...axis.

It is tempting to state that the frequency response, when it exists, is always a special case of the transfer function. However, it is possible (though somewhat unusual) that a system can have a well defined frequency response, but not have a well defined transfer function. An example of such a system is the ideal low pass filter.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...circle.

It is tempting to state that the frequency response, when it exists, is always a special case of the transfer function. However, it is possible (though somewhat unusual) that a system can have a well defined frequency response, but not have a well defined transfer function. An example of such a system is the ideal low pass filter.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.