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Laboratory generation of QPSK Abstract— Quadrature phase shift keying (QPSK), is a form of digital modulation. Demonstration of this modulation scheme in the laboratory is usually using Digital communication modules, or using trainer kits, or using the simulated platform of Matlab or the LabView. The attempt in this module is to demonstrate the generation of QPSK, using two widely available linear ICs: the sample-and-hold LF-398 and the operational amplifier LM-741, making it suitable for discrete component and linear IC implementation of QPSK in a typical Under-graduate laboratory course. The observed outputs at different stages of the modulator are given.

Keywords—Analog circuits, analog multiplier, analog integrated circuits, BPSK, digital modulation

1 . I ntroduction

Present Undergraduate Curriculum in the Electrical Sciences stream continues to have laboratory courses, which requires implementation and observation of certain concepts in communication. Laboratory implementation may be using transistors and diodes or using linear integrated circuits. It is also possible to use industry standard independent plug-in modules or the use ‘Virtual Instruments’. The choice from among the available methods depends on the complexity of implementation together with its quality. The preferred choice of method is usually implementation using discrete components, as the student understands the concept better than any of the other methods.

Digital modulation translates the base-band binary data stream onto a high frequency sinusoidal carrier. Some digital modulation schemes for binary data are: amplitude shift keying (ASK), binary phase shift keying (BPSK), frequency shift keying (FSK), quadrature phase shift keying (QPSK). Of these digital modulation schemes, students implement using components, the ASK, FSK, and the BPSK using discrete components. But, QPSK, is attempted using commercial communication modules, or the simulated environment. Use of commercial built-in modules, has little scope for the student to learn. The proposed method is simple and can be implemented as a laboratory experiment using discrete component. In this work, a simplified approach to the generation of the QPSK is attempted, which helps the student comprehend the concept of QPSK modulation scheme, through implementation using two commonly available ICs: the sample-and-hold LF-398 and the operational amplifier LM-741.

2. Quadrature Phase Shift Keying: QPSK

QPSK is characterized by the fact that the information is carried by the phase of the transmitted carrier. The signal constellation diagram for the QPSK consists of four points on a two-dimensional plane, as given in figure 1.

Figure 1

Figure 1: Signal Constellation diagram for the QPSK digital modulation scheme

For each dibit (two bits), one symbol is transmitted. One method of generating the QPSK waveform is by converting the input binary data stream into two streams: the odd- and the even bit streams consisting of the odd- and even numbered bits. Each of these binary streams can then be modulated using the BPSK, and then on adding we get the QPSK waveform. This method is shown below in figure 2.

Figure 2

Figure 2: One possible method of generating the QPSK waveform, for the binary data stream.

Direct implementation of the above blocks is not possible in a typical Under-Graduate laboratory session, as the number of blocks is high. Hence, direct implementation is usually not attempted, and observation by connecting commercial communication modules is attempted. The next Section discusses the proposed scheme simplified for direct implementation using discrete components and linear integrated circuits.

3. PROPOSED METHOD FOR QPSK generation

The proposed method of direct Laboratory implementation of the QPSK modulation scheme is based on the concept presented in figure 3. The input and output waveform of a quarter-cycle square wave delay block, contains two square waves which repeatedly generate all possible combinations of the dibits: 00, 10, 11 and 01. Choosing this delay-block eliminates the need to generate an arbitrary binary data stream, followed by the de-multiplexer of figure 2. The aim is to observe and comprehend QPSK, through direct implementation using discrete components, proposed in figure 3.

Figure 3

Figure 3: The input and output waveform of a quarter-cycle delay contains two square waves which repeatedly generate all possible combinations of the dibits: 00, 10, 11, 01.

Figure 4

Figure 4: The proposed method for QPSK generation

The QPSK modulator containing the delay-block is given in figure 4. The next Section gives the implementation and results of this proposed modulation scheme.

4. EXPERIMENTAL RESULTS

Each of the blocks of figure 4, can be generated in a number of ways. In this Section we give the method that we have used. The two delay blocks (i) quarter-cycle delay for the square wave and (ii) quarter cycle delay for the sine-wave, can be taken directly from Arbitrary signal generator if available (Tektronics: AFG3102 (100MHz) or AFG3022B (25MHz)- Dual-channel synthesizer with phase control, for example), else it can be generated using Flip-flops and RC elements respectively. The BPSK can be generated using transistors and transformers or using the sample-and-hold IC with the operational amplifier: two possible schemes are given in figure 5, with the corresponding BPSK output in figure 6. The outputs of the modulators are observed on a mixed signal oscilloscope (100MHz Agilent 54622D, capable of recording the output in ‘.tif’ format).

Figure 5

Figure 5: (a) and (b), circuits that can be used to generate BPSK

Figure 6

Figure 6: The BPSK output obtained using either of the circuits of figure 5.

Figure 7

Figure 7: (a) the two binary inputs to the BPSK generators, (b) the corresponding BPSK waveforms, and (c) the QPSK waveform

Using the individual blocks of figure 5 for the BPSK, and appropriate signals as input as specified in figure 4, the QPSK is implemented, and output at various stages of the proposed modulator is given in figure 7. It can be seen that the simplicity of the circuit makes it suitable for laboratory implementation, without the need of resorting to commercial communication modules, without any compromise on quality.

Acknowledgments

The author acknowledges Smt. Jayashubha, the laboratory Instructor, for patiently testing and recording the results of all the circuits.

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Last edited by B Kanmani on Sep 20, 2009 10:54 pm GMT-5.