Electrical – How to set the input signal to LTspice with equal intervals

LT spice should be consistent from run to run. Thermal noise is not simulated in a normal transient simulation. If your seeing differences from run to run they could most likely be explained from a numerical solver standpoint and differences on rounding, which some circuits can be quite sensitive to.

Another explanation may be that the models you are using have b-sources with a .rand statement in it, but this is unlikely.

If you are using a .rand statement, there isn't a way to set the seed (documented anywhere that I know of). If you need a .rand section to be repeated from run to run, use a PWL source, record the data as a .wav or PWL file and use that to make the runs consistent.

In general, SPICE uses an adaptive step-size in transient simulations to minimize simulation time while maintaining accuracy. When circuit variables (node voltages, for example) are changing rapidly, it will take shorter time steps, and when circuit variables are changing slowly, it will take longer time steps.

You can specify a "maximum timestep" in case you want to override the built-in algorithm (either because you find the built-in algorithm doesn't give accurate results or to produce denser points for plotting). In your case, specifying a maximum of maybe 0.1 or 0.2 us would likely result in this timestep being used for all your simulation runs, at the cost of longer simulation times. Specify this in the "Edit Simulation Command" dialog.

Alternatively, you could post-process the results to interpolate output values on whatever time intervals you want, or simply do an x vs y plot in Excel instead of the standard y vs category plot.