In this module you will learn about how inductors and capacitors behave in AC circuits, this is the basis of understanding filters and tuned circuits which are the fundamental building blocks of radio (Finally he mentions radio!) You will also learn about the Decibel measurement system.
I would also like you to read the chapter on radio wave Propagation in your course book Chapter 3.
Firstly study Inductors in AC circuits
See pages 5-15 in Your Course Book
Inductive Reactance
Note that even though reactance is measured in Ohms a Different symbol is used to represent it ( XL )
The Inductive Reactance Formula
You Will need to learn this formula. Note the relationship between frequency and Inductive reactance especially how Reactance increases with frequency, inductors are often used in radio circuits to block the passage of high frequency AC currents whilst allowing low frequency AC and DC current to flow, this forms the basis of a Low pass Filter.
Series and parallel XL
Note the similarities to series and parallel resistance's. You cannot however mix reactance ( X ) and resistance ( R ) in the same formula.
XL And ohm's Law
Current And voltage in XL Circuits
Pay careful attention to the phase angle it is important to remember that the Current always lags behind the voltage in XL Circuits
Power in XL Circuits
This is important as it explains why you must not use the power formula that you learned in Module 1 in Circuits containing reactance. You will also have to understand that there is no Average power consumed in reactive loads as all energy is returned to the circuit.
Now might be a good time to learn about the Q (or Quality factor) of an inductor
The capacitive Reactance Equation
Note the similarities to the Inductive Reactance Formula. You will need to learn this formula. Notice how reactance decreases with frequency, capacitors are often used in radio circuits to block the passage of DC and low frequency AC currents whilst allowing the passage of High frequency AC current, this forms the basis of a High Pass Filter.
Series and Parallel XC
Note that this is exactly the same method as used for Series and parallel XL but you must not add XC and XL in the same equation. We will deal with circuits containing both XC and XL as RLC Tuned Circuits.
Ohm's Law for XC
Current and Voltage in XC Circuits
pay careful attention to the Phase angle. It is very important to understand that the current of a capacitor always leads the voltage across the capacitor by 90 degrees.
Power in XC Circuits
You will also have to understand that there is no Average power consumed in reactive loads as all energy is returned to the circuit.
A circuit containing both an inductor and a capacitor and therefore, both inductive and capacitive reactance, is often called a tuned circuit.
There is a particular frequency at which the inductive and capacitive reactances are the same, that is, XL = XC. For most purposes, this is the resonant frequency of the circuit.
The resonant frequency is given by the formula
Where
f = frequency in hertz (Hz),
L = inductance in henrys (H),
C = capacitance in farads (F), and
pi
= 3.1416.
For most high-frequency (HF) radio work, smaller units of inductance and capacitance and larger units of frequency are more convenient. The basic formula becomes:
Where
f = frequency in Kilohertz (KHz),
L = inductance in micro henrys (micoH),
C = capacitance in picofarads (pF), and
pi = 3.1416.
Learn this formula as it forms the basis of many exam questions.
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A schematic diagram of a series-resonant circuit.
Although most schematic diagrams of radio circuits would show only the inductor and the capacitor, resistance is always present in such circuits. The most notable resistance is associated with losses in the inductor at HF; resistive losses in the capacitor are low enough at those frequencies to be ignored. The current meter shown in the circuit is a reminder that in series circuits, the same current flows through all elements. At resonance, the reactance of the capacitor cancels the reactance of the inductor. The voltage and current are in phase with each other, and the impedance of the circuit is determined solely by the resistance. As the frequency is shifted above or below the resonant frequency without altering component values, however, the reactive factor becomes significant, and the value of the current becomes smaller than at resonance. At frequencies far from resonance, the reactive components become dominant, and the resistance no longer significantly affects the current amplitude. The exact curve created by recording the current as the frequency changes depends on the ratio of reactance to resistance. When the reactance of either the coil or capacitor is of the same order of magnitude as the resistance, the current decreases rather slowly as the frequency is moved in either direction away from resonance. Such a curve is said to be broad. Conversely, when the reactance is considerably larger than the resistance, the current decreases rapidly as the frequency moves away from resonance, and the circuit is said to be sharp. A sharp circuit will respond a great deal more readily to the resonant frequency than to frequencies quite close to resonance; a broad circuit will respond almost equally well to a group or band of frequencies centred around the resonant frequency. Both types of resonance curves are useful. A sharp circuit gives good selectivity, The ability to respond strongly (in terms of current amplitude) at one desired frequency and to discriminate against others. A broad circuit is used when the apparatus must give about the same response over a band of frequencies, rather than at a single frequency alone. |
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Although series-resonant circuits are common, the vast majority of resonant circuits used in radio work are parallel-resonant circuits. Here is a typical parallel-resonant circuit.As is the case for series-resonant circuits, the inductor is the chief source of resistive losses, and these losses appear in series with the coil. Because current through parallel-resonant circuits is lowest at resonance, and impedance is highest, they are sometimes called antiresonant circuits. Likewise, the names acceptor and rejector are occasionally applied to series- and parallel-resonant circuits respectively. |
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Q in Tuned Circuits
As with inductor Circuits the term Q is used to present the 'goodness' of a tuned circuit, the main factor in determining the Q in most RF tuned circuits is the resistive losses and other inefficiencies associated with the inductor, it is unusual for capacitive losses to have much of an effect on the overall Q factor of a tuned circuit in normal radio circuits. It is often desirable to Broaden the bandwidth of a tuned circuit, for example where a tuned circuit is required to either pass or stop a range of frequencies |
Finally Please read about Decibels and the Decibel measurement system as used in radio, Please learn the most common ratios ( 3dB and 10 dB ) as this will enable you to perform most Decibel calculations relative ease.
Chapter 5-9 In your course book Covers Decibels