Quadrature Amplitude Modulation and Demodulation
QAM Modulator
A QAM modulator combines two amplitude modulated signals into a single signal that can be transmitted through a single channel. This allows for more efficient use of bandwidth.
QAM Demodulator
A QAM demodulator separates the two amplitude modulated signals from the combined signal received from a single channel. This allows for the original signals to be retrieved.
QAM Working Principle
The working principle of QAM is based on the combination of two amplitude modulated signals, where each
signal is shifted in phase by 90 degrees from the other. These two signals are then combined to form a
single signal with two separate amplitude and phase components.
The below waveforms are indicating the two different carrier signals of the QAM technique:-
Quadrature Amplitude Modulation Block Diagram
Quadrature Amplitude Demodulation Block Diagram
The block diagram of a Quadrature Amplitude Demodulator is as follows:
In the above diagram, the input signal is first passed through a bandpass filter to select the desired frequency band. The filtered signal is then split into two signals by the 90 degree phase shifter. These two signals are then multiplied with two local oscillators, one of which is in phase with the carrier signal, and the other is 90 degrees out of phase. The two resulting signals are then low-pass filtered and added to obtain the baseband signal.
Advantages of QAM
- Efficient use of bandwidth
- Ability to transmit large amounts of data
- Ability to transmit multiple signals through a single channel
- Low error rates
Disadvantages of QAM
- Complexity of modulator and demodulator circuits
- Greater susceptibility to noise and interference
- Higher power requirements compared to other modulation techniques
Analog QAM:
They are used to allow more than one signal to be carried. Analog QAMs and AM (Amplitude Modulation) are the same with two carrier signals transmitted together but are out of phase by 90 degrees.
Digital QAM:
Digital QAMs are generally described as a “ Quantized QAM ” and they’re generally used in radio dispatches systems ranging from cellular technology to Wi-Fi. Digital QAM can carry advanced data rates compared to both amplitude and phase-modulated schemes.
Quadrature Amplitude Modulation Applications
- Digital television broadcasting
- Wireless communication systems
- Satellite communication systems
- Cable modems
- Internet and data communication systems