Hi Violet,
In Simulink, the Zero-Order Hold (ZOH) block is used to model the discrete-time representation of a continuous-time system. The ZOH block holds the input value constant for the duration of the sample time and then updates it at the next sample time.
However, the ZOH block itself does not introduce any filtering action or affect the frequency response of the system. It simply represents the discrete-time sampling process.
If you are expecting some filtering action from the ZOH block, you may need to include additional filtering components in your model. The ZOH block is typically followed by a discrete-time filter, such as a low-pass filter, to achieve the desired filtering behaviour.
In your current setup with a P-controller (gain of 1) and the ZOH block, the resulting Bode plot being at 0 dB regardless of the sample time is an expected behaviour. The P-controller has a constant gain, and the ZOH block does not introduce any additional filtering. Therefore, the frequency response remains flat at 0 dB.
To introduce filtering action, you can modify your model by adding appropriate filter components after the ZOH block. This can include discrete-time filters, such as low-pass filters or other types of filters, depending on the desired frequency response and system behaviour.
