The demodulation process is the method of extracting the original signal, which carries the information, from the carrier wave. An electronic unit called the demodulator is used to extract the information from the modulated wave.
Since demodulation is the extraction of information, the output from a demodulator could be video signals or simply digital signals (binary data form). But the most common use is for audio applications.
Since modulation is used for radio transmission, demodulation is used for the same purpose. However, there are a lot of other systems that use demodulators. A common example is a modem, which is a modulator as well as a demodulator. It is used to extract digital data from a carrier signal.
Instead of demodulation, various terms like diode detector, synchronous detector and product detector. But demodulation is used far more widely, especially when it comes to extracting signals from carrier waves.
Demodulation techniques
There are many techniques used to demodulate amplitude modulation signals. There are different types because there are different applications concerning cost incurred and performance.
Diode rectifier envelope detector
This is the simplest form of the detector, as it only requires a single diode and a few other low-cost components. It is a good option for low-cost broadcasting radios. However, it does not meet the standards of other forms of demodulation.
Diode rectifier envelope detector’s disadvantage is that it has high levels of distortions and the performance under selective fading scenarios—such as those experienced on medium and short wave bands—is truly awful.
But due to its simplicity, the diode detector has been used for decades. It was extensively used for domestic or professional radios in earlier times. When the valves in radios were replaced using semiconductors, simple diode detectors were easy to use and implement.
Product detector
A product detector is used to receive a type of amplitude modulation called single-sideband or SSB. Demodulating SSB requires a circuit known as a product detector. In the single-sideband type of amplitude modulation, the carrier and one sideband are suppressed, leaving only one sideband functional.
The product detector uses an oscillator known as a beat frequency oscillator, which is also called a carrier insertion oscillator, to replace the carrier wave by reproducing the original modulated signal.
Interestingly, this circuit can also be used to intercept Morse code signals.
To demodulate, the receptor is tuned in such a way that there is a zero beat between the carrier wave of the amplitude modulation and the beat frequency oscillator. Thus, a demodulated audio shows up at the output of the detector. One crucial thing is that the receptor must correctly maintain its own frequency so that the BFO frequency is as precise as the incoming carrier wave. If this doesn’t happen, then a beat note will keep repeating itself.
Synchronous detection
A synchronous detector provides optimum performance for demodulating amplitude modulated signals. This detector is essentially a better product detector circuit and uses more components than a simple diode detector.
It is quite easy to include such a detector into a number of radio receivers for no cost.
The synchronous demodulator uses a product detector with a local oscillator signal. The local oscillator’s signal synchronises to the incoming signal carrier to ensure that no beat note is created with the incoming carrier wave. The sidebands of the amplitude modulated signal are then demodulated to get the output signal.
The synchronous detector provides superior performance. It’s easy to incorporate this detector into integrated circuits and is used in many broadcast receivers like radio communication equipment or walkie-talkies.
Conclusion
The most commonly used circuits (methods) for demodulating amplitude modulated signals are synchronous detection, product detector and diode rectifier envelope detector.
These demodulators can be used with any radio equipment used for amplitude modulated broadcast or radio communications. Although amplitude modulation usage is declining, it is still widely used in broadcasting on short, medium and long wavebands.
Their most significant use is for aeronautical radio communications, where it is used for ground communications.
