RF Blockset, S-Parameters plot correctly, but produce incorrect simulation response in Simulink

I have a 4-port s-parameter file from a full-wave simulation of a frequency multiplexer (3 channels) that I'm using in a simulink model. When I plot the response for all the channels from the block 's dialog box, I get precisely the channel responses I expect. When I run the simulink model, however, I see correct responses for two of the three channels.... with the single delinquent channel experiencing an unexpected ~30dB insertion loss.

All the Inport and Outport blocks are set to type Power and with a 50 Ohm source impedance.

I've included a narrow-band modulation spectrum for the three channels as they go into the channelizer and then the (unexpected) insertion loss in the middle channel. I've also included the "wiring diagram" from simulink and the simulink-produce response curves. From these you can see that the channelizer should provide fairly similar magnitude responses in each channel and are well matched at 50 Ohms.

I can re-create this behavior with any combination of channels being driven and even in the case of only the middle (effected) channel being excited.

This channelizer response has been validated (i.e. without the unexpected 30dB insertion loss) in Cadence, ADS, and HFSS. The Touchstone file format is identical for the three channels.

Answer 1

[MathWorks RF & Mixed-Signal Products Team]

Hi,

It is hard to comment without seeing the actual settings of the model. For example, some of the things to check are:

• What are the centre frequencies in the input/output ports?
• Is the S-parameter block terminated on 50Ohm? Are the ports of type "power"?
• Do you use time domain or frequency domain modelling for the S-parameters?

A useful example to validate S-parameters simulation can be found here: Analysis of Frequency Response of RF System

Also the sections in the documentation about Configuration / input port / output port might be useful.

If the problem persists, please contact MathWorks technical support.

Answer 2

[Jack Holloway]

Thank you for the reply. I have already reached out to the technical support team. Regarding your points:

- What are the centre frequencies in the input/output ports?

   -- I've used 0 as the center frequency. These are very broadband signals and systems.

Is the S-parameter block terminated on 50Ohm? Are the ports of type "power"? --- Yes, all the ports of the S-Parameter block are connected to either Inport or Outport blocks set to type "Power" and 50 Ohms of load impedance. This matches the Touchstone format's characteristic impedance of 50 Ohms.

- Do you use time domain or frequency domain modelling for the S-parameters? --- I've selected the frequency domain modeling of the S-Parameter block.

It's worth noting that when I swap the configuration of the S-Parameter block in question from frequency-domain to time-domain modeling, the issue is solved.

Thanks.