Experiment # (9) PSK modulator

Transcription

1 Islamic University of Gaza Faculty of Engineering Electrical Department Experiment # (9) PSK modulator Digital Communications Lab. Prepared by: Eng. Mohammed K. Abu Foul Experiment Objectives: 1. To understand the operation theory of PSK modulation. 2. To understand the signal waveform of PSK modulation. 3. To design the PSK modulator by using MC To understand the methods of measuring and adjusting the PSK modulation circuit. Experiment theory: In communication system, besides AM and FM, there is another type of modulation which is the phase modulation. In phase modulation, the amplitude and frequency remain the same, the only difference is the phase. The binary signal is used to switch the phase between 0 o and 180 o, which is called phase shift keying (PSK) modulation. Generally, in order to increase the transmission rate, we need to use more bandwidth. However, as for the variation of PSK modulation, the signal is hidden in the phase, therefore, the problem of the consumption of bandwidth will not occur. Figure 9.1(a) is the 1-bit transmission of the PSK modulation. If the variation located at the zero phase, it represents the data signal is zero. On the other words, if the variation located at the π phase, it represents the data signal is 1. Figure 9.1(b) is the 2-bit transmission of the PSK modulation, which it is also known as quadrature phase shift keying (QPSK). If the variation located at the zero phase, it represents the data signal is (00). If the variation located at the π/2 phase, it represents the data signal (01). If the variation located at the π phase, it represents the data signal is (10). If the variation located at the 3π/2 phase, it represents the data signal is (11). (a) (b) Figure 9.1 Constellation diagram of PSK and QPSK modulations. 1

2 Figure 9.3 is the simple circuit diagram of PSK modulator. At the data input port, input 5 V, then D 1, D 3 will ON, D 2, D 4 will OFF. The carrier signal inputs to the Carrier Input port will pass through T1 and couples to the second coil. After that the signal will pass through D 1, D 3 and reach to the first coil of T2. Then the signal will couple to the second coil of T2, at this moment, the phase of the waveform at PSK output terminal will similar to the phase of the carrier input, as shown in figure 9.2. On the other hand, if we input -5 V at the data input port, then D 1, D 3 will OFF, D 2, D 4 will ON. At this moment, the phase of the waveform at PSK output terminal will opposite to the phase of the carrier input. This type of modulation is known as PSK modulation. Figure 9.2 Signal waveforms of BPSK modulation Figure 9.3 Simple circuit diagram of PSK modulator. M-ary PSK can be expressed as: Where: m: index of the symbol. M = 2 n where n is the number of bits in the symbol 2

3 If the data signal is 1-bit, that is M=2. So, x PSK (t) will transmit binary bits signal and the phase shift of the modulated signal is 180 o out of phase. Figure 9.2 shows the signal waveform of binary phase shift keying (BPSK). The BPSK signal at logic 1 is represented as Acos(ω c t+π) and the BPSK signal at logic 0 is represented as Acos(ω c t+2π). Figure 9.4 shows the block diagram of PSK modulator. This block diagram is similar to the block diagram of ASK modulator in experiment 5. The only difference is the PSK modulator converts the unipolar data signal to bipolar data signal before sending the signal to the balanced modulator. Therefore, phase modulation can be achieved by using the balanced modulator. The bandpass filter will remove the high frequency signal, which make the PSK signal waveform more perfect. In this experiment, MC1496 is used to implement the balanced modulator. Figure 9.5 is the internal circuit diagram of MC1496 (As in experiment 5) Figure 9,4 Basic structure diagram of PSK modulator. Figure 9.5 Internal circuit diagram of MC1496 3

4 Figure 9.6 Circuit diagram of PSK modulator by using MC1496 Figure 9.6 is the circuit diagram of 1-bit PSK which the carrier signal and data signal are single-ended input. Pin 10 is the carrier input and data signal is passed through the unipolar to bipolar converter which is comprise by 74HCU04, 74HC126, 3904, 3906, D 1, D 2, D 3 and R 1 t0 R 8. The converted bipolar signal will be sent to pin 1 of MC1496. R 22 determines the gain of the circuit and R 23 determines the bias voltage of the circuit. If we adjust VR 1 or change the amplitude of the data signal, then we can prevent the PSK modulation signal from distortion. This signal will be sent to the filter, which is comprised by µa741, C4, C6, R26, R27 and R28. Then the high frequency signals, which are produced by the balanced modulator will be filtered and a better PSK signal will be performed. 4

5 Experiment items: Experiment 1: PSK modulator 1. Refer to the circuit diagram in figure 9.6 or refer to figure DCS15.1 on ETEK DCS module. 2. At the input terminal of modulation signal (Data I/P), input 5 V amplitude and 100 Hz TTL signal. By using oscilloscope, observe on the output signal waveforms of the unipolar to bipolar converter output terminal TP1, then record the measured results in table According to the input signal in table 9.1, repeat step 2 and record the measured results in table At the input terminal of modulation signal (Data I/P), input 5 V amplitude and 100 Hz TTL signal with 50 % duty cycle, i.e. data signal streams with "10". At the input terminal of carrier signal (carrier I/P), input 400 mv amplitude and 20 khz sine wave frequency. 5. By using oscilloscope, observe on the output signal waveforms of the output terminal of modulated PSK signal (PSK O/P). adjust VR1 and observe on the modulated PSK signal waveform until the waveform does not occur distortion. Slightly adjust the VR2 to avoid the asymmetry of the waveform. By using oscilloscope, observe on the output signal waveforms of TP1, bipolar signal test point (TP2), carrier signal (TP3), balanced modulator (TP4) and modulated PSK signal output port (PSK O/P), finally, record the output signal waveform in table According to the input signal in table 9.2, repeat step 4 to step 5 and record the measured results in table

6 Measured results: Table 9-1 Observe on the output signal of unipolar to bipolar converter by changing the frequency of data Data signal frequencies Data I/P Output signal waveforms TP1 100 Hz 1 khz 10 khz Table 9-2 Observe on the PSK modulation signal by changing the frequency of carrier signal (V c =400 mv, f Data =100 Hz. ) Carrier signal frequencies Carrier I/P Output signal waveforms TP1 TP2 TP3 20 khz TP4 PSK O/P NOTE: Repeat table 9-2 with carrier frequency 50 khz and 100 khz 6