Frequency division demultiplexing is the inverse of the multiplexing process. The receiver separates the multiplexed signals by means of appropriate band pass filters tuned at specific center frequencies. The demultiplexed signals are afterwards demodulated individually in a manner appropriate for the modulation scheme used for those signals. The signal at the receiver is sum of two modulated signals with different carrier frequencies. Figure 1 shows the received signal and output of band pass filtered signals at 2 kHz and 4 kHz.

Figure 1: spectrum of multiplexed received signal and band pass filtered outputs

The band pass filtered output signals have the information signal in their low frequency envelope. Therefore, in the implementation of demultiplexing in the FlexTrainer, these signals are rectified and low pass filtered to recover the original modulating signals.

The questions at the end of this experiment encourage students to generate signals of different frequencies and observe the demultiplexing of received signal and spectral leakage effects.