f1radiocommunication-islandphysicsf1无线电通讯岛物理.doc

F1 Radio CommunicationF1.1 ModulationModulation is when a high frequency carrier wave is made to vary in accordance with the amplitude of a signal wave. While there are many ways in which this can be achieved, each method essentially changes either the amplitude or the frequency of the carrier wave.F1.2 Carrier and signal waves The signal wave is the information which is to be sent from one place to another. It can be speech, music, video or computer information.The carrier wave is the high frequency electromagnetic wave which will transport the information as radio waves. It is important that the pure carrier wave is at a constant frequency so that its amplitude, frequency or phase can be moulded, or modulated with the signal wave form.F1.3 AM and FMAmplitude ModulationThe diagram below represents an Amplitude Modulated wave. In amplitude modulation, the magnitude of the carrier is varied in accordance with the amplitude of the information signal. This modulation system was widely used in radio transmission but is now being replaced by Frequency Modulation and the various methods of digital modulation. With an amplitude modulated wave the amplitude of the radio frequency carrier is varied in proportion to the audio frequency or information signal, as in the diagram above. The amount, or depth, of modulation depends upon the ratio of the amplitude of the information signal to the amplitude of the carrier wave signal.Depth of modulationConsider the diagram below: x is the modulating signal amplitude and y is the carrier wave amplitude.The modulation depth, m, of the resulting amplitude modulated signal is defined by:The modulation depth of the waveform in the diagram below is approximately 65% and can be verified by measuring the amplitudes of the waves in the diagram. The peak amplitude of the modulated wave is the sum of the modulating and carrier waves. If x is equal to y then the carrier is 100% modulated. If x is increased further then over modulation occurs and the region represented by y-x in the diagram becomes zero. However, if the modulation depth is too small then the received signal will be of poor quality because the signal-to-noise ratio will be reduced. The usual depth of modulation for good quality reception is approximately 80%.Diagram (a) below show an amplitude modulated wave at 100% and diagram (b) shows an amplitude modulated wave that is being over modulated (i.e. greater than 100%). Both diagrams assume the same carrier amplitude as for the diagram above.Frequency modulationAs the name suggests, the frequency of the carrier wave is varied in accordance with the variations in the information signal. This is shown in the diagram below.Each graph shows the variation of voltage against time. The first graph shows the high frequency carrier signal, the second the low frequency information signal and the third the effect of frequency modulation of the carrier by the information signal. It can be seen that as the information signal increases and becomes positive, the frequency of the carrier increases and as the information signal decreases and becomes negative, the carrier frequency is reduced. FM is used in high quality radio transmission and a typical FM carrier frequency (for radio transmission) is 100MHz and the maximum frequency deviation is limited, by international agreement, to 75kHz. There has to be a compromise between the improved quality that accompanies increased bandwidth and the restriction of the number of channels available in a frequency band.F1.4 Modulation and frequency problems.Please see the Student Work Book.F1.5 Power spectrum of an AM signal.The process of modulation results in the production of frequencies other than those of the carrier and the modulating signal. When a modulating signal of frequency fs, is combined with a carrier wave of frequency fc, a signal, comprising of three frequencies (fcfs), fc and (fc+fs), results. This is shown in the diagram below.It can be shown that for 100% modulation the amplitude of each side tone is half of the amplitude of the carrier wave and so since each side tone carries only a quarter of the power of the carrier. Since there is no information in the carrier wave signal and each side tone carries the same information, only one side tone is needed to recover the transmitted information. This means that 83.3% of the transmitted power is wasted in a standard AM transmission.F1.6 Sidebands and bandwidthSince the modulating signal usually consists of a band of frequencies, the resulting frequency spectrum of the modulated signal is shown in the diagram below.If the frequency of the modulating signal ranges from 100Hz to 5kHz then the lower sideband will range from (fc5000)Hz to (fc100)Hz, the upper sideband will range from (fc+100)Hz to (fc+5000)Hz, and the band。