Figure 3-13. Block diagram of a filter.
This next device in the lineup is called a filter (see Figure 3-13). The filter does just what you think it does—it filters out all the signals which are not wanted. Visually it can be viewed as a bunch of signals at different frequencies entering the filter on the left (f1, f2, and f3) and only the desired signal (at the desired frequency) coming out the right side (f2). Where does all this "stuff" which is unwanted come from? Everywhere.
The Filter's Function
There are tons (actually, they don't weigh anything) of these invisible RF waves cruising around in the air at any moment ...