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Electronic applications

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2.3 Types of interference

In the same way that Figure 5 shows simplified models of filters, there exist simplified models of the type of signals and noise we might want to apply filters to.

A common type of interference is adjacent channel interference, in which the interfering signal is in a frequency band above or below that of the wanted signal, as shown in Figure 6.

Figure 6 Adjacent channel interference: (a) below wanted signal; (b) above wanted signal

In each case, an appropriate filter can be used to reduce the interference. For example, you can see that in the case of Figure 6(b), where the adjacent channel interference is above the wanted signal, a low-pass filter can be used, with the passband coinciding with the wanted signal and the stop band coinciding with the interference. With this arrangement an ideal filter could, in principle, remove the interference altogether. (However, as you will see later, the reality is somewhat different.)

Life is trickier when the signal and interference overlap in frequency, as in the narrowband interference and wideband interference shown in Figure 7. Here, no form of filtering can give us what we would like, which is a noise-free signal with no adverse effect on the signal.

Figure 7 (a) Narrowband interference; (b) wideband interference

Note that, just as brick-wall filters are unachievable in practice, the brick-wall frequency bands and signal strengths of these various types of interference are not achievable in practice. In real-world situations, the boundaries are less clearly defined.

Now complete Activity 2 and apply the correct filter to the interference type.

Activity 2

Timing: Allow about 5 minutes

For each of the following types of interference, suggest a suitable filter to improve the signal-to-noise ratio, and say how the passbands and stop bands should be arranged. Explain any drawbacks.

• a.Narrowband interference
• b.Wideband interference

Answer

• a.For narrowband interference you can use a band-stop filter, with the stop band centred on the interference and of an equal width. This could in principle remove the interference. The drawback is that the stop band also removes some signal power.
• b.When you have wideband interference, a suitable remedy is to use a band-pass filter with the passband centred on the wanted signal and equal in width to the bandwidth of the signal. However, although this gives the best signal-to-noise ratio, it cannot remove all the interference.

Having seen the characteristics of ideal filters and the sorts of interference that you want to remove, the next section will look you what real filters are like.