Because the impedance of inductors and capacitors vary with frequency, we often make filters out of them. One of the most common is the T-network filter, so called because it looks like the letter T. An example is shown in figure E7C-1.
This particular filter is a T-network 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. 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.
QUESTION: Which of the following is a property of a T-network with series capacitors and a parallel shunt inductor? (E7C02)
ANSWER: It is a high-pass filter
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 low-pass, pi filter because it looks like the Greek letter π. 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. The reason the circuit acts as a low-pass filter 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.
QUESTION: How are the capacitors and inductors of a low-pass filter Pi-network arranged between the network’s input and output? (E7C01)
ANSWER: 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
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. 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.
QUESTION: How does an impedance-matching circuit transform a complex impedance to a resistive impedance? (E7C04)
ANSWER: It cancels the reactive part of the impedance and changes the resistive part to a desired value
QUESTION: What is one advantage of a Pi-matching network over an L-matching network consisting of a single inductor and a single capacitor? (E7C12)
ANSWER: The Q of Pi-networks can be controlled
A Pi network with an additional series inductor on the output is called a Pi-L network. They are often used for matching a vacuum-tube final amplifier to a 50-ohm unbalanced output. 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 than a T network.
QUESTION: Which describes a Pi-L-network used for matching a vacuum tube final amplifier to a 50-ohm unbalanced output? (E7C07)
ANSWER: A Pi-network with an additional series inductor on the output
QUESTION: What advantage does a series-L Pi-L-network have over a series-L Pi-network for impedance matching between the final amplifier of a vacuum-tube transmitter and an antenna? (E7C03)
ANSWER: Greater harmonic suppression
In addition to being used to control the frequency of oscillators, piezoelectric crystals are used to build filters. A crystal lattice filter is a filter with narrow bandwidth and steep skirts made using quartz crystals. The relative frequencies of the individual crystals is the factor that has the greatest effect in helping determine the bandwidth and response shape of a crystal ladder filter. The narrowness of the bandwidth and the steepness of the skirts are sometimes called the filter’s shape factor. The shape factor affects a filter’s ability to reject signals on adjacent frequencies.
QUESTION: What is a crystal lattice filter? (E7C09)
ANSWER: A filter with narrow bandwidth and steep skirts made using quartz crystals
QUESTION: Which of the following factors has the greatest effect on the bandwidth and response shape of a crystal ladder filter? (E7C08)
ANSWER: The relative frequencies of the individual crystals
QUESTION: Which of the following describes a receiving filter’s ability to reject signals occupying an adjacent channel? (E7C11)
ANSWER: Shape factor
Different types of filters have different characteristics. For example, a Chebyshev filter that has a sharp cutoff, but also ripple in the passband. An elliptical filter, on the other hand, has an extremely sharp cutoff with one or more notches in the stop band.
QUESTION: Which filter type is described as having ripple in the passband and a sharp cutoff? (E7C05)
ANSWER: A Chebyshev filter
QUESTION: What are the distinguishing features of an elliptical filter? (E7C06)
ANSWER: Extremely sharp cutoff with one or more notches in the stop band
Often, you’ll choose a filter type for a particular application. For example, a cavity filter would be the best choice for use in a 2 meter repeater duplexer.
QUESTION: Which of the following filters would be the best choice for use in a 2 meter band repeater duplexer? (E7C10)
ANSWER: A cavity filter
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