Now we're using higher frequencies it becomes easier to build transmitting and receiving antennas which work efficiently. In any given region we can also assign each transmitter a different carrier frequency. This can allow us to ‘tune’ the receiver to pick up just the transmission we want and reject all the others.
For the sake of example we can consider what happens when we choose to transmit information by amplitude modulating the signal carriers. From the analysis of AM in an earlier lecture we can expect a modulating signal which covers a frequency range, fmin to fmax to produce an AM wave which consists of a carrier at the chosen frequency, f, and a pair of sidebands which cover the frequency range from f-fmax to f+fmax.
The receiver uses passband filters to select one radio signal and reject all the others. For this to work properly we have to ensure that the sidebands of transmissions which have similar carrier frequencies don't ‘overlap’. Because radio waves can travel a long way the use of radio broadcasting is controlled by international agreement. The international agreements assign specific frequency bands for specific uses. They also control the carrier frequencies, powers, modulation methods, transmitter locations, and transmission bandwidths which can be used for broadcasting. For the sake of example here we'll look at the medium wave band Note that we'd find similar overall results if we considered other bands, but many of the figures would be different since they're set by international agreement not by fundamental physics!
The radio frequencies in the medium wave (525 — 1695 kHz) and long wave (150 — 285 kHz) bands are divided up into a series of Channels 9 kHz wide by specifying that:
This divides up the available frequency bands in the way illustrated in figure 16.4.
- All legal carrier frequencies must be an integer multiple of 9 kHz
- The modulation is AM and the modulating frequencies must always be less than or equal to 4·5 kHz
The second specification means that each broadcast uses a transmission bandwidth of no more than 9 kHz centred on the chosen carrier frequency. Since the carrier spacing is 9 kHz this means that transmitted sidebands won't overlap and make the receiver's task impossible. In effect, each modulated transmission is confined to its allocated channel. Theoretically this means that a receiver — if it is good enough — can always select the transmission it wants and reject all the rest. In practice, however, there aren't enough channels to go around so the same channel may be allocated to transmitters which are long way apart. This means that the receiver may pick up co-channel interference from far away transmitters which share the same channel.
The 4·5 kHz modulation bandwidth limit means that the broadcast signal isn't going to be popular with hi-fi fans who would like to hear audio signals up to about 20 kHz. However, a 20 kHz modulation bandwidth would mean, with AM, a transmission bandwidth of 40 kHz because we have to transmit two sidebands. This would mean we'd have to widen the carrier spacing to 40 kHz to avoid interference problems caused by sideband overlaps. This in turn would mean a 40/9 or approximately 4 times reduction in the number of transmitters we could fit in a given band in a given geographical area. Medium wave broadcasting grew up when very few people were interested in hi-fi. The 4·5 kHz modulation bandwidth was chosen purely to ensure that speech was decipherable! Now the medium wave & long wave bands are crowded with broadcasters, none of whom are willing to give up their transmitters just so someone else could use these bands for better quality transmissions!
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.