QAM Modulator & Demodulator

- the QAM modulator and demodulator are essential building blocks within an overall QAM system.

The QAM modulator and QAM demodulator are key elements within any quadrature amplitude modulation system.

The modulator and demodulator are used to encode the signal, often data, onto the radio frequency carrier that is to be transmitted. Then the demodulator is used at the remote end to extract the signal from the RF carrier so that it can used at the remote end.

As quadrature amplitude modulation is a complex signal, specialised QAM modulators and demodulators are required.

QAM modulator basics

The QAM modulator essentially follows the idea that can be seen from the basic QAM theory where there are two carrier signals with a phase shift of 90° between them. These are then amplitude modulated with the two data streams known as the I or In-phase and the Q or quadrature data streams. These are generated in the baseband processing area.

A diagram QAM modulator showing the oscillator, mixers and summing blocks as well as the 90 degree phase carrier phase shift between the two halves.
Basic QAM modulator diagram

The two resultant signals are summed and then processed as required in the RF signal chain, typically converting them in frequency to the required final frequency and amplifying them as required.

It is worth noting that as the amplitude of the signal varies any RF amplifiers must be linear to preserve the integrity of the signal. Any non-linearities will alter the relative levels of the signals and alter the phase difference, thereby distorting he signal and introducing the possibility of data errors.

QAM demodulator basics

The QAM demodulator is very much the reverse of the QAM modulator.

The signals enter the system, they are split and each side is applied to a mixer. One half has the in-phase local oscillator applied and the other half has the quadrature oscillator signal applied.

A diagram QAM demodulator showing the oscillator, mixers and splitter blocks as well as the 90 degree phase carrier phase shift between the two halves.
Basic QAM demodulator diagram

The basic modulator assumes that the two quadrature signals remain exactly in quadrature.

A further requirement is to derive a local oscillator signal for the demodulation that is exactly on the required frequency for the signal. Any frequency offset will be a change in the phase of the local oscillator signal with respect to the two double sideband suppressed carrier constituents of the overall signal.

Systems include circuitry for carrier recovery that often utilises a phase locked loop - some even have an inner and outer loop. Recovering the phase of the carrier is important otherwise the bit error rate for the data will be compromised.

By Ian Poole


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