To launch labAlive simulation applications you need a Java Runtime Environment supporting Java Web Start on your system. Here you can get more information about installing the right Java version.
|
|
An easy way to recover the message signal of an AM-modulated signal is the envelope detection. This non-coherent detection doesn't require a carrier recovery circuit. In its simplest form it consists of a rectifier diode and a low-pass filter.
The discovery of crystal detectors in the early 20th century enabled the development of simple radio receivers. The crystals capability to rectify signals caused amplitude modulation (AM) to become the prevalent method to send and receive messages. The amplitude of a high frequency carrier is modified by a low frequency information signal, which is detected afterwards by rectification. AM requires only a minimum effort of hardware.
This setup implements the transmission of a speech audio waveform by amplitude modulation.
- Explore how the audio signal modifies the amplitude of the carrier wave and it shapes the envelope of the waveform.
- The envelope or diode detector demodulates an AM modulated audio signal.
Start
A speech audio signal is AM modulated, transmitted and demodulated using a simple envelope detector.
Look at the transmission signal in the frequency domain and see the carrier frequency and two adjacent sidebands. Note that each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other. This standard AM is also called "double-sideband amplitude modulation" (DSB-AM) to distinguish it from further AM modulation methods.
|
|
Listen to the audio speech signal that is envelope detected after AM transmission. The audio playback starts automatically when the app is launched.
Experiment
Now that you are familiar with the envelope detection analyze what happens when one of the following parameters is changed:
| Change | Effect on demodulated signal |
|---|---|
| Increase the modulation index to |
 |
| Switch off the carrier |
|
| Change the carrier frequency to |
|
| Change phase of the carrier to |
|
Next
AM is inefficient in power usage as most of the power is consumed by the carrier signal. But it conveys none of the original information. Measure the power of the total transmission signal and the shares taken by the carrier and the two sidebands.
Take a look at the alternative AM demodulation technique Synchronous detector.
Check out Wikipedia for more information: Envelope detector, Crystal radio
 Modulating audio signal spectrum.
|
AM modulated transmission signal spectrum. Lower sideband (LSB) and upper sideband (USB) - each sideband is a mirror image of the other.
|