies. Think of them as extra requirements. Combline is selected.
Figure 4: Filter Implementation
On the right top side, the substrate definition. NuHertz needs this info to do the right calculations. You can click it. Select your transmission line type in the dialog box. Depending on what you select, the substrate parameters will change.
Figure 5: Substrate definition
Enter the conductor and dielectric thicknesses, the materials. Choose one from the list. If one can't find the right material, pick the closest one. In AEDT the user can change it. Choose the PCB material. Each material has its own tangent loss number. By enabling Er selection losses, one can change the tangent loss.
Save and close.
There are two tables to be filled out. Specify the maximum ripple in the passband, and the response at a frequency in the stopband in the first table. In the second table, Enter the center frequency, 3dB bandwidth, and stopband. The insertion needs to drop below -40dB at that stopband.
Figure 6: Filter definition
Provide the manufacturing design rules.
Minimum line width and the minimum gap. Don't pick the minimum, that'll make the design really sensitive. The dielectric thickness is selected so that the impedance of the RF lines doesn't change more than 10% with manufacturing tolerances. Set a maximum width and gap. If they're too strict, the software may have to increase the filter order. Make sure the values are big enough. Based on the numbers provided, the software will choose the order of the filters, but the user can impose the order.
At the bottom of table 2, select the band edge definition instead of using the center frequency. In the band edge option, the user specifies the 3dB bandwidth.
Add zeros to Tx. They fix the group delay to make it as flat as possible. In a future presentation, we'll show examples.
The last thing is asymmetric, is the filter symmetric or asymmetric. With asymmetric filters, the software can design a smaller filter with fewer stages.
Design Window:
We see a sample of the filter in the design window. The filter length, the width of each tunning stub, and the gap between the stubs. The capacitors on top of each stub are also given here. Tab points are where the input RF line connects to the first stub. The same for the Output.
Figure 7: Solution
The user can do in the design window:
1- Can print the graph to a PDF, OneNote, or Orcad file on the top bar.
2- Copy to clipboard, then paste into another application, like PowerPoint, Paint, or Word. NuHertz can generate a netlist.
3- Do Monte Carlo analysis, though. The MCarlo will be discussed later.
4- Manually edit the lines, gaps, and capacitors.
5- Display the layout
6- Display the design in 3D.
7- Click the other information to see the impedance of each stub. The software calculates the center impedance here.
8- Modify the center impedance.
9- Fit here, so the filter fits in the screen.
10- Digits, is simply the maximum number of digits we want to see in any number, including before and after the decimal point. Here, it is set to 4. All the numbers have 4 digits.
Result Window:
Figure 8: Filter response
1- Results window shows the S-parameters of the filter. The group delay can be activated or deactivated.
2- One can also change the x-axis to log.
3- Change the X-axis limits of the graph.
4- Print the graph to a PDF, OneNote, or Orcad file on the top bar.
5- Copy to clipboard, then paste into another application like powerpoint, paint, or word.
6- Change the Y-axis limits, and finally,
7- Display the numbers in text.
8- Zoom button.
9- shifting the graph.
10- Restore the graph's original shape by clicking this arrow button; turn off zoom and shift.
11- Display the insertion and return loss or deactivate one.
12- Display the base; the base is simply the response of the filter assuming you have infinite Q, no losses.
13- Plot the results in dB or mag.
14- Plot the smith chart,
15- Change the grid to the polar or to the Y-grid instead of the Z-grid.
16- Click the right button to enter a marker. If the user wants to change its location, left-click on it, then change the frequency location. Click the right-mouse button to remove the marker.
Figure 9: Markers
Back to Monte Carlo analysis
Click on the MCarlo button:
Figure 10: Monto Carlo
Choose which parameter to change: the width of the stubs and the gap. Add the length, the total length. Enter the manufacturing tolerance here and how many trials the software should try.
Select uniform distributions or Gaussian distributions with Monte Carlo. The Guassian just focuses on making small changes instead of big ones.
To see the variation, one can display all the records. The old results can also be kept when we do MCarlo more than once. When you do Monte Carlo, you get a new button "Restore", to go back to the nominal one back.
Exporting:
1- Export the S parameters, Z parameters, or Y parameters.
2- Export the design as a DXF
3- Export to Ansys electronic desktop. But first do the setup.
Figure 11: AEDT export setup
In the dialog box, the user can:
3.1- Send the design to circuit, HFSS, or 3D layout.
3.2- Ask AEDT to simulate the filter after exporting.
3.3- What should AEDT calculate: S-parameters, group delay
3.4- You can also specify graphs
3.5- Optimize the filter more. What are the goals? The information will be transferred to AEDT.
3.6- There are more options here for the side boundaries of the air, open or closed
3.7- Specify the substrate to extend to the edge of the metal.
3.8- Specify the format, is it direct to AEDT, or a Python file that will generate everything in AEDT. Choose python to learn how to do scripting in AEDT.
3.9- save and close this setup, or go back to the default configurations, cancel all changes and leave this dialog box,
4- The last form is 3D Data. This is a generic 3D file that can be used in lots of applications. It creates a txt file. The users can read this file directly into other applications or write a script to do it.
Figure 12: AEDT export setup
