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I've used the FFT analysis tool to see the spectrum of DM and CM EMI noise in the range of 150K to 30MHz.

But the analysis stops at about 3MHz automatically and there is no option to set the max and min frequencies. how can I have the spectrum at the desired frequency range (150k~30MHz)?

enter image description here

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    \$\begingroup\$ Maybe you .tran time steps are not fine enough. Did you set the max time step option in the .tran settings ? \$\endgroup\$
    – tobalt
    Jul 21, 2022 at 14:13
  • \$\begingroup\$ @tobalt, I didn't define it at all. what is the recommended value? BTW, the Run time would be very high with for example 100ns time step. \$\endgroup\$
    – WeTech
    Jul 21, 2022 at 14:21
  • \$\begingroup\$ Well for 1 GHz Nyquist frequency you would need 500 ps... \$\endgroup\$
    – tobalt
    Jul 22, 2022 at 4:18

1 Answer 1

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If you're doing an AC analysis in LT spice then you need to change the max sweep frequency in the .AC statement.

If you're doing a .trans analysis and converting it with the fft later then you need to increase the time resolution.

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