Learn About and Test Out FM Signal Generation and Demodulation Techniques

Testandmeasure.org is a new website that Digilent has launched in beta and it is aimed at being a valuable resource for WaveForms example workspaces, data, and projects. Today, I am highlighting a sample workspace for an FM Signal Generator/Demodulator!

This workspace is a step by step example of generating an FM signal and the demodulation process to recover the original signal using the Analog Discovery 2. It can be used to learn about FM generation and demodulation techniques, filter design and response, as a known input to test an application that requires or utilizes an FM input, or to compare to other FM signals. The signals can be viewed in WaveForms or exported as data for other means of analysis.

In this example workspace, the FM signal is 1kHz and the carrier frequency is 150kHz. Typically when designing filters, or capturing signals that require some sort of filter, the filter has to be designed in hardware. The oscilloscope instrument in WaveForms has the capability to add software filters to the data captured in the oscilloscope, in the event that a hardware filter isn’t convenient. As an example in this workspace the oscilloscope signal is passed through software filters that demodulate the FM signal. The first filter, “High Pass”, is a software-based 2nd order Butterworth filter with a cutoff frequency equal to the the carrier signal frequency, 150 kHz. The filter attenuates frequencies below this value, thereby creating an amplitude modulated (AM) version of the FM signal. From this point, the process for demodulating the signal is the same as for an AM signal. “H-W Rectifier” mathematically mimics a half-wave rectifier and passes the positive half of the signal through while nullifying the negative. “Low Pass” is a software-based 2nd order low pass Chebyshev filter with a cutoff frequency of 2kHz. This isolates the signal that is being transmitted through the frequency modulations. An actual demodulator would have subsequent steps, but this workspace demonstrates a signal conditioning technique for demodulating FM signals with WaveForms and the Analog Discovery 2.

If you have a signal you’d like to see on testandmeasure.org feel free to suggest it using the suggestion form! Click on the “HAVE AN IDEA?” link on the navigation bar and let us know!


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