How does source affect Simulink's frequency response estimation?

I'm supposed to create a Simulink model depicting the pupil contraction reflex. It is based on a model created in 1956 created by one Lawrence Stark. The model is very straightforward, it has negative feedback and a simple transfer function.

My task is to create a frequency response estimation of both the system's closed-loop state (with negative feedback plugged in) and open-loop state (with negative feedback cut off) by using Simulink's frequency response tools. You can look up on the original paper to see what kind of frequency response the open-loop state should create: [Stark 1956]. (It's on page 7 on the .pdf -file)

As I have tried to make the frequency response, I have noticed that depending on the source that I plug into the circuit, the frequency response is different each time. I have placed a Linear analysis input point right after the source block and an output point after the transfer function. I use the linear analysis tools to input a sine signal, but it's like the source on the circuit always interferes with the analysis.

I have tried plugging in a constant source of 1, the "In1" -block and the ground block, but these solutions make the frequency response into a straight line on both the amplitude and the phase shift plots. Nothing like the downward curves described on the original paper.

So the question is in short:

What source do I plug into the circuit, so that the source does not interfere with the frequency response analysis?

[Ryan G]

Can you post a pic of your model with the linear analysis points put in? This model sounds linear, do you need the frequency response estimation or could you just linearize it directly?

Answer 1

[Erman Korkut]

Hi Lauri,

This is a common issue frequency response estimation users encounter. Simulink Control Design offers ways to find and handle such blocks during frequency response estimation. Please see the following page:

http://www.mathworks.com/help/slcontrol/ug/effects-of-time-varying-simulink-source-blocks-on-frequency-response-estimation.html