MM0704, Lesson 2
undesirable frequencies find a low impedance path through the series-tuned circuits and a high impedance path through
the parallel-tuned circuits. Thus, the undesirable band of frequencies are rejected or suppressed. The narrow band of
desired frequencies find a high impedance path through the series-tuned circuit and a low impedance path through the
parallel-tuned circuit, which is passed on to the load.
T-section. In this circuit, undesired frequencies that manage to get past the first parallel-tuned circuit find a high
impedance path toward the load and a low impedance path through the series-tuned circuit to ground (figure 2-14).
Figure 2-14. T-Section Band-Rejection Filter
-section. This series-tuned tank circuit (figure 2-15) has maximum line current flow at the resonant frequency, and
the parallel-tuned tank has minimum current flow in the line at resonance. The combination of a parallel and two
shunting series tanks makes an easy path to ground for undesirable frequency currents. The -section filter with
carefully selected components, can reject a very narrow band of frequencies, because, as with all filters, the more tanks,
the sharper will be the sides of the pass band.
Figure 2-15. -Section Band-Rejection Filter.