In this way we get back the modulating signals x1(t) and x2(t)(message signals) at the receiving end.įREQUENCY SPECTRUM OF AMPLITUDE MODULATION (WAVEFORMS AND EQUATIONS DERIVATION)ĪMPLITUDE MODULATION (TIME DOMAIN EQUATIONS AND WAVEFORMS)ĪDVANTAGES AND DISADVANTAGES OF DIGITAL COMMUNICATION SYSTEMĪDVANTAGES OF OPTICAL FIBER COMMUNICATION the carriers at the transmitting and receiving ends must be in same phase. So at the outputs of these are low pass filters, we get the message signals.īut one important thing that should be kept in mind here is that, the oscillators used to produce carrier waves at the transmitting and receiving ends must be in coherence i.e. Now the outputs of these product modulators are passed through the Low Pass Filters (LPF) as depicted in the image. Now it's time to discuss the block diagram of QAM receiver. And at the receiving end we can easily detect these message signals. Therefore this allows us to use the same transmission channel. With this scheme we are simultaneously transmitting two amplitude modulated waves (in case of analog modulation) or two amplitude shift keying signals (in case of digital modulation). So this QAM scheme enables two modulated signals to occupy the same transmission channel and allows the separation of these two message signals at the receiver output therefore it is also known as 'Bandwidth Conservation Scheme'. Here the outputs of two product modulators are added and the resultant output is the multiplexed signal S(t). Now see the Output of the QAM transmitter. In this case it would be a combination of Amplitude Shift Keying (ASK) and Phase Shift Keying (PSK). This was the case for analog modulation, but if we talk about QAM with digital modulation, then, at the place of analog message signal, we use digital messages. So you can observe here that, quadrature amplitude modulation (QAM) is the combination of both Amplitude Modulation and phase modulation (As phase change of carrier wave takes place here). Now we will understand this concept with the help of block diagrams of QAM Transmitter and QAM receiver.īecause we know, when the product modulator is supplied with a message signal and the carrier wave, then we get amplitude modulated wave in the output. These modulated waves are summed at the output of the transmitter, therefore the final waveform is the combination of both PSK and ASK in digital case or PM and AM for analogue case. Therefore this scheme is known as quadrature amplitude modulation, and these carriers are known as Quadrature Carriers. Now we will understand, what is the meaning of 'Quadrature' in Quadrature Amplitude Modulation (QAM).Īctually in quadrature amplitude modulation, the two carrier waves are used, that are out of phase by 90 degrees. This modulation scheme is also known as 'Quadrature Carrier Multiplexing'. #PULSE MODULATION TECHNIQUES (PAM, PWM, PPM, PCM) Using QAM, it is possible to transmit two analog message signals or two digital bit sequences by modulating carrier wave's amplitude using analog modulation technique (AM) or digital modulation technique. If the message signal is analog in nature, then we use it with Amplitude Modulation (AM) but when the message signal is in digital form, then Amplitude Shift Keying (ASK) is used. Quadrature Amplitude Modulation (QAM) can be used, either with analog signals or digital signals. Introduction of Quadrature Amplitude Modulation (QAM) The block diagram of QAM transmitter and receiver also has been explained here. Here we will see, the basic concept of quadrature amplitude modulation and why it is known as quadrature amplitude modulation. In this post, we will discuss the Quadrature Amplitude Modulation (QAM).