Low pass filter( in LTspice simualtion)

It passes frequencies below the cut-off frequency and blocks all frequencies above it.

 

This is the schematic diagram of an RC low-pass filter.

The formula to calculate the cut-off frequency is:

$$f_\text{critical} = \frac{1}{2 \pi R C$$

The cut-off frequency of this low-pass filter is 1 kHz.

 

This is the frequency response plot of the low-pass filter.

There are two plots: the first is dB vs. frequency, and the second is output voltage vs. frequency.

The cut-off frequency of the low-pass filter is the point where the output voltage drops to 0.7 of its input voltage or where the output is at -3 dB. -3 dB is the point where the ouput power is half of input power.

In the plot, you can see that attenuation increases after 1 kHz, while there is no attenuation before 1 kHz.

The circuit passes frequencies below 1 kHz and blocks frequencies above 1 kHz.