THE CASCADING FILTERS
This is called cascading the arrangement of two or more circuits one next to another on the signal path, to multiply the effect that would be achieved with a single one. We deliberately leave out of consideration the effects that such a configuration would produce on the phase relations between the input signal and the output signal, dealing with the question theoretically.
Anticipating a matter that will be developed later, we'll say that a fourth-order filter is formed by the cascading disposition of two second-order filters. This produces, as already seen, a rolloff doubled if compared to that allowed by a single filter of half the order. But this event can be verified only if the two cascading filters are tuned on the same cut frequency. Suppose instead the case of two filters which cut frequencies are close but not coincident. For example, a second-order lowpass tuned to 150Hz next to a similar one tuned to 80Hz. The output signal will be characterized by a constant rolloff of 12dB/oct. immediately above the cut frequency of the first filter, becoming even steeper — exactly doubled — gone beyond 150Hz.
The cascading configuration , although of little utility in practical cases, helps to better understand the working principles of filters, tracing complex structures to the combination of elementary parts that have produced them. It's really with this attitude that, without getting too frightened, we can get to the heart of the tutorial examining in each type of filter the circuitry patterns