I'm doing my best to understand signals and systems. So I will give an example of what I'm talking about. you have a pressure transducer that has a 24vdc power supply and a voltage of 0-5vdc to measure the change in pressure, with the 0-5vdc connected to an analog to digital converter. With the analog to digital converter connected to a computer. Control systems use input frequencies to generate an output frequency. Is the analog to digital converter generating a sinusoidal input wave so the computer can generate a sinusoidal output? Pressure transducers can be bought with frequency outputs that rid the need of an analog to digital converter. Frequency will never be the same in a real system due to temperature or whatever else.