Under sampling is also known as aliasing effect in which the the sampling frequency is less than the maximum frequency of the signal and therefore the successive cycles of the spectrum overlap.
PWM, in general, reuires a greater average power than PAM systems. Also, the PWM system reuires a greater bandwidth than PAM.
One of the principle advantages of pulse position modulation is that it is an Mary modulation techniue that can be implemented noncoherently, such that the receiver does not need to use a phaselocked loop (PLL) to track the phase of the carrier.
This makes it a suitable candidate for optical communications systems, where coherent phase modulation and detecting are difficult and extremely expensive. The only other common Mary noncoherent modulation techniue is Mary freuency shift keying, which is the freuency domain dual to PPM.
The other advantages of pulse position modulation are:
Consider, for example, picture signal of a T.V camera. It has freuency spectra of DC to 5.5MHz.such a wide band of freuency can’t be propagated through ionosphere. However, if this signal is modulated with a carrier in VHF and UHF range, the percentage bandwidth becomes very small and the signal become suitable for trmission through atmosphere.
Amplitude Modulation is defined as the process in which the instantaneous value of the amplitude of the carrier is varied according to the amplitude of the modulating or base band signal.
It is possible to limit the propagation of errors to adjacent symbols, so that an error in measuring the differential delay of one pulse will affect only two symbols, instead of effecting all successive measurements.
Pulse Amplitude Modulation is the process by which the amplitude of the regularly spaced pulses varies according to the the amplitude of the modulating signal.
The drawbacks are:
Pulse position modulation (PPM) is the process in which the position of a standard pulse is varied as a function of the amplitude of the sampled signal.
Modulation may be defined as the process by which some parameters of a high freuency signal termed as carrier, is varie in accordance with the signal to be trmitted.
PPM may be used to trmit analog information, such as continuous speech or data.
Pulsewidth modulation (PWM) of a signal or power source involves the modulation of its duty cycle, to either convey information over a communication channel or control the amount of power sent to a load. PWM uses a suare wave whose pulse width is modulated resulting in the variation of the average value of the waveform is directly dependent on the duty cycle D.
It is primarily useful for optical communication systems, where there tends to be little or no multipath interference. Narrowband RF (Radio freuency) channels with low power and long wavelength (i.e., low freuency) are affected primarily by flat fading, and PPM is better suited.
The advantages are:
Length of antenna is shortened, signal loss is reduced, ease of radiation, adjustment of bandwidth, shifting signal freuency of the assigned value.
In PWM, the pulse amplitude is kept constant but the leading edge, trailing edge or both may be varied as a function of the amplitude of the sampled signal and care must be taken to ensure that the pulse don’t overlap in a TDM system.
Frequency Modulation can be defined as the frequency of the carrier (wc) is varied acc. to the modulating signal about an unmodulated frequency.
FM are of 2 types:
Freuency modulation is the process of varying the freuency of a carrier wave in proportion to the instantaneous amplitude of the modulating signal without any variation in the amplitude of the carrier wave.
The amplitude and the width of the pulse is kept constant in this system, while the position of each pulse, in relation to the position of a recurrent reference pulse is varied by each instantaneous sampled value of the modulating wave. This me that the trmitter must send synchronizing pulses to operate timing circuits in the receiver. The PPM has the advantage of reuiring constant trmitter power output, but the disadvantage of depending on trmitterreceiver synchronization.
Modulation is defined as the process in which some characteristics of the signal called carrier is varied according to the modulating or baseband signal. For example – Amplitude Modulation, Phase Modulation, Frequency Modulation.
In case of over modulation, the modulation index is greater than one and envelope distortion occurs.
In case of Nyquist rate, the sampling frequency is equal to the maximum frequency of the signal and therefore the successive cycles of the spectrum does not overlap.
PPM is employed in narrowband RF channel systems, with the position of each pulse representing the angular position of an analogue control on the trmitter, or possible states of binary switch. The number of pulse per frame gives the number of controllable channels available. The advantage of using PPM for this type of application is that the electronics reuired to decode the signal are extremely simple, which leads to small, lightweight receiver/decoder units. (Model aircraft reuire parts that are as lightweight as possible).
Aliasing can be avoided if:
PWM can be used to reduce the total amount of power delivered to a load without losses normally incurred when a power source is limited by resistive me. This is because the average power delivered is proportional to the modulation duty cycle. With a sufficiently high modulation rate, passive electronic filters can be used to smooth the pulse train and recover an average analog waveform.
PWM system gives a greater signal to noise ratio as compared to PAM but reuires a larger bandwidth to achieve this.
Multiplexing is defined as the process in which a number of message signals are combined together to form composite signals so that they can be trmitted through the common channel.
The two types of multiplexing are:
The phase deviation are usually small. The noise produces a smaller disturbing effect on the time position of the modulating pulse train and as a result, PPM waves have a better performance with respect to signal to noise ratio in comparison to PAM and PWM systems.
Sampling is defined as the process in which an analog signals are converted into digital signals. It me that a continuous time signal is converted into a discrete time signal.
Sampling Theorem is defined as : ’The continuous time signal that can be represented in its samples and recovered back if the sampling frequency (fs) is greater than the maximum frequency of the signal (fm) that is fs >2fm’.