Version 4
SHEET 1 1804 680
WIRE 64 160 -16 160
WIRE 208 160 144 160
WIRE -16 192 -16 160
WIRE 208 192 208 160
WIRE -16 320 -16 272
WIRE 208 320 208 256
WIRE 208 320 -16 320
WIRE -16 336 -16 320
FLAG -16 336 0
SYMBOL cap 192 192 R0
SYMATTR InstName C1
SYMATTR Value 10nF
SYMBOL res 160 144 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 10k
SYMBOL voltage -16 176 R0
WINDOW 3 24 152 Left 2
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value PULSE(0 1 0 1n 1n 0.5ms 1ms)
TEXT 448 168 Left 2 ;a) The simulation time of 20ms is an integer multiple of the signal’s period time (1ms). \nb) A maximum time step(in .tran command) of 100ns is corresponding to a minimum sample frequency of 1/100ns=5MHz\nThis gives a Shannon corner frequency of 0.5 x sample frequency, this frequency >> 1MHz(input frequency)\nc) The number of true samples is 20ms / 100ns = 200 000. So we enter  131 072(=2^17) sampled data points in time
TEXT -50 380 Left 2 !.options plotwinsize=0\n.tran 0 20ms 0 100ns

将以上内容保存成1RC_lowpass_fft.asc文件。

[Transient Analysis]
{
   Npanes: 2
   Active Pane: 1
   {
      traces: 1 {524290,0,"V(n001)"}
      X: ('m',0,0,0.002,0.02)
      Y[0]: (' ',1,0,0.1,1)
      Y[1]: ('_',0,1e+308,0,-1e+308)
      Volts: (' ',0,0,1,0,0.1,1)
      Log: 0 0 0
      GridStyle: 1
   },
   {
      traces: 1 {524291,0,"V(n002)"}
      X: ('m',0,0,0.002,0.02)
      Y[0]: (' ',1,0,0.1,1)
      Y[1]: ('_',0,1e+308,0,-1e+308)
      Volts: (' ',0,0,1,0,0.1,1)
      Log: 0 0 0
      GridStyle: 1
   }
}

将以上内容保存成1RC_lowpass_fft.plt文件，然后执行Run进行仿真，结果如下：

然后再Waveform Viewer中右键View–> FFT,打开FFT设置对话框，输入你需要的参数，如下图：

然后点击OK，会出现FFT之后的结果：