FM Signal Generator/Demodulator (Under Construction)

This project demonstrates how to generate FM signals with the Analog Discovery 2 and how to demodulate the signal to see the original waveform on the oscilloscope. The carrier frequency is 150kHz, the FM signal is 1kHz.

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.

• WaveForms
• Analog Discovery (Legacy)
• Analog Discovery 2
• ZIP Archive with WaveForms Workspace

Start by downloading and extracting the WaveForms workspaces from the ZIP Archive linked in the Inventory section, above.

Open WaveForms and plug in the Analog Discovery 2 via a microUSB cable. WaveForms will generate a prompt asking to switch to the device used with the workspace. Select No and navigate to Settings → Device Manager in the menu bar at the top of the window, to check that the active device is the Analog Discovery that was plugged in. When selecting the device in the Device Manager dialog, be sure that configuration #4 is selected.

Open the fm-signal-generator-demodulator workspace in WaveForms.

The workspace is ready to use and only requires that the Oscilloscope Channel 1 wires (solid orange and striped orange) be connected to the Waveform Generator Channel 1 (yellow) and ground (black), respectively.

To capture a sample of the FM signal, press the “Single” button in the Scope 1 screen on top, and then press the “Run All” button twice in the Wavegen 1 screen on bottom. Each stage of the demodulation will be graphed, and can be toggled on or off. The stages of the demodulation are in order from the top to bottom of the window.

Example results can be seen below:

For more guides and example projects for your Test and Measurement device, please visit its Resource Center, which can be found through this wiki's Test and Measurement page.

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