Teaching Resources: Possible Types of Extraterrestrial Intelligence Radio Transmissions

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Digitization and Nyquist Sampling

The Nyquist theorem states that to accurately digitize a signal without distortion, the sampling rate must be at least twice the highest frequency present in the signal. If this condition is not met, aliasing occurs, which is when higher frequency components of the signal are misrepresented as lower frequencies in the sampled data. In this Nyquist sampling demonstration, students use a flowgraph in GNU Radio to explore the principles of signal sampling and aliasing. The flowgraph consists of a signal source, throttle, and time and frequency sinks. By keeping the sample rate fixed and gradually increasing the signal frequency, students observe the onset of aliasing once the signal frequency exceeds half of the sample rate, illustrating the Nyquist theorem in action.

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Filters in Digital Signal Processing

Filters are essential tools in digital signal processing, used to isolate or remove specific frequency components from a signal. A bandpass filter, in particular, allows frequencies within a certain range to pass while blocking those outside the range. Filters are widely used in communication systems, audio processing, and data transmission to refine signals and eliminate noise. In this GNU Radio flowgraph, students use a signal source, various types of filters, and frequency sink to trace the frequency response of different filters. This visually demonstrates how the filter shapes the signal, showing its effect on the frequency spectrum by allowing only certain frequencies to pass and attenuating others.

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Amplitude Modulation and Demodulation (AM Radio)

Amplitude Modulation (AM) is a technique used in radio broadcasting where the amplitude (strength) of a carrier signal is varied in proportion to the information being transmitted, such as audio. In AM radio, the carrier frequency remains constant, while the audio signal modulates the amplitude, creating a combined signal that can be transmitted over long distances and easily received by AM radios.

In these GNU Radio flowgraphs, students work with two key components of AM radio. The first flowgraph takes an audio file and generates an amplitude-modulated signal. This signal is then virtually transmitted, simulating how AM radio broadcasts work. The second flowgraph acts as the receiver, capturing the modulated signal, and demodulating it to extract the original audio. These flowgraphs demonstrate how AM signals are created, transmitted, received, and demodulated, providing a hands-on understanding of both the modulation and demodulation processes that form the backbone of traditional radio communication.

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