Signals
Modulation

An analog signal, as was defined in the first chapter, is a variable signal continuous in both time and amplitude. If we were to graphically represent alternating current, it would appear as a wave, with voltage bouncing above and below the zero level. There are three factors to consider: frequency, amplitude and phase.

Frequency is the rate at which the current alternates above and below the zero current level.

When the current rises above zero, dips below zero and then returns to zero, we say the current has completed one "cycle". The name applied to the number of cycles per second is Hertz (Hz). Therefore, if there are 500 cycles per second for an analog signal, we say the frequency is 500 Hertz (500 Hz).

Amplitude would be viewed as the height (peak) and the depth (trough) of the graphic wave.

As analog data travels over distance, the amplitude of the wave decreases. This characteristic is called "attenuation". Analog waves are less susceptible to attenuation problems, but occasionally they have to be amplified. The amplitude of analog waves is measured in watts, amps or volts. The measurement decibel is often used to describe the power of a signal. A decibel (dB) allows us to understand the ratio of two different power levels of a signal. Decibel is logarithmic and dimensionless unit.

Finally, phase describes the difference in the start of the cycle of one signal to the start of the cycle of another. One signal acts as a reference signal; the other signal is the phased signal.

A phased signal is created by slightly delaying it in order to cause its peaks and troughs to be out of sync with the reference signal. The level of non-synchronization is measured in degrees. If a signal is 180º out of phase, it means that as the reference signal reaches zero voltage following a peak, the phased signal begins.

The importance in looking at frequency, amplitude and phase, lies in the fact that it is these components that can be varied in order to allow an analog signal to carry data.

Modulation is the process of conveying a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted.

The main purpose of modulation is to enable transmission of many signals in a channel with limited bandwidth. Signals are modulated based on requirement for certain sub-bandwidth availability. The main advantage is that one transmission medium (for example one optical cable) is shared by many signals.

In analog modulation, the modulation is applied continuously in response to the analog information signal. There are many types of analog modulations that can be used, one of the simplest ones are amplitude modulation (AM), phase modulation (PM) and frequency modulation (FM).

Amplitude modulation of analog and digital signal
Frequency modulation of analog and digital signal
Phase modulation of digital signal

AM works by varying the strength of the transmitted signal in relation to the information being sent. The carrier wave or carrier is a waveform (usually sinusoidal) that is modulated (modified) with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal. Carrier wave has its amplitude modulated by an input signal (information which needs to be transmitted) before transmission. The input waveform modifies the amplitude of the carrier wave and determines the envelope of the waveform.

FM conveys information over a carrier wave by varying its momentary frequency. In analog applications, the difference between the momentary and the base frequency of the carrier is directly proportional to the instantaneous value of the input signal amplitude.

PM is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave. We can say that modification in phase according to low frequency will give phase modulation. PM is not very widely used for radio transmissions. This is because it tends to require more complex receiving hardware and there can be ambiguity problems in determining whether, for example, the signal has changed phase by +180° or –180°. PM is used, however, in digital music synthesizers.

Demodulation is extracting the information signal from a modulated carrier wave. There are several ways of demodulation depending on how parameters of the signal (amplitude, frequency or phase) are transmitted in the carrier signal. For example, for a signal modulated with a linear modulation, like AM, we can use a synchronous detector. On the other hand, for a signal modulated with an angular modulation, we must use an FM demodulator or a PM demodulator.

Received modulated signal
Demodulated signal compared to original signal