ADVANTAGES OF CASCADING REFLECTIONLESS FILTERS

ADVANTAGES OF CASCADING REFLECTIONLESS FILTERS

The insertion loss curves for the conventional filter exhibit an expected increase in stopband rejection when two filters are cascaded in series. However, obvious ripple appears across the stopband in the two-section curve. This is due to the unstable phase relationship between the through-signal and reflected signal. Additionally, unwanted ripple is present in the passband close to the band edge of the two-section curve. This is a result of return loss degradation in the passband and reflections in the transition. By contrast, the insertion loss performance for the reflectionless filter repeats itself nicely when cascaded in 2 and 3 sections without any of the ripples or distortion seen in the case of the conventional filter.
Figures 5e through 5h show the effect on return loss when the two types of filters are cascaded in multiple sections. The conventional filter exhibits significant degradation in input and output return loss in the passband when cascaded in two sections – by as much as nearly 20 dB in some regions. When the reflectionless filter is cascaded in two sections, on the other hand, input and output return loss varies over the passband, but the same degradation is not evident, and return loss actually increases at some frequencies relative to that of a single filter. This illustrates that an improvement in return loss in the passband and the stopband can be realized by cascading reflectionless filters versus conventional filters.