Output CPM

Exporting and using Spice netlists in ADS 2016

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 Keysights Advanced Design System 2016 (ADS). You can find a similar guide for MicroWave Office 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 ‘ADS-test_filter’ and it gets the PSpice ‘.cir’ extension.

We are now done with CMS and the rest goes on in ADS.

3. For the purpose of this demo a new ADS workspace (CMS_Spice_test_wrk) has been created:

4. Select ‘File View’.

In the ‘File’ drop-down menu select ‘Import’>’Design…’:

5. In ‘Import file name (source)’ - browse to the location of the spice file in the CMS Netlists folder.

6. In ‘Options…’ specify parameters as below:

7. Press ‘OK’ a couple of times and select the ‘2 pins’ version of the sub-circuit:

8. The sub-circuit schematic is now created and the content of a logfile is displayed, which states that the netlist translation has completed:

9. The sub-circuit schematic should look like this:

10. Create a new schematic.

11. Select: ‘Insert -> Component -> Component Library’:

12. Click ‘Workspace Libraries’-> ‘ADS-Test_filter_lib. Then ‘double click’ the ‘ads-test_filter’ in the ‘Components’ column.

13. Insert the sub-circuit into the empty schematic page by a ‘click’ with the mouse:

14. Then insert an S-parameter simulation block and change the frequency sweep to that of the original CMS simulation. Finally insert S-parameter ports:

15. The setup can now be analyzed and plotted:

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

Making the spice parameters available for modification in ADS 2016

The parameters available for optimization are:

Center frequency: f0
Ripple Bandwidth: bw
Unloaded Q: qu
All couplings mij
Resonance frequency of each resonator: f_res1 - fresN

In order to make some - or all – of these parameters available for ADS, some adjustments to the sub-circuit schematic (point 9 above) are required.

A. First delete the sub-circuit symbol from the top level schematic:

B. Then open the sub-circuit schematic (ads-test_filter):

C. Select ‘Design Parameters’ in the ‘File’ drop-down menu:

D. The following window opens.

E. Select the ‘Cell Parameters’ pane:

In this window the filter parameters as defined in the spice netlist can be specified.

The exact names/speling of these parameters can be inspected by opening the netlist, which ADS has created from the original CMS spice netlist. This ADS equivalent has the same name as the spice file, but with the ‘.net’ extension, i.e. ‘ADS-test_filter.net’. In the present case it is stored in the workspace root directory. The exact location of this file was also displayed in the Netlist Translater log (see pt. 8 above) under ‘Output filename’

F. Opening this file in a text editor reveals all parameters and their values. The relevant part of this netlist is shown below:

G. With this print-out at hand one can now input the parameters in the ‘Design Parameters’ menu opened above. In the present case we choose only to define the unloaded Q parameter ‘qu’.

This is done by typing ‘qu’ in the ‘Parameter Name’ field and ‘3000’ in the ‘Default Value’ field.
Then press ‘Add’ to get the ‘qu’ parameter on the list. The same can be done for all the parameters, but for the demonstration we only choose ‘qu’.

H. Select ‘OK’ and insert the modified sub-circuit in the top level schematic as described in pt. 11-13 above.

Now the parameter ‘qu = 3000’ is displayed - and is available for tuning or optimization.

I. Changing this parameter to “qu = 300” and running the simulation gives:

The effect of the low Q is clearly visible.

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.