FREQUENCY DOMAIN ANALYSIS OF PERIODICALLY SWITCHED LINEAR CIRCUITS - Computer Methods for Analysis of Mixed-Mode Switching Circuits - page 271

Eq.(11.92) characterizes the average power spectral density of the re-
sponse of liner periodically time-varying systems with stationary inputs.
It can be used to compute the average output noise power of periodically
switched linear circuits. A few comments are made :

If the power spectrum of the input noise is broad-band, Nyquist the-
orem is violated. As a result, the sideband components of the input
noise are folded back to the base band. A pictorial illustration is given
in Fig.11.23 where the band width of the input noise is assumed to
be i.e.

The band width of the circuit is assumed to be infinite and its gain is
unity. Due to aliasing effect, the output noise power is 5 times that
of the input noise, i.e.

The contribution of the sideband components of the input noise
source clearly dominates the total output noise power.

For practical circuits with white noise sources, the total output noise
is computed from

where the maximum number of sidebands considered, N, is deter-
mined by the equivalent noise bandwidth of the circuits.

If there is no switching in the circuits, i.e. Eq.(11.92) simplifies
to the familiar expression for linear time-invariant circuits.