In this article we will discuss about:- 1. Introduction to Modulation 2. Types of Modulation 3. Analysis 4. Sidebands and Energy Consideration 5. Modulators 6. Types of Detection 7. Analysis of Amplitude and Frequency Modulation 8. Generation of FM Wave 9. Classification of FM Detectors 10. Classification of Radio Transmitters 11. Types of Receivers.
Contents:
- Introduction to Modulation
- Types of Modulation
- Analysis of Modulation
- Sidebands and Energy Consideration in Modulation System
- Modulators
- Types of Detection by Modulators
- Analysis of Amplitude and Frequency Modulation
- Generation of FM Wave
- Classification of FM Detectors
- Classification of Radio Transmitters
- Types of Receivers
1. Introduction to Modulation:
Our music or speech is usually in the form of low-frequency waves lying in the audio frequency range of 0 – 15 kHz approximately which cannot be transmitted directly at a long distance. For transmission of intelligence or message from one place to another we have to take the help of high-frequency or radio-frequency waves. Such a process of raising the frequency of the intelligence or the message by superposition over high-frequency voltage is known as modulation.
ADVERTISEMENTS:
Or, briefly we may define modulation as a process of superposing information on a radio-carrier wave. Thus it is seen that the process of modulation arose due to the human desire to establish communication over a long path. On the other hand, the reverse process of translating back the information from the radio wave to the original frequency is called demodulation or detection.
To illustrate the phenomena more explicitly let us consider the waveforms of Fig. 7.1. Here Fig. 7.1(a) reveals the electrical equivalent of a single musical tone which is simply an a.c. sine wave of the corresponding frequency while an RF carrier wave of constant amplitude and frequency is shown in Fig. 7.1(b).
In Fig. 7.1(c) the RF carrier is being amplitude modulated by the AF wave so that the amplitudes of both the half-cycles vary in accordance with the audio signal. In (d) of Fig. 7.1 finally the frequency of the carrier is varied according to the amplitude of the AF signal. Note that if the positive amplitude of the audio wave is higher, the greater is the frequency of the RF carrier.
2. Types of Modulation
:
Equation (7.2) clearly shows that the a.c. wave has two parameters which may be varied or modulated.
These parameters are:
ADVERTISEMENTS:
(i) The amplitude, and
(ii) The angle, causing the following two basic types of modulation:
(a) Variation of amplitude Am with time, giving the amplitude modulation (AM),
(b) Variation of the angle with time, giving the angle modulation which has two subgroups, viz., (i) variation of ʃωdt with time resulting frequency modulation (FM) and (ii) variation of ɸ, the phase angle, with time resulting phase modulation (PM).
ADVERTISEMENTS:
3. Analysis of Modulation
:
ADVERTISEMENTS:
(a) Amplitude Modulation:
For this case the angular velocity ω is a constant factor at ωc’ where ωc is the so-called carrier frequency.
Equation (7.2) can then be written as-
(b) Frequency Modulation:
For this case the angular velocity ω may be expressed as-
(c) Phase Modulation:
For this case ω is a constant factor at ω0. Equation (7.2) then gives the form-
4. Sidebands and Energy Consideration
in Modulation System:
Both amplitude and frequency modulation systems serve well the basic function of communication. However, AM is more widely used due to its greater power output, simplicity of operation, etc. We shall here confine our attention only to the case of amplitude modulation. We have found that the equation for an amplitude-modulated wave may be written as-
Thus we see that the required bandwidth for transmission of an amplitude-modulated signal is twice the highest modulating frequency.
Again from equation (7.14) it appears that for both the sidebands the amplitude is ma/2 times the carrier amplitude. Since power is directly proportional to the square of the voltage, we have-
5. Modulators
:
In order to get the amplitude modulation of a radio carrier, the modulating signal is injected into the RF power amplifier stages of a transmitter. There are several circuits for this purpose such as Modulated- (i) plate circuit modulation, (ii) control or screen or suppressor grid circuit modulation, etc. Of these plate circuit modulation as discussed below is most commonly used Load because of its higher efficiency and simplicity in adjustment.
The fundamental circuit for plate modulation is shown in Fig. 7.2. Here the RF carrier is inserted to the grid point of a tuned RF amplifier stage while the AF modulating signal is injected in series with the d.c. plate supply. Now during the positive half-cycle of the audio signal a positive voltage is induced in the transformer secondary causing an increase of RF voltage across the tank circuit while during the negative half-cycle the voltage induced in the secondary subtracts from the plate supply voltage and thus resulting a decrease of the RF voltage across the tank.
In this fashion the amplitude of the plate circuit is altered with the audio signal and the modulated carrier appears in the secondary of RF output transformer. To obtain 100 percent modulation the peak amplitude of the audio signal is made equal to the plate supply voltage.
6. Types of Detection by Modulators:
By detection, also known as demodulation, the audio component of a modulated wave is separated from the RF carrier component.
Devices used for detection are called detectors which are classified into two categories:
(a) Linear diode detectors, and
(b) Square law detectors.
In linear diode detectors a linear relation exists between the carrier amplitude and the detected output voltage while a square law detector utilizes the non-linear portion of the dynamic current voltage characteristic. Since in the latter type, current flows continuously through the detecting circuit element, its behaviour can be analyzed easily using the power series representation.
Square law detectors of the following types are generally used:
(a) Diode detectors, and
(b) Triode detectors.
These detectors are widely used in radio receivers as they are comparatively free from distortion and also very suitable for the automatic volume control.
The fundamental circuit diagram and waveforms of a diode detector are shown in Fig. 7.3. It is seen that the RF modulated signal is coupled here to the plate of the diode through a trans-former. When the plate is positive with respect to the cathode the tube conducts and consequently a pulse of current flows through the tube and the load resistor RL. But during the negative half-cycle of the carrier signal no current flows.
Thus at the output of the diode a series of positive half-cycles of the carrier wave are obtained which, however, become much fast to be made audible in a speaker. To smooth out the remaining RF carrier variations a filter circuit consisting of a capacitor C and a load resistor RL is used. The capacitor C bypasses all RF components while the average components develop a voltage across RL and thus producing the desired detected output.
Since the dynamic current voltage characteristic follows the square law relation, we may write-
Obviously, the second term on the right-hand side of equation (7.18) provides terms in frequencies ωm, 2ωm, 2ωc, 2(ωc + ωm) and 2(ωc – ωm). The RF terms are bypassed by using high- value capacitor while the terms in frequencies ωm, 2ωm are formed across RL. Out of these, the term in frequency 2ωm is the distortion term while ωm constitutes the desired output.
Square law triode detectors are of the following two types:
(i) Anode bend detectors, and
(ii) Grid leak detectors.
The circuit diagram of the anode bend detector is shown in Fig. 7.4. The triode is biased heavily near the cut-off value so that the operation takes place near the non-linear portion of the transfer characteristic. Here also the RF components are bypassed by the capacitor C so that only the low frequency components are developed across the load resistor RL. One great advantage of this circuit is that it amplifies the incoming signal besides the detection.
A circuit diagram of a grid leak detector is shown in Fig. 7.5. Here the grid bias is set up by d.c. rectified current passing through R and the AF voltage across CgRg combination is amplified. Detectors of this type are not very sensitive and a strong signal voltage is required for their proper operation. Their use is, therefore, limited mainly to the highly sensitive superheterodyne receivers.
7. Analysis of Amplitude and Frequency Modulation
:
In this type of modulation, the signal is used to vary the amplitude of the carrier. Thus the modulated carrier has an envelope in the shape of the information signal (Fig. 7.6). This can be achieved by varying the plate potential of the transmitter tube generating the carrier. It is one of the possibilities and called plate modulation.
Let the sinusoidal carrier be represented as A cos(ωct + ɸ) and let the information signal be p(t) not necessarily sinusoidal. Then the modulated signal s(t) can be written as-
From above it is clear that m lies between 0 and 1. Modulation depth should not be greater than 1, otherwise on demodulation the original signal will not be recovered. This means C + p(t) will always be positive.
Spectrum of s(t):
We are now interested in finding out the complete frequency spectrum of s(t) obtained by using conventional AM. This is essential because it gives us the BW required of the amplifiers, used in transmitters.
If the frequency modulation were to be defined as the substitution of a variable ωc + ap(t) for the constant ωc in C(t) = Acos(ωct + ɸ), then a ridiculous situation comes. To explain it let us assume p(t) to be periodic as p(t) = cos ωpt, then-
To Determine the Expression for s(t):
Let us use equation (7.25) such that-
Frequency Spectrum of FM Wave:
Where Jn (mƒ) is the Bessel function of the first kind and nth order with argument mƒ.
Equation (7.33) shows that an FM wave has infinite sidebands spaced from the centre frequency ωc, by ωp, and its harmonics. The amplitude of various frequency components depends upon mf and can be calculated using a table of Bessel’s functions.
The nature of frequency spectrum is indicated for mƒ = 0.5, 1.0, 2.0, 3.0 and 20.0 in Fig. 7.10, which shows that-
1. At a given modulation index, mƒ, the components for which n exceeds a certain value can be ignored. For example, for mƒ = 2.0, one can neglect all the components beyond the 4th sideband.
2. Larger the value of mf more and more sidebands will have to be taken into account, otherwise the received signal will be distorted.
3. For mƒ value less than 1 (e.g., 0.5), the second and higher order sideband components are relatively small and the frequency band required to accommodate the essential part of the signal is the same as in AM. This is the case of narrow band FM. (Here the only difference from AM is that the phase of the carrier relative to the sidebands differs by 90° in narrow band FM.)
A useful thumb-rule regarding the bandwidth required for an FM signal can be obtained from-
Here, ƒmax is the maximum frequency of the baseband and ∆f is the frequency deviation of the total modulated signal.
It is to be noted here that for small mf, BW is equal to 2ƒp or 2ƒmax since the frequency deviation ∆ƒ is small and for large mf the value of BW = 2∆ƒ.
8. Generation of FM Wave
:
To modulate a carrier in frequency or in phase it is only necessary to vary the capacitance of the frequency determining LC circuit in the oscillator or of a phase shifting filter after the oscillator. This is what is done in practice with a solid state varactor diodes which have voltage sensitive capacitance. It is to be noted that since information signal modulating the carrier leaves its amplitude unchanged in an FM wave, the massage or the information is thus contained in the zero-crossings of the signal.
Hence an FM wave is insensitive to amplitude errors and consequently there is no special requirements for the linearity of the output amplifier. For this reason an FM transmitter maybe much simpler than an AM transmitter, which is an important aspect in mobile installations.
FM Generation using Varactor Diode:
An FM signal, as pointed out above, can be generated by variation of the junction capacitance of a varactor diode operated in reverse bias across the oscillator-tuned circuit, as indicated in Fig. 7.11.
The capacitance of the junction varies, if the modulating signal p(t) is superimposed on the bias voltage as v = -VBB + p(t).
With the capacitance of the diode proportional to the square root of the applied voltage, and with peak amplitude of p(t) kept small, a linear variation in frequency can be obtained as-
Using the same principle as in diode FM generator; here the tank circuit of an ordinary oscillator is shunted by the plate cathode circuit of a pentode, called the reactance tube (Fig. 7.12).
This pentode is so arranged as to draw a reactive current that is varied in accordance with the information signal. In Fig. 7.12, the pentode amplifier draws a reactive current since the voltage Vgk is 90° out of phase with respect to Vpk obtained by means of an RC phase splitter, in which R<<jωC. Thus, the voltage Vgk applied to the control grid is 90° out of phase with the voltage Vpk and hence causes the amplifier to draw Ipk that is leading Vpk by 90°.
This is equivalent to shunting the oscillator plate-cathode circuit with a capacitive reactance. The modulating signal p(t) superimposed on the grid voltage of the reactance tube varies the trans-conductance of the tube in accordance with p(t); and so varies the equivalent resistance offered to Vpk and thus the instantaneous frequency of the oscillator.
Such modulators are essentially low level modulators and power is raised to the required level by means of class C amplifiers.
[The FM produced by above two methods has some incidental AM because there is always some resistive current component in the output of the reactance tube and this acts as load on the oscillator tank circuit which will vary the amplitude of the generated oscillation; at the same time the frequency is being varied. Such residual AM is eliminated by means of a limiter, i.e., a device that develops an output that is substantially independent of the input voltage.]
Phase Modulators:
Phase modulators are important in FM because an FM wave is obtained when the information signal applied to a phase modulator is inversely proportional to the modulating frequency.
A popular method of generating a PM or FM signal is the Armstrong method. Fig. 7.13 gives the block diagram of the system. The output of the balanced modulator is DSB-SC. It is combined with a somewhat larger unmodulated carrier wave shifted in phase by 90° from the carrier associated with balanced modulator. This results in an FM wave since the only difference between a narrow band FM (mf < 1) and a conventional AM is that of the 90° shift of the carrier with respect to the sidebands.
A linear relationship between the modulating voltage and phase shift exists up to an mp ≅ 0.5.
9. Classification of FM Detectors:
A discriminator or FM detector performs the following two functions:
(i) It converts the frequency- modulated voltage into corresponding amplitude-modulated voltage using one or more tuned circuits, and
(ii) It subsequently rectifies this amplitude modulated voltage in linear diode detectors to extract the original modulation frequency voltage.
Discriminators are of following types:
i. Single tuned circuit discriminator or slope detector.
ii. Stagger tuned discriminator.
iii. Phase difference discriminators of following types:
(a) Centre tuned discriminator (or Foster-Seeley discriminator).
(b) FM limiter-discriminator using no node.
(c) FM limiter-discriminator using gated beam tube.
iv. Ratio detector.
10. Classification of Radio Transmitters:
Radio transmitters may be broadly classified according to the-
(a) Type of modulation used,
(b) Carrier frequency involved, and
(c) Service processes involved.
According to Modulation:
There are amplitude, frequency and phase modulation transmitters.
According to Carrier Frequency:
Long, medium and short wave transmitters, VHF and UHF transmitters and microwave transmitters.
According to Service Processes:
Radio broadcast, radio telephone and telegraph, television, radar and navigational transmitters.
AM Radio Transmitter:
An AM radio transmitter using modulation at high carrier power level is discussed here briefly-
(i) A master oscillator,
(ii) A number of frequency multipliers,
(iii) Several power amplifiers, and
(iv) Modulating system.
The master oscillator generates oscillation at a desired frequency which is kept constant within a very close limit. If the frequency of the oscillator becomes less than the desired carrier frequency of the transmitter the frequency multiplier stages, called harmonic generators, are used to get it. In order to isolate the harmonic generators from the master oscillator a buffer stage is used as shown, so that the variation of load, if any, may not affect the oscillator.
The RF voltage thus generated by the oscillator has a low power which is raised to a sufficiently high value by a chain of class C amplifiers and is then modulated by using usually series plate modulation of high efficiency. At low power levels, however, grid bias modulation and suppressor grid modulation are also used sometimes.
The modulating amplifier, generally a class B pushpull type, feeds audio power into the modulated amplifier which is finally connected to the transmitting antenna, as shown.
11. Types of Receivers:
A radio receiver picks up any RF signal through a receiving antenna and then recovers from it the original modulating signal.
Radio receivers may be broadly classified as:
(a) Amplitude Modulation (AM) broadcast receivers,
(b) Frequency Modulation (FM) broadcast receivers,
(c) Television (TV) Receivers,
(d) Radar receivers,
(e) Communication receivers, and
(f) Code receivers.
The essential functions of a radio receiver are:
(i) To extract the desired RF signal from the electromagnetic waves,
(ii) To amplify the RF signal,
(iii) To demodulate the RF signal for getting back the original modulating voltage, and
(iv) To feed the modulating voltage in an indicator, like loudspeaker, etc., for receiving the original programme.
Both AM and FM broadcast receivers are usually used in home for entertainment purposes.
The following characteristics of a broadcast receiver are of primary importance:
i. Adaptability to Different Aerials:
A receiver is to be designed in such a manner so that it can be applied to any type of aerial.
ii. Operational Simplicity:
Since broadcast receivers are used by listeners with little technical knowledge, a simplicity of operation is desired. A receiver in its most elementary form, has therefore three controls. These are band switch, tuning control and volume control.
iii. Good Fidelity:
Good fidelity of a receiver means a uniform frequency response over the entire audio frequency band. For an AM receiver the maximum modulating frequency is 5 kHz while for an FM receiver this is 15 kHz and so the latter gives a better fidelity.
iv. Average Sensitivity:
The term sensitivity of a receiver is defined as the minimum input voltage necessary for producing a standard output voltage. A receiver should have reasonably high sensitivity to achieve good response over the low and medium strength signals. However, it should not be high enough, since then it will pick up even the undesired disturbances.
v. Good Selectivity:
By the term selectivity of a radio receiver is meant its ability to differentiate a desired signal of a particular frequency from other unwanted signals of slightly different in frequencies. Good selectivity depends on the sharpness of the resonance curves of different tuned circuits used in the receiver.
Superheterodyne Receivers:
Major Armstrong invented the superheterodyne receiver during the First World War. In this receiver all incoming carrier frequencies are converted to a fixed lower value, called the intermediate frequency at which the amplifier circuits can operate with maximum stability, sensitivity and selectivity. Such conversion to the intermediate frequency (IF) is made by beating or heterodyning the carrier frequency against a locally generated frequency.
A block diagram of the basic superheterodyne circuit is shown in Fig. 7.17.
Functions of different stages are discussed below:
i. Antenna:
It receives all the electromagnetic waves and the voltages so induced are communicated to the receiver input where a parallel tuned circuit responds only to voltage at the desired carrier frequency.
ii. RF Amplifier:
The voltage developed across the capacitor of the input tuned circuit needs to amplified for detecting weak signals. This is done by one or two RF voltage amplifiers designed to amplify only narrow band of frequencies using tank circuits. By varying the capacitance or the inductance of the tank circuits the desired signal is selected and then amplified.
iii. Mixer and Local Oscillator:
In a superheterodyne receiver the carrier frequency ƒc and the frequency of the local oscillations ƒ0 are fed to a device called mixer, at the output of which a voltage of frequency (ƒc – ƒ0) is obtained. This frequency difference is known as the intermediate frequency (IF) and may be designated as ƒi. The typical value of ƒi is 456 kHz.
iv. IF Amplifier:
It consists of one or more stages of tuned voltage amplifier designed to amplify only a narrow band of frequencies around a fixed centre frequency. Since this centre frequency has a value intermediate between the radio and audio frequencies, the amplifier is designated as an IF amplifier. Most of the receiver amplifications and selectivity are provided by this IF amplifier.
v. Detector:
Output of the IF amplifier is fed to the detector to separate the modulating signal from the carrier wave which was superposed at the transmitter. The detector is usually a linear diode detector, the output of which gives the audio frequency signal.
vi. AF Amplifier:
Since the audio frequency signal obtained at the output of the detector is of insufficient amplitude, it is fed to the AF amplifier to provide additional amplification. Generally one stage of audio voltage amplifier is applied followed by one or more stages of audio power amplifier.
vii. Loudspeaker:
Through an impedance matching transformer the audio output voltage is fed to a loudspeaker which reproduces the original programme.