Fri Jun 13 08:57:23 EDT 2014

"Lossless" is ambiguous: Transfer Function vs. Impedance

There is a big difference between a lossless transfer function and a
lossless electronic circuit as the "loss" refers to two different
things: signal level vs. signal energy.

A lossless transfer function needs to have a flat frequency response,
i.e. it's an allpass filter a.k.a. phase shifter.

A lossless circuit takes the form of a frequency dependent impedance,
e.g. an LC low-pass filter between power switcher and load in a class
D amplifier.

What is confusing in the latter is that a lossless circuit is often
used in combination with an ohmic load to create a freqency-dependent
signal transfer function.  There is obviously "loss" in the form of
dissipation in the load, however there is no loss in the L and C apart
from parasitics.