To experiment(out of curiosity) the effect of a low pass filter for an SMPS switching noise I wanted to try a filter where an LC cascaded by a linear regulator as shown in below diagram, I obtained the AC analysis in LTspice.
I don't know anything about the output impedance of the SMPS so I chose is as 1 Ohm. Then I swept the input between 0.1Hz to 1 Meg Hz.
But why in the Bode plot Vout starts from -180dB but not 0dB?
Following some comments I superimposed the noise on a 24V DC as shown below:
The AC coupling caps C4 and C3 are there to view the noise component at the input and output.
Here is when 1V 100 kHz square wave noise(with 1p rising and falling edges) is superimposed on the 24V DC:
Here is when 1V 100 kHz sine wave noise is superimposed on the 24V DC:
Below is when 1V 0.1 Hz sine wave noise is superimposed on the 24V DC:
And finally here is the AC response when superimposed on the 24V DC:
It seems in the AC response at 0.1 Hz the y axis shows around -70dB. This corresponds to a voltage ratio of 3162 and for 1V the output would be expected as 320uV which almost matches to the 0.1 Hz transient response noise amplitude.
Are these results expected in practice? If so it seems to me the linear regulator works as a very very strong low pass filter.