This particular filter has the characteristic of being a high-pass filter. That is to say it will pass frequencies above a certain frequency, called the cutoff frequency, and block frequencies below that frequency. A T-network with series capacitors and a parallel shunt inductor has the property of it being a high-pass filter. (E7C02) The reason the circuit acts this way is that as the frequency of a signal increases, capacitive reactance decreases and inductive reactance increases, meaning that lower-frequency signals are more likely to be shunted to ground.
A circuit containing capacitors and inductors can also form a low-pass filter. A low-pass filter is a circuit that passes frequencies below the cutoff frequency and blocks frequencies above it.
The circuit shown in figure E7C-2 is called a pi filter because it looks like the Greek letter pi. The capacitors and inductors of a low-pass filter Pi-network are arranged such that a capacitor is connected between the input and ground, another capacitor is connected between the output and ground, and an inductor is connected between input and output. (E7C01) The reason the circuit acts this way is that as the frequency of a signal increases, capacitive reactance decreases and inductive reactance increases, meaning that higher-frequency signals are more likely to be shunted to ground.
Pi networks can also be used to match the output impedance of one circuit to the input impedance of another or the output impedance of a transmitter to the input impedance of an antenna. An impedance-matching circuit transforms a complex impedance to a resistive impedance because it cancels the reactive part of the impedance and changes the resistive part to a desired value. (E7C04) One advantage of a Pi matching network over an L matching network consisting of a single inductor and a single capacitor is that the Q of Pi networks can be varied depending on the component values chosen. (E7C13)
A Pi network with an additional series inductor on the output describes a Pi-L network used for matching a vacuum-tube final amplifier to a 50-ohm unbalanced output. (E7C12) One advantage a Pi-L-network has over a Pi-network for impedance matching between the final amplifier of a vacuum-tube transmitter and an antenna is that it has greater harmonic suppression. (E7C03)
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