F carrier. An RF carrier is an unmodulated RF signal of some specified frequency. In theory, because it is not modulated, it has an indefinitely small bandwidth. In practice, of course, it does have some measurable bandwidth, although very narrow. Such a carrier derives from a simple frequency source such as an oscillator or a more complex source such as a synthesizer, which can generate a stable output in a range of frequencies. A simplified block diagram of an FDM link is shown in Figure 4.18. 4.5.2.2 Mixing. The heterodyning or mixing of signals of frequencies A and B is shown as follows. What frequencies may be found at the output of the mixer? Both the original signals will be present, as well as the signals representing their sum and their difference in the frequency domain. Thus at the output of the illustrated mixer we will have present the signals of frequency A, B, A + B, and A - B. Such a mixing process is repeated many times in FDM equipment. Let us now look at the boundaries of the nominal 4-kHz voice channel. These are 300 Hz and 3400 Hz. Let us further consider these frequencies as simple tones of 300 Hz and 3400 Hz. Now consider the following mixer and examine the possibilities at its output: 300-3400 Hz Output Mixer 20 kHz First, the output may be the sum or 20,000 Hz 20,000 Hz + 300 Hz + 3,400 Hz 20,300 Hz 23,400 Hz A simple low-pass filter could filter out all frequencies below 20,300 Hz. Multiplex Demultiplex Four-wire Four-wire voice channels (send) Transmission medium (receive) voice channels (send) (receive) Demultiplex Multiplex Four-wire Four-wire voice channels (receive) Transmission medium (send) voice channels (receive) (send) Figure 4.18 Simplified block diagram of an FDM link. 4.5 INTRODUCTION TO MULTIPLEXING Now imagine that instead of two frequencies, we have a continuous spectrum of fre- quencies between 300 Hz and 3400 Hz (i.e., we have the voice channel). We represent the spectrum as a triangle: 3400 Hz As a result of the mixing process (translation) we have another triangle, as