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Modulation is the procedure where a Radio Frequency or Light Wave's amplitude, frequency, or phase is changed in order to transmit data. The uniqueness of the carrier wave is that it instantaneously varied by another "modulating" waveform. 
There are many ways to modulate a signal:
Amplitude Modulation Frequency Modulation Phase Modulation Pulse Modulation
The phase modulation additionally classified into few types which are; 
Pulse Modulation (PM)
With Pulse Modulation, a "snapshot" (sample) of the waveform is taken at regular intervals. There are a variety of Pulse Modulation schemes:
Pulse Amplitude Modulation Pulse Code Modulation Pulse Frequency Modulation Pulse Position Modulation Pulse Width Modulation 
Pulse Position Modulation (PPM)
PPM Pulse Position Modulation definition:-
Also known as Pulse Time Modulation, PPM is a method where the pulses of equal amplitude are generated at a rate proscribed by the modulating signal's amplitude. Again, the random arrival rate of pulses makes this unsuitable for transmission using TDM techniques. The amplitude and width of the pulse is kept invariable in the system. The location of each pulse, in relation to the location of a recurrent reference pulse, is varied by each instantaneous sampled value of the modulating wave. PPM has the benefit of requiring constant transmitter power since the pulses are of constant amplitude and duration. It is widely used and it needs synchronization between transmitter and receiver.
SOME DETAILS ABOUT PPM:-
The electronic scheme in the transmitter scans the positions of the sticks, pots and switches and creates eight pulses. Pulse length is linked to stick position, for example; all the way to the left 1 ms, middle 1.5 ms and all the way to the right 2 ms. Seven or Eight pulses according to the channels of the radio control system are put together and completed by a very long starting pulse to complete the frame-length of 22.5 ms. The starting pulse can vary between 22.5 -8*2 = 6.5 ms and 22.5-8*1 = 16.5ms,(Depending upon the channels which may be 7 or 8,the calculation will be accordingly). The 1-2ms pulses consist of 0.7 -1.7 ms High-Phase and 0.3 ms Low-Phase. The High-Phase is related with the f+b transmitter frequency, the Low-Phase with the f-b frequency. The transmitter switches between f+b and f-b. 
In order for a model to be proscribed in flight, the modeler must be able to input control movements which are sent to the model and translated into control surface movements. This is the main purpose of the radio control system. The radio transmitter has an input device, an encoder, a radio frequency (RF) section, and an antenna. Each part serve a specific purpose and each is required for the total operation of the system.
An input device may be a knob, a switch, or a slide control. Basically, each of these devices set a particular resistance value that represents a particular type of movement or input. The reality of an input device is emulated in some way at the model depending on the installation.
The input devices are connected to potentiometers or Gyro, which convert the positions or rate into voltages. A fanatical IC called an encoder reads these voltages and produces a stream of pulses that is then sent to the RF section. The pulses are set to specific values as determined by the value of the input device and kept in a stream called a pulse train.
The pulse train is a collection of square wave pulses that are about 300 microseconds in width. For a standard 7-channel transmitter, there will be eight positive going pulses. With all surface controls in neutral and the throttle control at midpoint, all of the pulses will be spaced approximately 1.5 ms apart which is measured from the leading edge of the first pulse to the leading edge of the next and on down the line to the last pulse.
Assume the base line is zero volts and the positive going pulses are at about +5 volts. When the irregular edge of the last pulse in the train drops down to zero volts, the output of the encoder stays at zero volts for a period of time that is called "sync pause". After the output has been at zero volts for a period of time, another set of pulses called a "frame" of information or data is generated. The purpose of the sync pause is to reset the decoder IC in the receiver to start reception of another set of pulses. Most PPM transmitters have a frame period of about 20 ms. 
As an input is given, the distance changes between the pulses. If Channel 1 is moved to the full upper extreme, the pulse width will be approximately 2.0 ms wide. The length of time can vary from 1.0 to 2.0 ms depending on the stick position of the radio control system. The pulses are 1.0ms in length when the switch is in the off position and 2.0 ms in length with the switch in the on position. The pulses are 1.0 ms in length when the switch is in the first position, 1.5 ms in length with the switch in the next position and 2.0 ms in length with the switch in the last position.
The sync pause will vary in length so the frame width will be same. The sync pause is long enough so with all controls in an 8-channel system at 2.0 ms, there is still have enough time left in the sync pause to reset the decoder in the receiver.
The RF section is the part of the transmitter that generates the radio signal. The pulse train is translated by the RF section and a particular amplitude or frequency variation is generated to represent the pulse train. The radio signal is sent to the antenna and radiated by the transmitter.
The receiver contains a radio receiver, a decoder, and a servo buss. Each component is accuracy made and each is required for proper translation of the radio signal.
The radio receiver part receives the radio signal. All RF is eliminated from the signal. The signal is then demodulated into a pulse sequence. The decoder generates a large +ve going pulse for each receiver channel. The decoder then transmits the pulse to a particular port or connector on the servo buss.
The servo is the component that actually does the work in the system. It is basically a bi-directional motor that receives the pulse from the port in which it is plugged. It has a circuit that directs the rotation of the motor to a particular position based on the pulse signal of the channel. This location determines the current position of the corresponding control surface. 
ABOUT RADIO CONTROL SYSTEMS:-
There are four basic components of Radio control system.
[Transmitter]-The device which takes the input from the user through the gimbals or sticks, encodes it, and sends it to the aircraft
[Receiver] -The device that receives the signal, decodes it, and routes it to the appropriate servo
[Servo] -The device that converts the decoded signal to mechanical force to operate a control surface
[Batteries]-The device that provide power for the other devices to operate
The radio system can transmit and receive on either AM or FM. FM radios seem to be less infected by the interference than the AM although those using AM radios often have problems with interference. Some radio systems use one of two types of modulation systems to help to minimize the interference. These are called pulse position modulation (PPM) and pulse code modulation (PCM). Each has its advantages and limitations. 
Block Diagram of a PPM system:
Decoder for Multiplex Channels and PPM Decoder
The multiplex channel output from one modified port of receiver is shown below.
As each Pulse width defines one channel. We have seven channels so we see seven pulses these pulses are related with the channels of the radio control system. It is important to note that the large gap after 7th channel is known as synchronization gap or reset gap. This gap (the long duration) or width of pulse is used to synchronize all the channels. This width is variable which is almost [10ms < Sync Width < 15ms] and all the channel widths are between [1ms < Ch < 2ms].
The responsibility of the decoder is to de multiplex all the channels and places them in different registers present in microcontroller. As we see that all the pulse width is uneven and changes their width according to command so this scheme is called PPM.
As in real time applications every thing must be error free because in case of flight of the UAV a minor error leads to disaster. The purpose of frame checking algorithm is to check that either the frame we receive is free of errors or not before transmitting the frame to PWM generator. The Frame checker works as if the frame is received it check channel seven pulse width as channel seven is the channel of switch so pulse is either 1ms (ON) or 2ms(OFF) and is not variable between these two readings so if this condition is satisfied the frame is consider to be error free.
Block Diagram of all PPM Units
The Servo Control Signal
Each channel in radio control system can be used to provide control over one aspect of the behavior of a (RC) model, for a system with only two channels, might be the elevator and motor speed. Each channel needs a separate signal in order to effective control over the model and, typically, this is a short pulse whose width is made to vary with the transmitter stick movement. There is generally a standard for the pulse width range to ensure inter-operability between the products of various manufacturers and a typical signal is represented in Figure 1.8.
The full ±0.5ms range of pulse widths is shown in the figure above although most systems do not use it all. It is also the pulse repetition rate, or frame rate, of 20ms which means that each channel is updated 50 times per second. A servo, when controlled by this signal, will get a position resolute by the width of the pulse with, for example, 1ms representing fully anti-clockwise and 2ms fully clockwise.
The Multi-Channel Frame
In order to have control over more than one servo the information relating to each channel is sent in a 'serial' manner, i.e. one after the other.
Figure 1.9 shows this in diagrammatic form for a system with 4 or 7 channels and, as can be seen, the information for each channel repeats in the same place in every frame. When this mixed up signal, or multi-channel frame, is separated back into 4 or 7 separate servo control signals each will resemble that shown in Figure 1.9. The space between the pulse relating to the final channel, channel 4 or 7 in the figure shown, and the start of the next frame is known as the synchronization time, or sync time for short. This time is, the 'dead time' and is changing with the number of channels and also with the channel content. However, even for a system with 8 channels, it is much longer than the time between successive channel pulses and the receiver uses this to synchronize itself to the serial pulse train. This ensures that the positional information for channel 1 always drives the correct servo. 
The Transmitted Signal
Figure 1.2.0 shows the concept of sending multiple information channels over a single RF link. In reality the signal transmitted differs somewhat as shown in the various sections of Figure 1.2.0 below.
Figure 1.2.0a shows that a single channel section of the transmitted waveform is made up of two different parts. These are a fixed time, shown as Tf (fixed time) and typically 0.5ms in length, and a variable time, Tv (variable time), which ranges from 0.5ms to 1.5ms depending on the stick position. mixing these two times gives a total channel time that can vary between 1.0ms and 2.0ms.Figure 1.2.0b shows 4 of these individual channel times, with varying lengths, added into a multi-channel frame and shows the 'time-slice' related to each one. It can be seen that the location, relative to the start of the frame, of each consecutive Tf time depends on the information content of the preceding channels and it is this characteristic that gives the description of pulse position modulation, or PPM, for the data encoding method. The key to understanding the final decoding procedure, to be described in the final part of the series, is to know that the receiver uses the falling edge at the start of each channel time to also indicate the end of the previous channels information .Figure1.2.0c shows the transmitted RF carrier for an AM system transmitting the information shown in Figure 1.2.0b. Note that rather than just reducing the amplitude of the carrier for the duration of each Tf period it is actually switched off for a moment. This is done to convey the maximum change in signal level to the envelope detector, described in the first part of the series, and results in the fastest response and hence the best possible timing accuracy.
The transmitter performs the functions of interpreting the various stick and channel switch location, encoding these into a repeating train of pulses and impress this pulse train onto an RF carrier.
The RC Time Constant
In context to understand how the transmitter interprets the stick positions it is necessary to introduce the RC time constant. This is a characteristic of a simple circuit made up ofa resistor and a capacitor.
Figure 1.2.1, overleaf, shows the two basic measures with Figure 1.2.1a showing a capacitor charging circuit, and Figure 1.2.1b, a discharge circuit. The time constant, T, is defined as the duration of time taken, in seconds, for the capacitor to charge, or discharge, to approximately 63% of its final voltage and is calculated as the product of the two component values. The final voltages, as shown in the circuits of Figure 1.2.1, are +V for charging and 0V for discharging and are reached up till 5xT.Figure1.2.1c gather the two circuits and adds two switches. When closed switch, SW1, will charge the capacitor, C, via resistor, R1, and when open will cause the capacitor to hold its charge kept that switch, SW2, is also open. Closing SW2 will emancipation the capacitor provided SW1 is open. The action is, thus, similar to that of the envelope detector with the capacitor acting rather like a bucket with two taps, one to fill it and the other to drain it. For a capacitor with a given value the rates of charge and discharge are set by the values of R1 and R2, the smaller the resistor the faster the action. Figure 1.2.1d shows the response of the circuit to a specific switch operating sequence .The value for R1 is smaller than that for R2 and, therefore, the capacitor is charged more shortly than it is discharged. 
Like the transmitter the receiver has many sections and is almost the mirror image of the Tx. There is an RF section often known as the 'Front End' to receive the incoming signal from the (Tx) transmitter, a Crystal Oscillator, a Mixer, an IF (Intermediate Frequency) Strip, a detector and a decoder. The RF section is tuned to receive the signal from the Tx while the Crystal Oscillator (local oscillator) generates a similar signal but at a lower frequency (455Khz lower), to be mixed with the incoming Tx signal. The distinction in frequency between the two signals is then passed to the IF Strip. The IF Strip is a filter which will only permit signals of the IF frequency to pass through it. With single conversion PPM receivers this frequency is 455 kHz which is the difference between transmitted signal frequency and the local oscillator frequency presumptuous of that both are operating on the same channel number after the IF Strip the signal is then rectified i.e. transformed to a DC signal similar to that of the Encoder and passed to the Decoder. The Decoders function is that of a postman's that is to post each piece of channel information to the correct output channel socket for onward dispatch to the servos. It must do this 100% perfectly every time and irrespective of the number of channels the Tx or Rx has such that an 8 channel Rx will work with a 4 channel Tx and vice versa etc. It is added in this by the synchronization pulse inserted by the Encoder which tells the Rx when one chain of information is complete and another is about to start. The result of this pulse is to tell the Decoder counter to stop counting and go back to zero again. 
This literature review include the usage of one of the pulse modulation technique which is called pulse position modulation, this technique is used mostly in radio control systems this literature review will help the reader to enhance knowledge about the usage and importance of pulse position modulation technique in radio control systems, which is in fact related with the flight control. It include how the transmitter, receiver, and stick positions are related with the pulses of PPM and how PPM is used for these signals to detect and to give the response or output to the inputs in pulse form, from speedometer or gyro or from the rudder, elevator, etc of the autopilot.
- B.P.Lathi, modern digital and analog communication systems,2nd& 3rd edition
- www.tpub.com ;( technical publications)