UNDERSTANDING TELEPHONE ELECTRONICS

By LINDA DAVIS

LOW RF AND IN MANY CASES AUDIO AND MIDRANGE FREQUENCIES BELOW MHZ THIS MAKES THE CIRCUITS LESS EXPENSIVE LESS CRITICAL AND MORE ADAPTABLE TO TRADITIONAL BREADBOARDS AN EFFORT HAS BEEN MADE TO USE THE OLDER DIP ICS RATHER THAN THE MORE COMMON SURFACE MOUNT DEVICES SO COMMON IN WIRELESS AND COMMUNICATIONS GEAR TODAY FINALLY BY USING THE LOWER FREQUENCIES COMMONLY AVAILABLE TEST EQUIPMENT IS TYPICALLY ACCEPTABLE BREADBOARDS AND TRAINERS STANDARD BREADBOARDS AND TRAINERS MAY BE USED FOR MOST OF THE EXPERIMENTS DESCRIBED IN THE MANUAL THE KEY TO MAKING THE CIRCUITS WORK IS TO KEEP THE COMPONENT LEADS AND INTERCONNECTING WIRES SHORT AND THE COMPONENTS GROUPED CLOSER TOGETHER TO MINIMIZE THE STRAY INDUCTANCE CAPACITANCE AND COUPLING ALL POWER SUPPLIES SHOULD BE BYPASSED WITH 0 1 F CERAMIC CAPACITORS WHERE THEY ATTACH TO THE BREADBOARD TO MINIMIZE NOISE COUPLING AND OTHER PROBLEMS A GOOD CHOICE FOR A GENERAL PURPOSE TRAINER IS THE NATIONAL INSTRUMENTS NI ELVIS SEE FIGURE 1 IT CONTAINS THE USUAL BREADBOARDING SOCKETS AS WELL AS MULTIPLE POWER SUPPLIES AND A FUNCTION GENERATOR AND IT IS DESIGNED TO CONNECT TO A PC OR LAPTOP RUNNING NI LABVIEW SOFTWARE THAT CONVERTS THE COMPUTER INTO A GROUP OF VIRTUAL INSTRUMENTS VIS WHICH INCLUDE A MULTIMETER AND OSCILLOSCOPE THIS COMBINATION IS HARD TO BEAT SINCE THE COMPLETE LAB STATION BREADBOARD TRAINER POWER SUPPLIES GENERATORS AND INSTRUMENTS ARE CONTAINED IN TWO UNITS THE ELVIS TRAINER ITSELF AND THE PC ALL OF THE EXPERIMENTS HAVE BEEN TESTED ON THE ELVIS TRAINER I HIGHLY RECOMMEND IT AS THE AVAILABILITY OF THE LABVIEW SOFTWARE PROVIDES USEFUL VIRTUAL INSTRUMENTS AND ALLOWS THE DEMONSTRATION OF DSP AND OTHER CONCEPTS THAT ARE IMPOSSIBLE WITH COMMON HARDWARE VIRTUAL INSTRUMENTATION TRAINERS SOME COLLEGES ARE NOW USING VIRTUAL TEST INSTRUMENTS INSTEAD OF THE TRADITIONAL BENCH TEST INSTRUMENTS A VIRTUAL INSTRUMENT IS ONE THAT IS IMPLEMENTED ON A COMPUTER SPECIAL CIRCUITRY AND SOFTWARE IS USED TO CREATE A DMM OSCILLOSCOPE FUNCTION GENERATOR SPECTRUM ANALYZER AND EVEN POWER SUPPLIES BY USING A DESKTOP OR LAPTOP COMPUTER THE COMPUTER SCREEN SERVES AS THE READOUT FOR THE METER SCOPE AND ANY OTHER INSTRUMENTS AVAILABLE ON SCREEN KNOBS SWITCHES AND OTHER CONTROLS ARE MANIPULATED WITH THE MOUSE THE ELVIS TRAINER USES VIRTUAL INSTRUMENTS TWO OTHER LESS EXPENSIVE VIRTUAL INSTRUMENT PRODUCTS ARE THE NATIONAL INSTRUMENTS MYDAQ AND THE DIGILENT DISCOVERY SEE FIGURE 2 THE MYDAQ CONNECTS TO THE COMPUTER VIA A STANDARD USB PORT AND DERIVES POWER TO SUPPLY DC VOLTAGES OF 5 V AND 15 V TO EXPERIMENTAL CIRCUITS IN ADDITION THE SOFTWARE REFERRED TO AS LABVIEW IMPLEMENTS A DMM SCOPE FUNCTION GENERATOR AND A FEW OTHER INSTRUMENTS A CONNECTOR ON THE SIDE OF THE MYDAQ PROVIDES CONNECTIONS TO THE POWER SUPPLIES THE INSTRUMENTS AND AN EXTERNAL BREADBOARDING SOCKET ANOTHER VIRTUAL INSTRUMENT IS THE DIGILENT DISCOVERY IT TOO HAS BUILT IN POWER SUPPLIES METER SCOPE AND FUNCTION GENERATOR THE DEVICE ALSO USES A USB CONNECTION TO THE COMPUTER SOFTWARE CALLED WAVEFORMS IS USED ON THE COMPUTER TO IMPLEMENT THE INSTRUMENTS WIRE LEADS CONNECT TO THE BREADBOARDING SOCKET WHERE THE EXPERIMENTAL CIRCUIT IS BUILT A POSITIVE FEATURE OF THESE PRODUCTS IS THAT THEY ARE VERY AFFORDABLE AND TYPICALLY COST LESS THAN A TEXTBOOK MANY STUDENTS CAN AFFORD THEM IF A STUDENT OWNS A LAPTOP HE OR SHE MAY WANT TO INVEST IN ONE OF THESE DEVICES AS THEN THEY WILL BE ABLE TO RUN MANY OF THE EXPERIMENTS IN THIS MANUAL AT HOME IN THE DORM OR ANYWHERE WHEN IT IS CONVENIENT THESE TRAINERS WORK WELL IN THE LAB AND THEY ARE AFFORDABLE IF THE LAB ALREADY HAS COMPUTERS FIG 1 THE ELVIS TRAINER COURTESY NATIONAL INSTRUMENTS FIG 2 THE NATIONAL INSTRUMENTS MYDAQ LEFT AND DIGILENT ANALOG DISCOVERY RIGHT TRAINERS USE VIRTUAL INSTRUMENTS ON A LAPTOP TO PROVIDE A LOW COST BASE FOR MOST LAB ANY STANDARD DIGITAL MULTIMETER IS ADEQUATE FOR MAKING TESTS AND MEASUREMENTS IN THESE EXPERIMENTS IT SHOULD BE CAPABLE OF MEASURING DC AND AC VOLTAGE AND CURRENT AND RESISTANCE A GOOD ANALOG METER IS ALSO SUITABLE ALMOST ANY OSCILLOSCOPE USED IN COLLEGE LABS IS SUITABLE FOR USE WITH THE EXPERIMENTS IN THIS MANUAL A DUAL TRACE CHANNEL SCOPE IS RECOMMENDED THE BANDWIDTH SHOULD BE AS BROAD AS AFFORDABLE I USED BOTH 25 MHZ AND 100 MHZ SCOPES IN CREATING THESE EXPERIMENTS HOWEVER THE VIRTUAL OSCILLOSCOPE IN THE NI ELVIS TRAINER DESPITE ITS UPPER BANDWIDTH LIMIT RESTRICTED BY THE SAMPLING RATE OF THE DATA ACQUISITION CARD USED IS CERTAINLY ADEQUATE FOR SOME LOW FREQUENCY EXPERIMENTS GOOD SCOPE PROBES ARE ESSENTIAL FOR PROPER MOST OF THE EXPERIMENTS REQUIRE ONE OR MORE SIGNAL SOURCES SINCE MOST SCHOOLS HAVE GENERAL PURPOSE FUNCTION GENERATORS THAT CAN PRODUCE SINE AND SQUARE WAVES UP TO ABOUT 2 MHZ THESE ARE SPECIFIED IN MOST CASES SOME EXPERIMENTS HAVE BUILT IN SIGNAL SOURCES TO MINIMIZE THE NEED FOR EXCESSIVE EQUIPMENT THE ELVIS TRAINER CONTAINS A FUNCTION GENERATOR CAPABLE OF GENERATING SINE SQUARE AND TRIANGULAR WAVES UP TO 250 KHZ AN RF SIGNAL GENERATOR IS HIGHLY RECOMMENDED FOR SOME EXPERIMENTS IN GENERAL ANY GOOD RF SIGNAL GENERATOR WILL WORK BUT MOST OF THEM ARE VERY EXPENSIVE I STRONGLY RECOMMEND THE MFJ ENTERPRISES MFJ 269 THIS INSTRUMENT IS DESIGNED FOR ANTENNA AND TRANSMISSION LINE TESTS AND MEASUREMENTS HOWEVER IT CONTAINS A VERSATILE SIGNAL GENERATOR THAT COVERS THE RANGE FROM 1 8 MHZ TO MHZ AND 415 470 MHZ IT ALSO HAS A BUILT IN FREQUENCY COUNTER AND CIRCUITS FOR MEASURING SWR IT CAN MEASURE CALCULATE AND DISPLAY COMPLEX IMPEDANCES ITS LOW COST MAKES IT A GOOD INVESTMENT FOR ANY COMMUNICATIONS COURSE AND LAB ANY STANDARD FREQUENCY COUNTER WILL WORK FINE WITH THE EXPERIMENTS DESCRIBED IN THIS MANUAL THE HIGHER THE FREQUENCY CAPABILITY THE