Phase Modulation -The modulation where the phase of the carrier signal is changed according to the low frequency of the message signal is called phase modulation.
The Phase Modulation is a modulation in which the phase of the carrier wave changes according to the instantaneous amplitude of the modulating signal keeping amplitude and the frequency as constant. The Phase Modulation and the Frequency Modulation are similar but in Phase Modulation frequency of the carrier signal is not increased. It is used in mobile system. Phase of the carrier wave is modified in order to send the data or information.
This modulation is widely used especially for data transmissions is Phase Modulation (PM). Phase and frequency are inextricably linked both forms of modulation are often referred to by the common term angle modulation.
How phase modulation works:-
First it is necessary to give an explanation of phase. The Radio signal consists of an oscillating carrier in the form of a sine wave. Amplitude follows this curve moving positive and then negative and by returning to the start point after one complete cycle. it can also be represented by the movement of a point around a circle . Phase at any given point being the angle between the start point and point on the waveform as shown in Figure
The Pictorial Way to View the Generation of FM and PM
The idea of signal phase modulation and signal frequency modulation and difference between them are often a source of confusion. Similar for the related idea of phase distortion. Here, we’ll try to clarify things. For understanding phase distortion recall that any waveform can be considered to be a sum of sine waves of different amplitudes and the frequencies. The Phase distortion arises when some frequencies get through a system more slowly than other frequencies. Effect generally known as group delay is to modify the shape of the resulting waveform but not its fundamental frequency. The change in shape is considered a phase distortion. To complex matters somewhat the term phase distortion is sometimes applied to the case where a reactive component changes the relationship between instantaneous current and the voltage. The load is purely resistive applied voltage and the current are in phase. If the load is reactive meaning that it is either capacitive or inductive the voltage and current waveforms are no longer in phase. The point to note about the phase shift caused by purely reactive components is that the shape of the pure sine wave doesn’t change.
The phase modulation of the signal changes the phase from what it would have been if no modulation were applied. Similarly the speed of rotation around the circle is modulated about the mean value. This it is necessary to achieve to change the frequency of the signal for a short time. In this way when phase modulation is applied to a signal there are frequency changes and vice versa. The Phase and the frequency are inseparably linked as phase is the integral of frequency. The Frequency modulation can be changed to phase modulation by simply adding a CR network to the modulating signal that integrates the modulating signal. In this way this information regarding sidebands bandwidth and this also holds true for phase modulation as it does for frequency modulation bearing in mind their relationship.
Phase shift keying
The Phase modulation may be used for the transmission of data. The Frequency shift keying is robust and has no ambiguities because one tone is higher than the other. So this PSK has several benefits in terms of efficient use of bandwidth and this form of modulation chosen for many cellular telecommunications applications.
The phase shift keying is the basic form known as Binary Phase Shift Keying (BPSK) or occasionally Phase Reversal Keying (PRK). The digital signal alternating between +1 and 1 (or 1 and 0) will create phase reversals e.g.180° phase shifts as the data shifts state.
Binary phase shift keying.
This PSK is the problem that receiver cannot know the exact phase of the transmitted signal to determine whether it is in a mark or space condition. This is not even possible as transmitter and this receiver clocks were accurately linked because the path length would find the exact phase of the received signal. For overcoming this problem PSK systems use a differential method for encoding the data onto the carrier. It is accomplished by e.g. Creating a variation in phase equal to a 1 and no phase change equal to a 0. Improvements can be made upon this basic system and the number of other types of phase shift keying have been developed. Simple improvement can be made by making a change in phase of 90° in one direction for a 1 and the 90° the other way for a 0. They retains the 180° phase reversal between the 1 and the 0 states but gives a distinct change for a 0. The basic system not using this process it may be possible to lose synchronization if the long series of zeros is sent. This is due to reason that the phase will not change state for this occurrence.
There are many changes on the main idea of phase shift keying. It has its own advantages and disadvantages enabling system designers to choose the one most applicable for any given circumstances. Some common forms include Quadrature Phase Shift Keying (QPSK) where four phase states are used each at 90° to the other 8 PSK where there are eight states and so forth.
We often convenient to represent a phase shift keyed signal and other types of signal, using a Phasors or constellation. The scheme the phase of the signal is represented by the angle around the circle and the amplitude by the distance from the origin or center of the circle. This way signal can be resolved into quadrature components representing the sine or I for In-phase component and cosine for the quadrature component. The Phase shift keyed systems use a constant amplitude and points appear on one circle with a constant amplitude and changes in state being represented by movement around the circle. In this binary shift keying using phase reversals the 2 points appear at opposite points on the circle. Some forms of phase shift keying may use different points on the circle, and it can be more points on the circle.
As plotted using test equipment, errors may be seen from the ideal positions on the phase diagram. The errors may appear as the result of inaccuracies in the modulator transmission and the reception equipment or the noise that enters the system. It is imagined that as the position of the real measurement when it is compared to ideal position becomes very large then data errors will appear because the receiving demodulator is not able correctly to detect the required position of the point on the circle.
A constellation view of the signal enables quick faultfinding in a system. This difficulty is related to phase the constellation will go around the circle. And if the problem is related to magnitude the constellation will spread off the circle either towards or away from the origin. The graphical techniques assist in isolating problems much faster than when using other methods.
The QPSK is used for the forward link from the base station to the mobile in the IS-95 cellular system and the absolute phase uses position to represent the symbols. And there are 4 phase decision points and when transitioning from one state to another it is possible to pass through the circle’s origin, indicating minimum magnitude.
Similarly On the reverse link from mobile to base station Offset Quadrature Phase Shift Keying (OQPSK) is used to prevent transitions through the origin. The components that make up any particular vector on the constellation diagram as X and the Y components. As normally both of these components would transition simultaneously causing the vector to move through the origin.
Minimum shift keying
The binary data containing sharp transitions b/w 1 and 0 states vice versa potentially create signals that have sidebands extending out a long way from the carrier this is not ideal from many aspects. The filtering can overcome part of the signal while transitions in the data become progressively less sharp as the level of filtering is increased and bandwidth is reduced. For overcoming this a form of modulation called as Gaussian filtered Minimum Shift Keying (GMSK) is widely used e.g. This has been adopted for the GSM standard for mobile telecommunications. It is derived from a modulation scheme known as Minimum Shift Keying (MSK), which is what is known as a continuous phase scheme. Here there are no phase discontinuities because frequency changes occur at the carrier zero crossing points.
Let’s take the example shown in Figure. It can be observed that the modulating data signal changes the frequency of the signal there are no phase discontinuities. This increase as a result of the unique factor of MSK that the frequency difference between the logical 1 and logical 0 states is always equal to half the data rate. it can be expressed in terms of the modulation index and which is always equal to 0.5.
This way appears to be fine in fact the bandwidth occupied by an MSK signal is too wide for many systems where the maximum bandwidth equal to the data rate is required.
The capacitive circuit current leads the voltage. Which happen that the impedance in a capacitive circuit is minimum when rate of change of the applied voltage is greatest and maximum when rate of change is least. The capacitive circuit reacts not to the voltage level but to the rate of change. This is why current is said to lead voltage in a capacitive circuit. In this way an inductive circuit impedes current flow the most when rate of change is greatest and impedes the least when rate of change is lowest. This way to think of it is that as applied voltage in an inductive circuit rises energy is temporarily stored in the magnetic field that is being established around the conductive elements and falls that energy is fed back into the conductive elements in both cases opposing the change in voltage. This why current is said to lag voltage in an inductive circuit. The technique called phase distortion synthesis introduced by Casio in its music synthesizers. This concept can be a bit confusing because the Casio modulated a carrier with another waveform but so in a manner that the distorted each cycle of the carrier without changing its frequency. Modulators were different angular waves that distorted the carrier by changing the point in time of its first zero crossing within each full waveform period. Similarly technique is now available in various music generator ICs.Which brings us to phase modulation and frequency modulation. Both modulation techniques are classified as angle modulation techniques. Angle modulation affects the phase angle and thus the frequency of the signal. With the phase modulation the change of the phase angle is proportional to the message that is to be modulated onto the signal. Contrary the instantaneous change in frequency is proportional to the message that is to be modulated onto the signal in frequency modulation.
The phase modulation the modulation index h (i.e. the degree of modulation) is directly proportional to the modulating voltage only while in the frequency modulation. This modulation index is also inversely proportional to the modulating frequency.
PM h = Kθ
K is constant of proportionality and θ is modulating voltage amplitude.
FM h = ∆f/F
The ∆f= maximum frequency deviation and F = the modulating frequency. Hence when modulating frequency changes the PM modulation index will remain constant while the FM modulation index will rise as modulating frequency diminishes vice versa. A few misunderstandings about FM and PM that may lead to some confusion. The belief is that FM solely modulates the frequency and keeps a constant phase angle while PM keeps a constant frequency and the phase angle is modulated. As a practical matter it is tough to keep a constant frequency while changing the phase angle and vice versa. The change in phase angle always causes an instantaneous change in frequency and the change in frequency must cause an instantaneous change of the phase angle. Here the word instantaneous is important as rate of change and the instantaneous rate of change are 2 completely different things. The Phase modulation is capable of conveying analog information but more suitable for digital transmission in which it is widely used. This comprises the contextual framework for schemes that include phase shift keying (PSK) binary phase shift keying (BPSK) quadrature phase shift keying (QPSK) eight-point phase-shift keying (8PSK) sixteen point phase shift keying (16PSK) and offset phase shift keying (OPSK).
In many kinds of modulation techniques modulation index is called as the change in the modulated variable in correspondence with its unmodulated variable. It corresponds to the deviations in phases of the carrier signal. Which is denoted as
Here M.I = ∆ ∆ϴ represents the peak phase difference.
Forms of Phase Modulation
The PM is used in analog transmissions which is widely used as a digital type of modulation and it is controls among dissimilar phases which is called as phase shift keying and there several forms are available in this.
Which is still possible to merge phase shift keying and the amplitude keying in a type of modulation is also called as quadrature amplitude modulation.
The FM forms that are used are listed below.
- Phase Modulation (PM)
- Phase Shift Keying (PSK)
- Binary Phase Shift Keying (BPSK)
- Quadrature Phase Shift Keying (QPSK)
- 8-Point Phase Shift Keying (8 PSK)
- 16-Point Phase Shift Keying (16 PSK)
- Offset Phase Shift Keying (OPSK)
The list is some of the forms of PM which are frequently used in the applications of radio.
Phase Modulation Advantages and Disadvantages
The phase modulation advantages include the following.
- The Phase modulation (PM) is a simple contrasted to Frequency modulation (FM).
- PM is used to find out the velocity of a target by removing Doppler data. Which needs constant carrier which is achievable during phase modulation however not in FM (frequency modulation).
- The advantage of this modulation is signal modulation because it permits computer for communicating on high-speed using a telephone system.
- The information is being transmitted without intrusion then the speed rates can be observed.
- PM is improved immunity toward the noise.
Phase modulation disadvantages include the following.
- The Phase modulation needs two signals by a phase variation among them. Through this both the 2 patterns are required like a reference as well as a signal.
- The modulation requires hardware which obtains more complex due to its conversion technique.
- The Phase ambiguity arrives if we exceed index pi radian of modulation.
- The Phase modulation index can be enhanced by employing frequency multiplier.
Applications of Phase Modulation
The phase modulation applications include the following.
- Modulation is useful in radio waves transmission, and it is an essential element in several digital transmission coding schemes.
- The PM is widely used for transmitting radio waves and an element of many digital transmission coding schemes that help an ample range of wireless technologiessuch as GSM Satellite television and Wi-Fi.
- PM is used in digital synthesizers for generating waveform and signal
- Phase modulation is used for signal and waveform generation in digital synthesizers like Yamaha DX7 for phase modulation synthesisimplementation, and Casio CZ for sound synthesis which is called as phase distortion.
Difference of PM from Frequency Modulation
The PM and FM varies in the below characteristics
- The PM phase changes are linearly related to the modulating signal while FM frequency changes are linearly related to the modulating signal.
- The Noise immunity lies in between AM and FM for phase modulation and in FM noise exemption is more than that of PM.
- The signal to noise ratio is not much better than that of frequency modulation in PM and FM signal-to-noise ratio is more.
- The Mobile radio services mostly implement phase modulation and FM is utilized for commercial radio broadcastings.
Difference of from Amplitude Modulation
|S.NO.||AMPLITUDE MODULATION||PHASE MODULATION|
|1.||Amplitude Modulation the frequency remains the same.||Phase Modulation the amplitude remains the same.|
|2.||Simple circuit.||Complex circuit.|
|3.||Modulation index varies from 0 to 1.||Modulation index is always greater than one.|
|4.||The Noise immunity is poor than in PM.||Noise immunity is better than in AM.|
|5.||Amplitude Modulation the phase remains the same.||Phase Modulation the frequency remains the same.|
|6.||Amplitude Modulation is widely used.||Phase Modulation is used in mobile system.|
|7.||Signal to noise ratio is better than Phase Modulation.||Signal to noise ratio is poor than Amplitude Modulation.|
|8.||The amplitude of the carrier wave is modified in order to send the data or information.||The phase of the carrier wave is modified in order to send the data or information.|
|9.||In AM, received signal is of low quality.||In FM, received signal is of high quality.|
|10.||It has better sound quality.||It has poor sound quality.|
PHASE MODULATION, WORKING, GENERATION, PSK, BPSK, QPSK, OPSK