I am working with open Modelica now a days and i wanted to know if there is any way to change the frequency of the sine voltage source e.g the frequency of the source doubles or triples every 3 seconds so that when i draw the graph for a low pass filter i could see the magnitude of the output voltage decrease with time. Is there any way to model such voltage source? Please reply as soon as possible.
Vary frequency of sin voltage source in Modelica
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If I understand your question, you are trying to sweep a system with an excitation frequency and measure its response. A Bode Plot is almost certainly a better way to visualize the characteristics of your filter.
But if you wish to try and do everything int he time domain, you need to make sure that the rate at which you sweep the frequency is significantly longer than the time constant of your system. Otherwise, the transient response of your system will completely confuse any attempt to visualize its frequency domain characteristics.
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When simulating in time domain (as you probably do), you can also refer to the paper on Recording of Model Frequency Responses and Describing Functions in Modelica presented during 8th Modelica Conference.
You can probably use a when or if statement that sets the frequency of the source at certain sample interval. See when/reinit in the Modelica specification.