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 they 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, 5 V representing Logic 1 and 0 V 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.

The 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 1 V and 4 V. The digital signal (red) is either Logic 1 or Logic 0.

The diagram shows an A.C. analogue signal (purple). It is A.C. because it varies between +12 V and −6 V. The digital signal (red) is either 5 V (High) or 0 V (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

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:

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 17 V 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 17 V. This is because the D.C voltage is constant, the A.C. voltage is repeatedly rising from zero to the peak value of 17 V 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 = 12 V. An A.C Voltage with a peak value of 17 V and a D.C. Voltage of 12 V have the same heating effect in that, on average, they transfer the same amount of energy per second.

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

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

Frequency and Time Period

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:

For example, the A.C. mains has a frequency of 50 Hz in the UK. The time for one complete cycle of the A.C. mains is therefore 1 ÷ 50 = 0.02 s or 20 ms.