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QAM - GENERATIONQAM - GENERATIONQAM - GENERATIONQAM - GENERATION

modulesmodulesmodulesmodulesbasic: ADDER, AUDIO OSCILLATOR, 2 x MULTIPLIER

preparationpreparationpreparationpreparationConsider the block diagram of Figure 1. It is a quadrature modulator.

output ωωωω cos t +

Q

I

QAM

message A

message B

Figure 1: a quadrature modulator

There are two messages, A and B. Whilst these are typically independent when they areanalog, it is common practice for them to be intimately related for the case of digitalmessages. In this case the modulator is often called a quadrature amplitudemodulator (QAM).

This Lab Sheet investigates an analog application of the modulator. The system is thendescribed as a pair of identical double sideband suppressed carrier (DSBSC) generators,with their outputs added. Their common carriers come from the same source, but are inphase quadrature. The two DSBSC are overlaid in frequency, but can be separated (by asuitable receiver) because of this phase difference.

Note that the two paths into the ADDER are labelled ‘I’ and ‘Q’. This refers to the phasingof the DSBSC - inphase and quadrature.

experimentexperimentexperimentexperimentFigure 2 shows a model of the block diagram of Figure 1.

The 100 kHz quadrature carriers come from the MASTER SIGNALS module. Note thatthese do not need to be in precise quadrature relationship; errors of a few degrees makenegligible difference to the performance of the system as a whole - transmitter, channel, and

Emona-TIMS QAM - generation

TIMS Lab Sheet 2/2

receiver. It is at the demodulator that precision is required - here it is necessary that thelocal carriers match exactly the phase difference at the transmitter.

The two independent analog messages come from an AUDIO OSCILLATOR and theMASTER SIGNALS module (2 kHz).

sin tωωωω ωωωωcos tsin tµµµµ

QAMOUT

Figure 2: QAM generation - the model of Figure 1

Setting up is simple. Choose a frequency in the range say 300 to 3000 Hz for the AUDIOOSCILLATOR (message ‘A’).

Confirm there are DSBSC at the output of each MULTIPLIER. Adjust their amplitudes tobe equal at the output of the ADDER, by using the ADDER gain controls (remove the ‘A’input when adjusting ‘g’, and the ‘B’ input when adjusting ‘G’).

Since the QAM signal will (in later experiments) be the input to an analog channel, itsamplitude should be at about the TIMS ANALOG REFERENCE LEVEL of 4 volt peak-to-peak.

What is the relationship between the peak amplitude of each DSBSC at the ADDER output,and their sum ?

To what should the oscilloscope be triggered when examining the QAM ? Is the QAM of a‘recognisable’ shape ? For the case when each message could lie anywhere in the range300 to 3000 Hz, what bandwidth would be required for the transmission of the QAM ?