Quadrature Amplitude Modulation and demodulation in detail
In this article, I will explain Quadrature Amplitude Modulation and demodulation in detail along with the necessary circuit diagram and frequency spectrum. This article is easy to understand, clear and with all the information. Suitable for study and training.
Generation of QAM signal
Modulation methods are used for diverse signal transduction. It can always modulate a specific carrier frequency only one information signal. To be modulated on the same carrier at the same time a second information according to the same modulation method, as can be after demodulation the two information no longer separate. You have additively superimposed and formed a new composite signal.
The signal separation is possible, however, when two different carriers are modulated with a respective one of the signals. The carriers have different, even if they have the same frequency, a fixed phase shift to each other. A simple and easily understandable way provides the phase shift of 90 °. The support 1 with the reference phase of 0 ° corresponds to the sine wave. The second carrier at the same frequency is obtained from it by a 90 ° phase shift and presents itself as a cosine wave.
Both carriers can be separated properly even after modulation by suitable circuits. At the time points where a carrier wave passes through zero, the other support is at its maximum. For all other times, the amplitude values are calculated from the geometric addition of the values in x-and y-direction.
The amplitudes of each other by 90 ° phase shifted carrier are located in all four quadrants of the rectangular coordinate system. The amplitudes of the unmodulated carrier to be a circle. The image, known as Lissajous figure is obtained on the oscilloscope in X -> Y mode, in which the internal time base is replaced by the signal of a channel.
The sine and cosine carriers are modulated in amplitude. The procedure is called quadrature amplitude modulation, QAM abbreviated because the sum pointer can be separated into two perpendicular components. The method described herein is the analog QAM, for example, comes in the analog color television engineering. There, the two color difference signals (B-Y) and (R-Y) amplitude modulate the same carrier frequency 4.43 MHz. The color carrier is for the (B-Y) signal in its 0 ° phase sine wave before and for the (R-Y) signal in the coupled fixed 90 ° phase angle as a cosine wave. The resultant chrominance signal F is the QAM signal.
Since the carrier itself contains no information, it can be suppressed in the modulation method. The AM method with suppressed carrier will have a better power balance. The energy saved by the transmitter for the support can be made available to the sidebands. For error-free in the demodulation characteristics of the receiver, the transmitter-side carrier must be known. The transmitter must send a trigger coupled to the original carrier signal. This auxiliary signal, the auxiliary carrier oscillator in the receiver is synchronized.
The block diagram shows how an analog QAM signal. As modulators or ring modulators Voltage multiplier diodes are suitable. An RF oscillator, the carrier oscillation is available. It is in the modulator ZM1 directly from the information signal for NF1 AM modulated with suppressed carrier (red). The carrier passes through the phase shifter and the cosine signal is referred to as the second information signal from the modulator ZM2 amplitude modulator NF2 (blue). Both AM signals form said QAM signal (green) in an adder.
The following flash movie is interactive QAM signals after entering various information represents the modulation frequencies used simulation product modulators and ZM-generated signals. The output signal can be switched into the IQ representation. Inline (I) axis f = 0 ° corresponds to the sine carrier and the quadrature (Q) axis of the cosine of f = 90 °. The IQ diagram showing that the needle of the analog QAM signal can occupy all the points within the circle.Demodulation of QAM
For demodulation must be in the transmitter, a receiver corresponding sine and cosine. Two identical demodulator circuits receive the QAM signal as an input signal. A demodulator receives the subcarrier with the 0 ° phase position of the other demodulator is fed = 90 ° rotated in the phase by f subcarrier. At the outputs of the separate demodulated information signals are available. The phase rotation makes a suitably sized OPV integrating possible. The block diagram shows the QAM demodulation.
The signal transmitted from the auxiliary transmitter carrier signal is filtered out by a bandpass filter in the receiver may use a phase-locked loop phase-locked to prevent the carrier oscillator in the receiver at the suppressed carrier transmission in the QAM signal. This ensures the correct demodulation in the two AF signals.
In the PAL color television method, the V component is changed by 180 degrees after each line periodically. Thus, a separation in the two components ZM must be performed before demodulation in the QAM decoder runtime. Simultaneously, the corresponding carrier is switched by 180 °. The two demodulators receive the separate U-ZM and ZM ± V signal with the correct phase subcarriers.
No doubt, there is a lot of information here, but most of the sentences are incomplete or contain grammatical errors. It makes the content really difficult to understand.