Hands-On Digital Communication Episode 2: SystemVue Basics and Simulation of a Crystal Radio

Transcription

1 Hands-On Digital Communication Episode 2: SystemVue Basics and Simulation of a Crystal Radio By Dennis Silage, K3DS k3ds@arrl.net A hands-on computer simulation of digital communication geared toward Amateur Radio helps you understand digital modulation methods with acronyms like ASK, FSK and PSK. In this second installment you ll look at how the PC simulation software SystemVue works and what you can begin to do with it! A new textbook, Digital Communication Systems Using SystemVue, available on the ARRL website, features PC based simulations of the digital modulation methods in use with Amateur Radio operation with acronyms like ASK, FSK and PSK. The SystemVue simulation software builds a virtual digital communication system as simple interconnected tokens. In the first installment of this series of web-based articles, Hands-On Digital Communication, you took a quick trip to see what SystemVue is all about with something that we all know, a simulation of an amplitude modulation (AM) transmitter and a virtual crystal radio as a receiver. In this installment you ll begin to look at how the PC simulation software SystemVue Textbook Edition works and what we can do with it. You should install the SystemVue software from the CD-ROM in the textbook on your PC to get started on the journey. SystemVue Basics A SystemVue simulation of a communication system opens with the Design Window, which is shown in Figure 1. The Design Window has a standard Windows Menu, which provides the File operations of opening and printing and should be familiar from your experience in other Windows applications. You should use the File operation to open the SystemVue simulation of a double sideband amplitude

2 modulation (DSB-AM) transmitter and a crystal radio as a receiver by its name Fig1-67.svu in the Examples\Chapter 1 folder. Figure 1. SystemVue design window with the DSB-AM transmitter and a crystal radio as a receiver. Although some Edit operations are available, the SystemVue Textbook Edition software is limited and does not allow a simulation model to be saved or the tokens to be copied or deleted. But that won t bother you at all! The SystemVue models on the CD-ROM are already built and you ll have plenty to do to explore and learn from the what-ifs of communication system design by changing token parameters. Put your Windows cursor over the light blue Token 4, the DSB-AM modulator, right click the mouse button to get the pop-up window in Figure 2 and then left click to get the parameter window in Figure 3. Here I m assuming my mouse buttons are set up like yours are. The parameters of the DSB-AM modulator token in Figure 3 include the modulation index (Mod. Index in Figure 3) but you ll come back to learn what that is and what happens to the transmitted and received signal when it s changed in a later installment.

3 The SystemVue simulation is run by left clicking the Run System button on the toolbar (the green triangle). Since the DSB-AM simulation has a.wav audio file input Token 0 and output Token 11 your default Windows media player, if setup on your PC, opens in another window so that you can hear the before and after results. In that s not the case, then put your cursor over Token 0 and Token 11 and right click the mouse button to get the pop-up window with the option to Play Audio. Figure 2. SystemVue token parameter editing. The dark blue Tokens 12, 14, 15 and 16 are Analysis Windows where the time waveforms can be viewed, as on a digital storage oscilloscope. After the SystemVue simulation is run left clicking on the Analysis Window toolbar button (to the left of the button that looks like a stopwatch) opens the display shown in Figure 4. The waveform displays are identified by their token number and here you see the input speech and recovered output.wav audio files from Token 16 and Token 12. Left clicking the rightmost button on the toolbar in the Analysis Window returns you to the Design Window. You might try manipulating all the waveform displays for the AM-DSB analog communication system now but let s back up and learn more about SystemVue simulation.

4 Figure 3. SystemVue parameter window for the DSB-AM Modulator.

5 Figure 4. SystemVue analysis window. SystemVue Simulation of Half Wave Rectification You can learn more about SystemVue simulations and what they can do with a simpler example. Close the AM-DSB system and open the half-wave rectified sinusoid system in the Design Window by its name Fig1-55.svu in the Examples\Chapter 1 folder, which is shown in Figure 1.55 (the figure number in the textbook).

6 Figure 1.55 Half wave rectified sinusoid. The system simply consists of a sine wave source, Token 0, and a half-wave rectifier, Token 1. Two analysis sinks, Token 2 and Token 3, connect to each of them to complete the system. The sinusoid source has parameters of an amplitude of 1 V, a frequency of 1 khz and 0º phase offset, which is the phase angle that the sine wave has when it starts up. Analog signals are simulated in SystemVue simulation by sampled data values that are uniformly spaced in time. The interval between data is the time spacing in seconds and its inverse is the sample rate in Hertz. Signals only look continuous in a display because the samples are close together and are connected by straight lines. The sample rate here is set to 50 khz, which is well above the 1 khz frequency of the sine wave so that the signal looks smooth to you. The SystemVue simulation runs from a start time until a stop time is reached. Usually the start time is set to 0 seconds. The start and stop times and either the time spacing or sample rate is set by left clicking the toolbar button that looks like a stopwatch in the Design Window. Figure 5 shows what the parameter window looks like. The number of samples can be entered separately in the parameter window but is actually the stop time divided by the time spacing. Finally, the frequency resolution in Hertz indicates what you can expect to be able to separate when viewing the spectrum of the signal. The frequency resolution is the sample rate divided by the number of samples and here is about 200 Hertz, which is not adequate to see much detail in the spectrum. However, you ll change that in the next installment of the web-based article. The numbers in the parameter windows are often given in scientific notation in which, for example, e-3 represent 10 3 or A quick way around this is to just think that e-3 is milli (or thousandths, as in millivolts or millisecond) and e-6 is micro (millionths, as in microvolts or microsecond). Going the other way, e+3

7 is kilo (thousand, as in kilohertz) and e+6 is mega (million, as in megahertz). You can easily learn to read other numbers between these common values by scaling the number in front of e. A shift of the decimal point of the number to the right (as in 2.5 to 25) makes the whole number after e smaller (as in e 2 to e 3). A shift of the decimal point of the number to the left (as in 8 to 0.8) makes the whole number after e larger (as in e+5 to e+6). Here are some examples of what you may see for parameters in scientific notation. Try you hand at reading them: 5e+3 = e 3 = e+2 = 0.4e+3 = 400 6e+6 = e 2 = 25e 3 = e+5 = 0.8e+6 = e+4 = 40e+3 = e 4 = 0.55e 3 = Of course, a search of the Internet gives you many websites like this to learn more about scientific notation. Figure 5. System time parameter window. But for now, run the SystemVue simulation and open the Analysis Window. If all of the waveforms you want to see are not displayed, left click on the Window menu and select which dark blue sink token you want by its number as in Figure 6. Figure 1.56 in the textbook shows you what you should see in the Analysis Window for these two signals.

8 Figure 6. Window menu in the SystemVue analysis window. Although you can size and position the displays as you would in any Windows application, there are three green menu buttons on the Analysis Window toolbar (the 8th, 9th and 10th from the left) which tile the displays vertically, horizontally or as a cascade (overlap). Another green menu button (the 14th from the left) opens all the displays. If you see a blinking blue toolbar button (the 1st from the left) it means that the displays have not been updated after the last simulation that was run. Left click on it and new displays will come up. What-ifs 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. For the half-wave rectified sinusoid try the following: Change the voltage where the ideal half-wave rectifier conducts in its parameter window (called the zero point ). Try a threshold of 0.1 V and 0.5 V. The portion of the sine wave that is passed by the rectifier is less. The amplitude may look the same but its not. This happens because the display is automatically scaled. What happens if the threshold is greater than 1 V, the peak voltage of the sinusoid? No signal is passed by the rectifier. Change the sample rate from 50 khz to 10 khz in the system time parameter window or a decrease by a factor of 5. Make sure that the stop time remains at 5 milliseconds (5e-3 seconds). What do the signals looks like to you? The signals are choppy because the sample rate is too low and straight lines connect the sampled data. The Analysis Window toolbar button with the yellow dots connected by a line (the fifth from the left) turns on and off the display of the sample points in the active display window.

9 The same choppy effect on the signals can be seen if the sample rate is 50 khz and the frequency of the sinusoid is changed to 5 khz or an increase by a factor of 5. Make sure that the stop time is set to 1 milliseconds (1e-3 seconds) so that there are 5 complete cycles of the now 5 khz sinusoid. Of course, remember to re-run the simulation in the Design Window and update the display in the Analysis Window after a change has been made The Digital Communication Systems Using SystemVue textbook has much more to say about what you see here, of course! The next installment of the web-based article continues your journey but now to the world of the frequency domain of a signal. If you have any questions, you can me. I hope to see you on the trip!