How do you implement amplitude modulation and demodulation using software-defined radio (SDR)?

1 What is SDR?

SDR is a radio system that can perform signal processing tasks such as modulation, demodulation, filtering, and encoding using software. SDR can use a generic hardware platform, such as a computer or a smartphone, to process radio signals. SDR offers many advantages, such as flexibility, adaptability, and cost-effectiveness. You can use SDR to create and test different radio applications without changing the hardware.

2 How to generate AM signals using SDR?

To generate AM signals using SDR, you need a software tool such as GNU Radio. This open-source framework provides a graphical user interface and a library of signal processing blocks. To generate AM signals, you must use a signal source block to create the information signal, such as a sine wave or an audio file. Then, you can use a multiply const block to adjust the amplitude of the information signal. Additionally, a signal source block must generate the carrier signal, such as a cosine wave with a higher frequency than the information signal. Subsequently, you need to use a multiply block to combine the information and carrier signals to produce the AM signal. Lastly, you must employ a sink block to output the AM signal to a file, speaker, or transmitter.

4 What are the types of AM and demodulation?

Different types of Amplitude Modulation (AM) and demodulation methods exist, depending on how the amplitude of the carrier signal is varied by the information signal. Common types include Double Sideband (DSB) AM, Single Sideband (SSB) AM, Vestigial Sideband (VSB) AM, and Quadrature Amplitude Modulation (QAM). The demodulation methods for these types of AM signals vary in complexity and performance. Envelope detection extracts the envelope of the AM signal using a rectifier and a filter, but has poor noise immunity and distortion. Synchronous detection multiplies the AM signal by a local oscillator that matches the frequency and phase of the carrier signal, producing a baseband signal with better noise immunity and fidelity. Coherent detection is used for QAM signals and multiplies the AM signal by two local oscillators that are 90 degrees out of phase, resulting in two baseband signals that are combined using a phase detector. This method has the best performance and efficiency.