QAM Modulator & Demodulator

The modulation and demodulation of quadrature amplitude modulation, QAM utilises circuits that are able to accommodate both I and Q components.

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Quadrature Amplitude Modulation, QAM Tutorial Includes:
Quadrature amplitude modulation, QAM basics QAM theory QAM formats QAM modulators & demodulators

Modulation formats: Modulation types & techniques Amplitude modulation Frequency modulation Phase modulation

In any system that uses quadrature amplitude modulation, QAM, there will be modulators and demodulators.

These QAM modulators and demodulators are required to provide the capability to modulate both the in-phase and quadrature components of the modulating signal onto the carrier.

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 QAM modulator circuit diagram showing the oscillator, mixers and summing blocks as well as the 90 degree carrier phase shift between the two halves — Basic QAM I-Q modulator circuit

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 QAM demodulator circuit showing the oscillator, mixers and summing blocks as well as the 90 degree local oscillator phase shift between the two halves — Basic QAM I-Q demodulator circuit

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.

The circuits shown above show the generic IQ QAM modulator and demodulator circuits that are used in a vast number of different areas. Not only are these circuits made from discrete components, but more commonly they are used within integrated circuits that are able to provide a large number of functions.