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ELEKTROTEHNIKA

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Signals

A signal is a function that carries information. Signals are transmitted by the transmitter and received by the receiver. In electrical engineering, the fundamental quantity of representing some information is called a signal. Any quantity whose variation in time or variation in space can be controlled may be used as a signal, such as voltage as a function of time, or current, or electric field, etc. In fact any quantity measurable through time over space or any higher dimension can be taken as a signal. A signal could be of any dimension and could be of any form.
In circuit boards, we usually use functions of voltage, and sometimes functions of current, for signals. Wireless communications typically use electric fields or magnetic fields as signals. Fiber-optic communications use light intensity as a function of time as a signal.

Signals can be periodic meaning that they have a pattern that repeats over time, or they can be aperiodic or irregular. Signals can also be continuous, where they have a value defined for every instant of time (these signals are defined over continuous independent variables; they are difficult to analyze, as they carry a huge number of values and they are very much accurate due to a large sample of values), or they can be discrete, where they are only defined at certain times (Digital signals are very easy to analyze; they are discontinuous signals - they are the appropriation of analog signals. Digital signals are less accurate then analog signals because they are the discrete samples of an analog signal taken over some period of time. However digital signals are not subject to noise). Human voice is an example of analog signals. When we speak, the voice that is produced travel through air in the form of pressure waves and thus belongs to a mathematical function, having independent variables of space and time and a value corresponding to air pressure, while, on the other hand, whenever a key is pressed from the keyboard, the appropriate electrical signal is sent to keyboard controller containing the ASCII value of that particular key. For example the electrical signal that is generated when keyboard key a is pressed, carry information of digit 97 in the form of 0 and 1, which is the ASCII value of character a.

Analog signals are denoted by sin waves, while digital signals are denoted by square waves.

Basic aperiodic signals

Two common aperiodic (irregular) signals are the step function and the pulse function. The step function describes a signal that starts in one state, and at a particular time it changes to another state. An example of this is a light bulb turning on. It is off until we flip the switch, and then it turns on. The pulse function describes a signal that is in one state, and at a particular time it changes to another state, and then at a later time it goes back to the original state. Additional common aperiodic signals are the exponential rise and the exponential decay. These functions occur with the charging and discharging of capacitors and inductors.

Basic periodic signals

Common periodic signals are the sine wave, the square wave, and the triangle wave. Sine waves are extremely important - it was demonstrated, by Mr. Fourier, that all periodic signals can be described as a collection of sine waves. Digital signals in electronics are square waves. And, triangle waves show up in timing circuits and switching power converters.
Period is the time for one cycle of the signal.
Frequency is the inverse of Period [Cycles per second, Hz].
Phase is the position in a cycle. A full cycle is 360 degrees or 2π radians. A quarter cycle is 90 degrees or π/2 radians. One can also talk about the “phase difference” or “phase shift” between two signals. Or, that two signals are “out of phase”, like it is so in resistor, inductor and capacitor in an electric circuit.
Amplitude is the magnitude of the signal. Amplitude is described as peak amplitude, peak-to-peak amplitude, or root mean square (RMS) amplitude.
Peak Amplitude is measured from the centerline of the wave to the positive peaks.
Peak-to-Peak Amplitude is measured from the negative peaks to the positive peaks.
RMS Amplitude is the square root of the mean average of the squared value of the peak amplitude. It should be mentioned that the RMS amplitude and the average value of the signal are not the same. RMS amplitude is different depending on the shape of the periodic signal. RMS amplitude for a sine wave is the peak amplitude divided by square root of 2. RMS amplitude of a square wave is equal to the peak amplitude.
Duty cycle is the fraction or percentage of a cycle in which the signal is active (ON). Typically, this is just applied to square waves where it is the percentage of a full cycle (period) occupied by the positive (on) pulse. So, if the positive (on) pulse is 25% of the period, and the negative (off) pulse is 75%, then the duty cycle is 25%. The width of the on pulse added to the width of the off pulse equals the full period (100%). A 100% duty cycle means the signal is always on and so is no longer a periodic signal.

Offset is how far the centerline of the waveform is from zero. If y = 2 sin(x) + 3 for example, then the offset is 3.