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I have a question based on low pass filtering. If I input a signal to this low pass filter, why does the output file have an AC (of very low frequency).

LTSpice Circuit

Frequency Response of LPF

The input signal is$$ v_i(t) = 2 + sin(2*pi*100Mhz*t)$$ According to the filter response, the $$ sin(100Mhz*t) $$ should be attenuated by a very large number, thus resulting in a coefficient << 0 and leaving the DC component of 2 left. The output however is $$ v_o = 2 + sin(2*pi*0.1t) $$


Where does that AC signal come from?

Time Domain of Capacitor Voltage

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    \$\begingroup\$ Change "M" to "Meg". \$\endgroup\$
    – jonk
    Commented Aug 5, 2020 at 16:59
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    \$\begingroup\$ In other words, your source is running at 100 millihertz = 0.1 Hz, which is obvious from the 10-second period. \$\endgroup\$
    – Dave Tweed
    Commented Aug 5, 2020 at 17:09
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    \$\begingroup\$ @Aaron The triangle wave is likely due, instead, to "numerical behaviors" related to truncation/rounding in floating point notation. You are using a very fast sine wave and a long period. If you'd like to see more "normal" plotting, try reducing the frequency by a factor of 100 and reduce the time by a factor of 1000 and see how that plots out. \$\endgroup\$
    – jonk
    Commented Aug 5, 2020 at 17:36
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    \$\begingroup\$ @jonk yes a triangle signal is almost equal to a sine signal with very low frequency. Convert DC to AC doesn't produce a perfect sine wave, it is just little changes over time but not continuous discrete. \$\endgroup\$ Commented Aug 5, 2020 at 17:36
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    \$\begingroup\$ @HelenaWells I believe that all conforming Spice implementations use the name notation. So downloading a newer version won't help. \$\endgroup\$
    – jonk
    Commented Aug 5, 2020 at 17:37

2 Answers 2

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Where does that AC signal come from?

The AC signal comes from the source, a passive linear filter can only change the amplitude and the phase, it cannot change the frequency of the waveform that it is filtering.

So the AC diagram shows the amplitude for all frequencies, and the chart below (in the OP) shows the output for one frequency.

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Where does that AC signal come from?

The AC signal comes from the source. Filters do not change the type of input signal, they change the amplitude and the phase of the input signal.

If you want it to convert it to DC you will need a rectifier as long as with a filter and then the filter will 'filter out' the input signals converting some of them to output DC with a detectable amplitude.

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