Hands-On Digital Communication Episode 1: Introduction and Simulation of Analog DSB-AM

Transcription

1 Hands-On Digital Communication Episode 1: Introduction and Simulation of Analog DSB-AM By Dennis Silage, K3DS Amateur Radio operators are using digital techniques to communicate with acronyms like ASK, FSK, PSK, QAM, FHSS and DSSS. Many of us grew up with AM, SSB and FM analog modulation and these new methods might seem like a challenge to understand. In this first installment of a series of web-based articles you ll be introduced to a hands-on computer simulation of digital communication geared toward Amateur Radio could change that for you. Amateur Radio operators have embraced digital techniques for efficient data communication. From amplitude shift keying (ASK) as in CW, to radioteletype (RTTY) and packet radio using frequency shift keying (FSK) and to phase shift keying (PSK) in the aptly named PSK31, carrier based digital modulation are popular modes of operation. But for many of us who grew up learning the radio arts by using analog amplitude or frequency modulation (AM and FM), understanding digital modulation and even spread spectrum techniques may seem like a daunting task. A new textbook, Digital Communication Systems Using SystemVue, with PC based simulations could change all that for you. This text and its approach of using computer simulation, rather than difficult equations, static block diagrams and drawn waveforms, is a direct outgrowth of my experience in teaching digital communication systems. You should find that your understanding improves by implementing a digital communication system as simple interconnected tokens in a computer simulation. For example, difficult equations provide calculated solutions to the frequency spectrum of modulated signals, but are these spectra really what occurs? There is something rewarding

2 in assembling a digital communication system from tokens, executing a simulation and then obtaining the spectrum of a modulated signal, all without benefit of any equations at all. SystemVue is a product of Agilent Technologies and the text provides not only a student version of the professional software but pre-configured models of digital communication systems for you to explore. A SystemVue simulation is a software brassboard to explore the what-ifs of operation with channel noise and glitches. SystemVue can easily animate the typical block diagrams of digital communication system, which are offered in textbooks as if their appearance somehow validates the results discussed. Audio.wav files can also be used as an input to a SystemVue simulation to provide a perceptible assessment of the performance of a digital communication system. For example, µ-law companding (compression and expansion) of a speech signal for pulse code modulation (PCM) is often discussed in other texts. However, here in this text a µ-law companding PCM system is simulated and the result of the speech processing is audible. The SystemVue Textbook Edition software and the simulation models are provided on the accompanying CD-ROM. The software allows the manipulation of the parameters of these simulation models to explore the what-ifs of digital communication system design. This series of web-based articles, Hands-On Digital Communication, will serve as your guide through this new realm. Armed with the textbook, available on the ARRL website, we ll explore the Amateur Radio digital modulation techniques that you have read about and are probably using. You ll find the trip most rewarding! Take a Quick Trip with SystemVue To start the trip and for you to see what SystemVue is all about with something that we all know, here is a simulation of an amplitude modulation (AM) transmitter and a virtual crystal radio as a receiver. A more complete description of the SystemVue simulation and AM modulation is in Chapter 1 of the text. The SystemVue simulation model consists of tokens that are dragged-and-dropped onto a grid and interconnected with what can be considered as wires, as shown in Figure 1.67 below (the figure number in the textbook). The tokens have identifying numbers that we ll used to describe them and operate with parameters that can be set in a pop-up window. We ll revisit the SystemVue design environment, what parameters to use and how to change parameters in later web-based articles. Token 0 inputs an audio.wav file that says SystemVue in a male voice. The dark blue Tokens 12, 14, 15 and 16 are Analysis Windows where the time waveforms can

3 be viewed, as on a digital storage oscilloscope. The burgundy Token 3 is a gain and offset token that appropriately scales the large amplitude audio.wav file to a more reasonable audio signal. The light blue Token 4 is a double sideband amplitude modulator (DSB-AM) with nominal amplitude of 10 V and a carrier frequency of 25 khz. Figure 1.67 A SystemVue simulation of a DSB-AM transmitter and a crystal radio as a receiver In a SystemVue simulation of a communication system the intent is not to necessarily build a realistic system with a carrier frequency in the MHz range. Such a high carrier frequency increases the execution time and computer memory and disk storage requirements for the simulation. The important concepts can be easily seen with a lower carrier frequency like 25 khz and the simulation then executes quickly. The output of the DSB-AM modulator is inputted to the communication channel with additive noise. The simple channel is the yellow Token 5, an adder, and the pink Token 1, which generates white noise. The virtual crystal radio starts with the green Token 6 which is a Butterworth analog bandpass filter (BPF) with sharp cutoff frequencies of 17 KHz and 32 KHz. A Butterworth filter can be constructed with inductors and capacitors but here is designed and simulated in SystemVue. The cutoff frequencies are centered ± 8 khz about the carrier frequency of 25 khz which is greater than the nominal bandwidth of the DSB-AM signal. The input

4 audio.wav file has a bandwidth of less than 8 khz and the DSB-AM signal then has a bandwidth of less than twice that, the double sideband, or 16 khz. The semiconductor diode of the typical crystal radio is simulated in SystemVue by an ideal half-wave rectifier function, the red Token 17. The green Token 8 suppresses the unwanted DC component of the rectified DSB-AM signal and the double sideband spectral components centered at twice the carrier frequency or 50 khz. These unwanted spectral components are predicted but the SystemVue simulation clearly shows that they exist without any equations, as shown in Figure 1.65 below. SystemVue computes the power spectral density (PSD) or spectrum of the received waveform after half-wave rectification but before the final BPF. The PSD is measure in decibels (db) referenced to one milliwatt (mw, but here shortened to m ) per Hertz. Token 8 is the final Butterworth BPF with cutoff frequencies of 80 Hz and 8 khz. As can be seen for the spectrum before this BPF in Figure 1.65, only the desired audio signal less than 8 khz would be passed by Token 8. Figure 1.65 The SystemVue simulation of the spectrum of the received DSB-AM signal after half-wave rectification

5 The burgundy Token 9 rescales the receiver output as an appropriate audio.wav file. Finally Token 10 provides a change in the SystemVue simulation rate ( samples/sec) so that the audio.wav file is outputted at the proper data rate (8 000 samples/sec) for playback. Token 0 and Token 11 can be played with any media player on the computer. Do the What-Ifs If all you were to do is to look at the spectrum and hear the audio output your insight would probably be limited. Doing the what-ifs of communication system design in a SystemVue simulation provides more understanding and brings other references, like the ARRL Handbook for Radio Communications, to life. Here are some of the what-ifs you can try for DSB-AM in this SystemVue simulation. Looking at the changed spectrum and hearing what happens to the audio output is the idea behind the learning here. How and by how much to change the parameters of the SystemVue tokens and what it all means is discussed in later web-based articles. Change the percentage of modulation of the DSB-AM signal with Token 4. Add more noise to the communication channel with Token 1. Change the center frequency and bandwidth of Token 6 Change the bandwidth of Token 8. Change the type of filter and roll-off of Token 6 and Token 8. Change the voltage where the ideal half-wave rectifier conducts in Token 17. The Road Ahead This quick trip with a SystemVue simulation of DSB-AM gives you an idea of what lies ahead on the journey. You should get a copy of Digital Communication Systems using SystemVue on the ARRL website. The future web-based articles will guide you and provide further discussions of digital communication systems in Amateur Radio operation to supplement the textbook. We ll spend more time with simulations of familiar DSB-AM and FM communication systems in Chapter 1 of the textbook to learn more about SystemVue. Next stop will be simulations of the baseband (without a carrier) digital communication techniques of pulse amplitude modulation (PAM) and delta modulation (DM) in Chapter 2. Here you ll learn about the bit error rate (BER) and the best way to detect digital signals corrupted by noise in the communication channel. You ll also look right into the eye diagram of these baseband digital signals.

6 Chapter 3 introduces you to the bandpass (with a carrier) digital communication techniques of ASK, FSK, PSK and quadrature amplitude modulation (QAM) and how to best detect them when noise is added by the channel. You ll also look at the stars in a constellation plot of these baseband digital signals. Chapter 4 considers the practical problems of carrier frequency and phase and digital data symbol synchronization and more complicated communication channels than the simple additive noise channel in our quick trip with DSB-AM. Efficient digital data transmission over wideband communication channels is provided by the techniques of time and frequency division multiplexing (TDM and FDM). When carrier-based digital communication systems present a problem, frequency hopping and direct sequence spread spectrum (FHSS and DSSS) systems are used. You ll also see the far-out world of orthogonal frequency division multiplexing (OFDM) used in DSL and wireless LANs (IEEE ). You ll learn about them in Chapter 5. Finally our journey ends in Chapter 6 where you ll learn about digital data sampling and quantizing as performed by an analog-to-digital converter (ADC) and the reverse process done by the digital-to-analog converter (DAC) in a SystemVue simulation. Here you ll see the practical PCM digital communication system with companding, the basis for modern wired telephony, and digital line codes used to send data serially. You can read more about SystemVue simulation of digital communication systems and its extended use in undergraduate and professional Electrical Engineering education at my website. There you will find the table of contents of the textbook, background materials, lecture slides and laboratories. The textbook was reviewed in QEX and you ll find those comments there also. If you have any questions about the journey, you can me. I hope to see you on the trip!