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Analogue Signals

Introduction

A signal transfers information and can be in the form of a voltage or a current. It is more usual to consider voltages as there are easy to measure and so amplifiers, for example, may be described in terms of input and output voltages. In logic circuits binary is expressed in terms of voltages with, for instance, 5V representing Logic 1 and 0V representing Logic 0.

A.C. and D.C.

D.C. stands for Direct Current - this is the type of current that you get from a battery. Direct Current only flows in one direction. The current is not necessarily constant but it does not change direction. The charge carriers in the circuit move slowly around the circuit from one terminal of the battery to another.

A.C. stands for Alternating Current - this is the type of current that you get from a generator. Alternating Current repeatedly changes direction. The charge carriers in the circuit move backwards and forwards as the polarity of the power supply terminals changes. The motion of the charge carriers is still due to the influence of the power supply and they still transfer electrical energy to other parts of the circuit.

Analogue and Digital

Electrical signals can either be analogue or digital, the difference is illustrated in the diagrams below.

Analogue signals can take any value between the maximum and minimum determined by the power supply. Digital signals, on the other hand, have only two specific values which are usually referred to as ON and OFF or HIGH and LOW. It is always assumed that digital signals change from one state to another very (infinitely) quickly.

Analogue signals can be either positive or negative. Digital signals are always positive. Both analogue and digital signals may have a frequency and time period

analogue and digitalThe diagram shows a D.C. analogue signal (blue) - it is D.C. because it is always positive and analogue because it is varying between about 1V and 4V. The digital signal (red) is either Logic 1 or Logic 0.


analogue and digitalThe diagram shows an A.C. analogue signal (purple) - it is A.C. because it varies between +12V and -6V. The digital signal (red) is either 5V (High) or 0V (low). None of the signals shown are regular or periodic and so none of the signals shown have a well defined period or frequency.


Amplitude

sine wave The amplitude of any regular symmetrical periodic signal is the maximum value of the signal measured from zero (either positive or negative). The term 'amplitude' is often applied to waves in Physics and is less common in Electronics. In Electronics we refer to the peak value and the RMS value. The diagram shows the peak and RMS voltages of a sinusoidal (like a sine wave) electrical signal.

The peak value is the maximum value achieved (measured from zero). RMS stands for 'Root Mean Squared' and it is not necessary to understand the mathematics at this level. It is always the case that:

RMS Value = Peak Value ÷ √2


The physical significance of RMS voltage and peak voltage is related to effective power output ... i.e. how well an A.C. source can light a bulb or heat a heater compared to a D.C. source.

Consider an A.C. supply of 17V peak voltage. If this was attached to a bulb, the bulb would glow. If a D.C. voltage was attached to the bulb so that it glowed with the same brightness (i.e. it dissipated the same power) then the D.C. voltage would have to be less than 17V ... this is because the D.C voltage is constant, the A.C. voltage is repeatedly rising from zero to some peak value and back to zero again. The RMS voltage is the equivalent D.C. voltage that would produce the same average power output ... in this case 17 ÷ √2 = 12V. An A.C Voltage with a peak value of 17V and a D.C. Voltage of 12V have the same heating effect in that, on average, they transfer the same amount of energy per second.

Example: The mains voltage is 230V, however, what this actually means is the A.C. mains voltage has an RMS value of 230V - if the mains voltage was D.C. it would be 230V. What is the peak value of the mains voltage?

Peak Value = RMS Value × √2

Therefore the peak value of the mains is actually 230 × √2 = 325V - the mains voltage varies between 325V and −325V at 50Hz and the average effect is the same as a constant 230V.

Frequency and Time Period

sine wave Consider a periodic signal such as the sine wave shown or a regular square wave etc. The Period (T) is the time taken for one complete cycle (measured in seconds). The Frequency (f) is the number of cycles each second (measured in Hertz). The relationship between Frequency and Time Period is:

Frequency = 1 ÷ Time Period