Electronics Engineering

In communications, modulation is the process of "mixing" one signal (the one you intend to transmit, called the "message" and often simplified as being a simple sinusoid) with another (called the "carrier" and also often simplified as being a sinusoid) in some form. In Amplitude Modulation (AM), the two are simply linearly multiplied, ie:

u(t) = Ac(1 + k*m(t))*cos(2*pi*fc*t)

where Ac represents the amplitude of the carrier signal, k is a modulation index, fc is the carrier frequency, and u(t) represents the modulated signal. Through the trigonometric properties of sinusoids, it is possible (and in the case of AM fairly straightforward) to recover the original message signal m(t) in the absence of noise.

Both Frequency Modulation (FM) and Phase Modulation (PM) are forms of Angle Modulation, in which your signal of interest m(t) modulates the angle of the carrier wave, which is a type of nonlinear modulation. This can be generalized as:

u(t) = Ac*cos(2*pi*fc*t + p(t))

where p(t) is linearly related to m(t), your message, and itself represents an angle shift. (For now it doesn't matter whether p(t) is modulating frequency or phase.)

Assume p(t) described above is a sinusoid out of phase with the carrier by 90 degrees, specifically that the carrier is a cosine wave and the angle modulating message p(t) is a sine wave. Using the simple trigonometric identity

cos(a + b) = cos(a)cos(b) - sin(a)sin(b)

we can rewrite u(t) in its in-phase quadrature form

u(t) = Ac[ cos(p(t))*cos(2*pi*fc*t) - sin(p(t))*sin(2*pi*fc*t) ]

Trigonometrically speaking (see first-order Taylor Series approximation for further reading), for very small (close to zero) values of t in cos(t), cos(t) is almost 1, and sin(t) is almost t. If we assume that p(t), the angle modulating signal message, always has a very small value (nearly zero), we can reasonably simplify the modulated signal to the form:

u(t) Ac[ cos(2*pi*fc*t) - p(t)*sin(2*pi*fc*t) ]

which, if you compare with the form of the AM signal, is very similar. In fact, this "narrowband" angle modulation, which assumes a narrow range of angles possible, is nearly identical to the functionality of AM and therefore consumes almost the same amount of signal bandwidth and is analyzed in a very similar manner. This is because a first-order approximation (which narrowband is an example of) is linear and therefore is fundamentally the same as AM.

Physically speaking, however, using a narrowband angle modulation technique is not reliable and provides little benefit over an AM technique. It consumes the same amount of signal bandwidth as AM and is just as susceptible to noise. (Consider some additive spectral noise variable, taken with our assumption that p(t) was extremely small, will indicate that the received signal will be unrecognizably different than the transmitted signal.)

Wide band angle modulation, on the other hand, does not make this simplifying assumption that angles are small (first-order approximation). Without these assumptions, signal analysis is much more complex, and involves solving Bessel functions for multiple values of the message signal across the intended spectrum. However, because of its true nonlinearity, wide band angle modulation is much more resilient to noise than is narrowband/AM and consumes much more bandwidth.

Function of a thyristor

A thyristor - also known as an SCR (silicon controlled rectifier) - is like a very fast static switch and is good for controlling large amounts of power (called power regulation) and for controlling the speed of dc motors. Another typical application is to make dimmers for lighting circuits.

How it works

A thyristor is semiconductor device having 3 electrodes:

• an anode
• a cathode
• a gate

No current can travel from the anode to the cathode until a pulse which has the right amount of voltage (called the "trigger voltage") has been applied to the gate for the right minimum amount of time (called the "trigger duration") which causes the thyristor to switch on to allow current to flow through it from the source to the load.

After being triggered, current continues to flow through the thyristor from the source to the load until either:

• the load gets disconnected from the thyristor by some other means

  or

• the supply of current from the source to the thyristor gets turned off by some other means.

Ball & Roller Bearings both belongs to the category of radial contact bearings.

In case of ball bearings spherical balls are used however in case of roller bearings cylindrical rollers are used.

due to balls used in ball bearings there is a point contact is made whether it is line contact in roller.

The analog signal goes through an ADC (Analog to Digital Converter) integrated circuit chip which analyzes it by taking samples in discrete steps which is a process called "quantizing": at each step the signal is rapidly matched against 255 digital levels.

Each of the 255 levels has a matching eight bit digital (0 or 1) word associated with it. Thus the sigbal level at each step is converted into a binary number to represent that level, which is called "encoding", and the resulting stream of encoded steps is sent onwards as the digital signal.

That process is also called PCM (Pulse Code Modulation).

Further more detailed answers

There are ICs called ADC for analogue digital converter. The mechanism is simple: there is a precision reference voltage of any value. This reference voltage is compared to the input voltage. The comparator's bits output are scaled to represent a digital value in binary code for a 12 bits up to 2047. If the reference value is 10 volts then the least significant bit is 10/2048=4.9 x10-3 volts or 4.88 mV or thereabout. The least significant bit (LSB) can be monotonic.

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An analogue signal is compared to a fixed DC reference voltage. The comparator will add transition bits, each equivalent to a digital unit, [i.e. one bit: a 0 or a 1] until full scale is reached. An analogue value of xxx volts will be converted to xxxx bits for a 4 bit converter and into xxxxxxxxxxxx bits for a 12 bit convertor. The IC used is called an ADC or Analog to Digital Converter.

Note

A Digital to Analogue Converter (DAC) does it the other way around with the same precision and results.

The prime (and possibly only significant) difference is in the kernel scheduler, which determines which processes get run, with what priority, and for how long.

In a typical "ordinary" OS, the scheduler is some sort of "fair-use" implementation, which insures that no one process monopolizes all the resources if other processes are waiting for CPU time. That is, this kind of scheduler is optimized for interactive use, and the ability to make sure all applications run in a reasonable amount of time.

In a RT OS, all applications are given strict priorities, and the scheduler is designed to give certain characteristics to each priority rating. For instance, it is entirely possible for a RT scheduler to allocate all resources to a single process, and not give anything to any other process, until that process is complete (or does something like block on I/O wait). The based concept behind a RT OS is that certain process priorities are more important than others, and this importance is paramount over any concept of "fair" resource use. In essence, a RT OS prioritizes work in such a manner that certain processes can preempt others and hog resources in ways that a typical OS would consider extremely "unfair". RT OSes are typically optimized for embedded controllers, which are designed to require certain actions be completed immediately, regardless of other demands on the system.