Digital Communications Lab. Prepared by:
Eng. Mohammed K. Abu Foul
Experiment # (5)
1. In figure 5.6, what are the purposes of VR1 and VR2? 2. In figure 5.6, what are the purposes of R13 and R14? 3. Why we use modulation to transmit the baseband signals?
1. 2. 3.4. To understand the operation theory of Amplitude Shift Keying (ASK). To understand the signal waveform of the ASK modulation. To implement the ASK modulator by using XR2206 IC and MC1496. To understand the methods of measuring and adjusting the ASK modulation circuit.
In the wireless digital communication, it is not easy to transmit the digital data directly. This isbecause it needs to pass through the modulator and modulate the carrier signal in order to send the signal effectively. One of the easiest ways is to use the different data stream to change the amplitude of carrier, this kind of modulation is called amplitude modulation, and we call it as amplitude shift keying (ASK) modulation in digital communication. Figure 5.1 is the basic circuit diagram of ASKmodulator. Let the input data be 5V, when the signal pass through the buffer, he switch S1 will switch to point A, at this time the ASK output waveform is f1. When the input data is 0V, when the signal pass through the buffer, the switch S1 will switch to point B, at this time the ASK output waveform is DC 0V. the abovementioned is the basic theory of ASK modulation. ASK modulation signal can beexpressed as: XASK(t) = Ai cos(Wct+Øo); 0 ≤ t ≤ T, Wc : Cutoff frequency. Øo : Phase i=1,2,……,M (5.1)
In equation (5.1), the values of amplitude Ai have M types of possible change, the Wc and Øo denote the cutoff frequency and phase, respectively. If we choose M=2, the XASK(t) signal will transmit the binary signal, therefore, the values of A are A1=0 and A2=A, A is the arbitrary constant so wecan obtain the binary ASK modulated signal waveform as shown in figure 5.2. when input logic is 1, then the signal is transmitted out. When the input logic is 0, then no signal is transmitted, so this also called on-off keying (OOK), this type of method is used in the past time.
In this experiment, we utilize 2206 IC waveform generator and MC1496 multiplier to produce the modulated ASKsignal. First of all let's introduce the characteristics of XR2206 IC. XR2206 IC is a waveform generator, which is similar to 8038 IC. Figure 5.3 is the circuit diagram of the ASK modulator by using 2206 IC. In figure 5.3, the resistor R2 and R5 comprise a voltage divided circuit. The main function of the voltage divided circuit is to let the negative voltage waveform of the 2206 IC operatesnormally. The oscillation frequency of XR2206 IC is determined by resistor R1 and the resistor located at pin 8. Its oscillation frequencies are:
Where R is the resistor at pin 8. If R=∞, then the frequency f2 equal zero. There is an internal comparator in 2206 IC. Assume that when the input is 5 V, the output frequency is f1, and when the input is 0 V, the output frequency is f2. We can utilize theTTL signal at pin 9 to control the output frequency to be f1 or f2. This type of structure is similar to the structure in figure 5.1. Therefore, by using the characteristic of this structure, we can achieve ASK modulation easily.
Figure 5.1 Basic circuit diagram of ASK modulator.
Figure 5.2 ASK modulation signal waveform.
Figure 5.3 Circuit diagram of ASK modulator by using 2206IC.
Figure 5.4 The basic block diagram of ASK modulator.
Figure 5.4 is the basic block diagram of ASK modulator, which the balanced modulator can meet the objectives of amplitude modulation, and the bandpass filter will remove the high frequency signal to make the ASK signal waveform perfectly. We use the MC1496 to implement the balanced modulator in this experiment. Figure 5.5 is the...