How can I set the input signal to LTspice with equal intervals?
To 'sample' nodes in LTspice you can use the .WAVE command to create a WAV audio file. The maximum resolution is 32 bits and maximum sampling frequency is 4096MHz.

The plot below was produced from your LTspice schematic. It was created in LTspice with .wave fltrtest22.wav 16 50 V(in) V(out) which specifies 16 bits and 50sps, then converted to csv text data using Sound eXchange (command line: 'sox fltrtest22.wav -t dat fltrtest22.csv'), loaded into Openoffice Calc as merged space-delimited text, and finally plotted on an x-y scatter chart.