Narrowband Data Transmission ASK/FSK

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1 Objectives Communication Systems II - Laboratory Experiment 9 Narrowband Data Transmission ASK/FSK To generate amplitude-shift keyed (ASK) and frequency-shift keyed (FSK) signals, study their properties, and to recover the data sequence back from them. Apparatus 1. MCM31 board of the Elettronica Veneta (EV) training kits. 2. DC power supply. 3. Oscilloscope. 4. Connecting wires. Theory 1. Amplitude Shift Keying (ASK) The simplest digital modulation technique is amplitude-shift keying (ASK), where a binary information signal directly modulates the amplitude of an analog carrier. ASK is similar to standard amplitude modulation except there are only two output amplitudes possible. Amplitude-shift keying is sometimes called digital amplitude modulation (DAM). Mathematically, amplitude-shift keying could be described as (1) where, ( )( ) = amplitude-shift keying wave ( ) = digital information (modulating) signal (volts) A/2 = unmodulated carrier amplitude (volts) = analog carrier radian frequency (radians per second, 2 ) The modulated wave ( )( ), is either cos( ) or 0. Hence, the carrier is either ON. or OFF, which is why amplitude-shift keying is sometimes referred to as on-off keying (OOK). Figure 1 clarify the signal waveforms of Digital amplitude modulation. The demodulation can be coherent or non-coherent. In the first case, more complex as concerns the circuits but more effective as against the noise effect, a product demodulator multiplies the ASK signal by locally regenerated carriers. In both cases the detector is followed a low pass filter, which removes the residual carrier components, and a threshold circuit which squares the data signal. Figure 2 clarifies coherent ASK demodulator. Figure 3 describes a non-coherent ASK demodulator. The minimum bandwidth requirement of ASK modulation scheme is equal to the bit rate, which is equal to 1/, where is the bit duration. 1 5

2 Figure 1 Digital amplitude modulation: (a) input binary; (b) output OOK waveform. Figure 2 Coherent ASK demodulator. 2. Frequency Shift Keying (FSK) Figure 3 Non-coherent ASK demodulator. Frequency-shift keying (FSK) is another relatively simple, low-performance type of digital modulation. FSK is a form of constant-amplitude angle modulation similar to standard frequency modulation (FM) except the modulating signal is a binary signal that varies between two discrete voltage levels rather than a continuously changing analog waveform. Consequently, FSK is sometimes called binary FSK (BFSK). The general expression for FSK is where, ( ) = binary FSK waveform = peak analog carrier amplitude (volts) = analog carrier center frequency (hertz) = peak change (shift) in the analog carrier frequency (hertz) ( ) = binary input (modulating) signal (volts) (2) From Equation 2, it can be seen that the peak shift in the carrier frequency,, is proportional to the amplitude of the binary input signal ( ). Figure 4 confirms the idea. 2 5

3 Figure 4 FSK in the frequency domain. The minimum bandwidth can be approximated as = 2( + ), where is the input bit rate (bps). Figure 5 shows FSK modulator. ( ) must be NRZ unipolar format for this type of FSK modulator. Figure 5 FSK modulator. FSK demodulation is quite simple with a circuit such as the one shown in Figure 6. The FSK input signal is simultaneously applied to the inputs of both bandpass filters (BPFs) through a power splitter. The respective filter passes only the mark or only the space frequency on to its respective envelope detector. The envelope detectors, in turn, indicate the total power in each passband, and the comparator responds to the largest of the two powers. This type of FSK detection is referred to as noncoherent detection; there is no frequency involved in the demodulation process that is synchronized either in phase, frequency, or both with the incoming FSK signal. Figure 7 shows the block diagram for a coherent FSK receiver. The incoming FSK signal is multiplied by a recovered carrier signal that has the exact same frequency and phase as the transmitter reference. However, the two transmitted frequencies (the mark and space 3 5

4 frequencies) are not generally continuous; it is not practical to reproduce a local reference that is coherent with both of them. Consequently, coherent FSK detection is seldom used. Figure 6 Noncoherent FSK demodulator. Figure 7 Coherent FSK demodulator. The most common circuit used for demodulating binary FSK signals is the phase locked loop (PLL), which is shown in block diagram form in Figure 8. A PLL-FSK demodulator works similarly to a PLL-FM demodulator. As the input to the PLL shifts between the mark and space frequencies, the dc error voltage at the output of the phase comparator follows the frequency shift. Because there are only two input frequencies (mark and space), there are also only two output error voltages. One represents a logic 1 and the other a logic 0. Therefore, the output is a two-level (binary) representation of the FSK input. Generally, the natural frequency of the PLL is made equal to the center frequency of the FSK modulator. As a result, the changes in the dc error voltage follow the changes in the analog input frequency and are symmetrical around 0 V. Figure 8 PLL-FSK demodulator 4 5

5 Procedure Part I: OOK Modulation and Demodulation 1. Set the circuit in ASK mode, with 24-bit data source and without data coding (connect J1c-J3d-J4-J5-J6a; set SW2=Normal, SW3=24-bit, SW4=1200, SW6=ASK, SW8=Bit, ATT=min, Noise=min). 2. Set an alternated data sequence 00/11 and push START. 3. Display and sketch the input binary sequence along with the ASK modulated signal. 4. Adjust the phase of the carrier to make the zero of the sine wave corresponds to the starting of the bit interval. 5. Display and sketch the signals before and after the communication channel. 6. Note the effect of the communication channel on the ASK signal. 7. Display and sketch the signals at TP23, TP24 and TP29. Map those signals to Figure 3 in theory. Part II: FSK Modulation and Demodulation 1. Set the circuit in FSK mode, with 24-bit data source and without data coding (connect J1c-J3a-J4-J5-J6b; set SW2=Normal, SW3=24-bit, SW4=1800, SW5=1200/0 o, SW6=FSK, SW8=Bit, ATT=min, Noise=min). 2. Set an alternated data sequence 00/11 and push START. 3. Display and sketch the input binary sequence along with the FSK modulated signal. 4. Adjust the phase of the 1200 Hz carrier to get continuity of FSK signal in the passage between the two frequencies (this kind of modulation is known as Minimum Frequency Shift Keying). 5. Display and sketch the signals before and after the communication channel. 6. Not the effect of the communication channel on the FSK signal. 7. Display and sketch the signals at TP23, TP24 and TP29. Describe those signals. Discussion 1. With a unipolar NRZ baseband data format of 1 Mbps transmission rate, determine the minimum required ASK and FSK transmission bandwidth. 2. Describe continuous-phase FSK (CP-FSK) and compare it to ordinary FSK. References [1] W. Tomasi, Advanced Electronic Communications Systems, Pearson Education Limited, Sixth Edition, [2] EV-MCM31 Training Kit Manual, Elettronica Veneta & Inel Spa, Treviso, Italy. Prepared by: Yaseen A. Mohammed (Communications Lab. Instructor) Dr. Lubab A. Salman (Communications Lab. Supervisor) 5 5

DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING ECOM 4101 (ECE 4203) COMMUNICATIONS ENGINEERING LAB II SEMESTER 2, 2016/2017 EXPERIMENT NO.

DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING ECOM 4101 (ECE 4203) COMMUNICATIONS ENGINEERING LAB II SEMESTER 2, 2016/2017 EXPERIMENT NO. 2 ASK MODULATION NAME: MATRIC NO: DATE: SECTION: Objectives To