Exporting and using Spice netlists in Microwave Office

In CMS it is possible (from v.5.0) to save filter designs as PSpice netlists for later import into a circuit simulator. 
Most circuit simulators can import P-Spice netlists.
 
Reasons for exporting CMS designs to circuit simulators could be for combining several filters into di- or multiplexers or for modification/optimization of couplings etc. using the facilities of the circuit simulator.

To demonstrate ‘how to’ we now want to transfer the following CMS design into a model which can be used in Microwave Office (MWO):  
 (You can find a similar guide for ADS here)

 

 
1. To create a PSpice netlist we simply select the ‘Make PSpice Netlist’ entry in the ‘Tools’ drop-down menu.
 

 
2. Then specify a name and save the spice netlist in the ‘Netlists’ folder in the CMS installation directory. The P-Spice netlist for the filter in question is called ‘MWO_test_filter’ and it gets the PSpice ‘.cir’ extension.
 

 
We are now done with CMS and the rest goes on in MWO.
 
3. For the purpose of this demo a new MWO project (CMS_Spice_Test) has been created:
 

 
4. In “Project Options” in the project tree, enter the same frequency range as was used in CMS:
 

 

5. Right-click on “Netlists” in the project tree and select “Import Netlist...”

Then browse to the “Netlists” folder in the CMS installation directory and select the “CMS_Spice_Test.cir” file (usually located  in: C:\Program Files (x86)\Guided Wave Technology\CMS\Netlists).

MWO then imports the PSpice file .
 
6. Scrolling down to the bottom in the netlist it can be seen that the filter has been given the name “MWO_Test_Filter”, which is the name which we gave the netlist when creating it in CMS.
 

 

7. Next, right-click on “Circuit Schematics” in the project tree and select “New Schematic...”

In the present case the schematic is called “MWO_test_filter”.
MWO now creates an empty schematic.

 
8. Choose the “Subcircuit” button in the top menu – or alternatively - select “Add Subcircuit” from the “Draw” fall-down menu:
 

 
Click OK to add a subcircuit element and place the sub-circuit on the schematic.
 

 

The subcircuit with all parameters is now visible in the schematic:

 

 
9. Then add ports by choosing the “port” button in the top menu:
 

 
The Netlist has now been imported into MWO. All couplings and other parameters are now available for further treatment in MWO.
 

10. To plot the filter characteristic in MWO we need to insert the plots:

  • In the project tree right-click on “Graphs” and select “New Graph...” and create a rectangular graph.

  • On the just created graph, “right click” and choose “Add Measurement..”

  • Select “MWO_Test_Filter” as data source and specify S21 in dB as shown below:

  

 
11. Then add a new measurement to the same graph and select S11 this time.
 

12. Everything is now ready for running the simulation. 
Do this by selecting the “lightning” symbol in the top menu. The result is shown below, where scaling of the y-axis has been adjusted to match that of the corresponding CMS plot:

 

 

It can be verified by comparison that the MWO generated plot is close to identical to the original from CMS.

By selecting the “Variable Browser” entry in the “View” fall-down menu, one can get an overview of all the available parameters in the netlist model. Here one can also specify which parameters should be available for further optimization etc.

The available parameters are:

  • Center frequency (f0)

  • Ripple Bandwidth (BW)

  • Unloaded Q (QU)

  • All couplings (Mij)

  • Resonance frequency of each resonator (F_RESn)

 
Final remarks to the netlist model
 

The couplings in the netlist model have been implemented using lumped admittance inverters based on ideal inductors.

An inductor is a frequency dependent (physical) device, which used in admittance inverters will give frequency dependent couplings. The netlist model in ADS is therefore more “physical” as compared to the pure mathematical model used in CMS, where couplings are 100% independent of frequency.

For this reason differences will exist between CMS and the netlist model – especially when the bandwidth starts to increase and exceeds 10% relative bandwidth.
For the same reason transmission zeroes may not be placed exactly at the frequencies specified in CMS.

 

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