In practice, TRF systems are rarely used. One of the most important of these can explained by considering figure 16.6. To do this we would need a filter with a 'flat top' and steep 'edges'. The ideal filter shape would be like rectangular brick. In fact it is possible to prove that it is physically impossible to make filters which have this perfect behaviour.
All real filters have an imperfect frequency response.
The filter shown in 16.6 shows typical behaviour. The passband shape isn't flat, hence the filter tends to attenuate sideband components which are at relatively high modulation frequencies. The filter's response also isn't zero everywhere outside the wanted channel. As a result some signal 'leaks through' from transmissions at nearby frequencies. The performance of a filter is normally specified in terms of a Rejection Ratio .