Frequency division multiplexing involves spacing multiple data signals apart from each other across a frequency spectrum so they can be combined as a single signal before transmission to better utilize the transmission medium. Digital filtering is employed on the receiving side of the system to single out one of the original signals and return it to its sampling filters. The technique of decimation was also employed to enhance the visualization of the filtering results. The filtered results were then demodulated by multiplying the time domain digital values with time domain values from a 15,000 Hz signal. A simple system was set up to show form of multiplexing as well as to show different digital filtering techniques.
To demonstrate this process, circuitry will be built using a Motorola MC2833 transmitter chip and a Motorola MC13136 FM receiver chip. The data signals varied between 200 and 400 Hz. The carrier frequencies were 2000, 3500, and 5000 Hz. Each data signal was combined with a carrier signal, then those results were broadcast on a 49 MHz common carrier. The received signal was then sampled by the computer and processed with different digital band pass filters to isolate the desired frequency. The filter derived by frequency sampling performed better than the filter derived by pole placement. The data filtered by the frequency sampling filter was then decimated to fill out the Nyquist interval. To show these steps the received signal was Fourier transformed to show the three carrier signals and the associated data signal(s). The filtered data was then Fourier transformed to show only one remains. The decimated data was transformed to show it has been spread out over the Nyquist interval. The demodulation signal was created by using the cosine function in a spreadsheet.
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