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İzmir University of Economics EEE 205 Fundamentals of Electrical Circuits Lab EXPERIMENT 8 Signal Waveforms and Measurements A. Background A.1. Basic Signal Waveforms Although the term DC stands for "direct current", DC often is used to refer "constant" signals. It is possible to refer a constant voltage as DC voltage, or similarly a constant current as DC current. The waveform of a 10 volt DC voltage is plotted in Fig.8.1. 10 VCC, volts t, sec Fig. 8.1. DC Voltage waveform In electrical circuits, besides DC signals, most commonly used signal types are sinusoidal, triangular and square waveforms as shown in Fig. 8.2. xS(t) Sinusoidal t, sec xS(t) Triangular t, sec xS(t) Square Wave t, sec Fig. 8.2. (a) Sinusoidal Waveform, (b) Triangular Waveform, (c) Square Waveform 8-1 Above waveforms are periodic. Usually the time reference t=0 sec is not reachable. For periodic signals, the period must be defined. The period T is measured as shown below in Fig. 8.3. xS(t) T Vp Vp-p t, sec Fig. 8.3 It is usually more convenient to measure peak-to-peak voltage instead of peak voltage. The peak-to-peak voltage Vp-p is defined in Fig. 8.3. If the periodic voltage has zero average value, then the peak voltage is just the half of peakto-peak voltage, i.e., Vp = Vp-p/2. A.2. Oscilloscope Oscilloscope is a device that is used to display periodic waveforms. (If a signal is not periodic, then sample values are taken and then displayed.) The oscilloscope that is used in the laboratory is given in Fig. 8.4. Fig. 8.4. Tektronix TDS100 Oscilloscope An oscilloscope has always a display section (monitor) on the front panel to display the time waveform. Only voltage waveforms are measured and displayed using an oscilloscope. If current waveforms are needed to be measured, a voltage that is proportional to the current must be generated. The display details of the screen of the oscilloscope used in the laboratory is given on Fig. 8.5. The display section has 10 horizontal (time) divisions and 8 vertical (voltage) divisions. It is possible to measure two signals (Channel 1 – CH1 and Channel 2- CH2) simultaneously using TDS1000. 8-2 Fig. 8.5. Display Section of Tektronix TDS100 Oscilloscope The voltage value (500mV) on the right to CH1 indicates the voltage scale of each vertical division for CH1 (yellow waveform). According to this scaling, CH1 has a peak-to-peak voltage of approximately 3.6x500mV=1.8 V. For Channel 2 (light blue waveform) has peak-to-peak voltage of approximately 2.6 V since the voltage scale for CH2 is 1.00V as given on the display. For time measurements, the scale of the each division is given on the right of M parameter. For the above signals, each time division is 25.0nsec. Then the period of CH1 is 2x25.0nsec=50 nsec. Similarly the period of CH2 is just 25.0 nsec. The frequency of a signal is determined using T as f=1/T. The unit of the frequency is hertz (Hz). For CH1 voltage waveform, f = 1/50 nsec = 20x106 Hz = 20 MHz as displayed on the screen as 20.0002MHz. For CH2 f = 1/25 nsec = 40x106Hz = 40 MHz. The signs 1 and 2 indicate the zero voltage axis of the voltages at CH1 and CH2 respectively. 8-3 B. Preliminary Work 1. Consider the voltage waveforms given below in Fig. 8.6. Determine the peak-to-peak voltage, period and the frequency of each waveform. 100 mV/div 10 nsec/div Vp-p = ………. T = ……….. f = ………… 1 V/div 10 sec/div Vp-p = ………. T = ……….. f = ………… 10 V/div 100 nsec/div Vp-p = ………. T = ……….. f = ………… Fig.8.6 2. Consider the inverting amplifier circuit given in Fig. 8.7. Circuit Parameters R2 VCC Vi R1 - Vo R1 = 10 k R2 = 100 k VCC = 12 V -VCC = -12 V + -VCC Fig.8.7 8-4 i. Determine the voltage gain . ii. Determine and plot VO corresponding to the following sinusoidal input Vi. Vi, volts 2 100 sec/div -2 VO, volts 20 10 100 sec/div -10 -20 Fig.8.8 8-5 C. Experimental Work 1. Using the signal generator of your laboratory set-up, generate a sinusoidal voltage with 4 volt peak-to-peak and frequency of 4 kHz. Plot the oscilloscope display on Fig. 8.9. Indicate the zero voltage level on the plot. Channel 1 scale: ………… ……./div Time division: ………… ……./div Vp-p = …………… T = ………………. f = ……………….. Fig.8.9 2. Using the signal generator of your laboratory set-up, generate a triangular voltage with 2 volt peak-to-peak and frequency of 1 kHz. Plot the oscilloscope display on Fig. 8.10. Indicate the zero voltage level on the plot. Channel 1 scale: ………… ……./div Time division: ………… ……./div Vp-p = …………… T = ………………. f = ……………….. Fig.8.10 8-6 3. Using the signal generator of your laboratory set-up, generate a square wave voltage with 5 volt peak-to-peak and frequency of 10 kHz. Plot the oscilloscope display on Fig. 8.11. (Important: Indicate the zero voltage level on the plot.) Channel 1 scale: ………… ……./div Time division: ………… ……./div Vp-p = …………… T = ………………. f = ……………….. Fig.8.11 4. Construct the circuit given in Fig. 8.12. Circuit Parameters R2 Vi VCC R1 - Vo R1 = 10 k R2 = 100 k VCC = 12 V -VCC = -12 V + -VCC Fig.8.12 8-7 i. Apply a sinusoidal with amplitude 4 volt peak-to-peak and with frequency 1 kHz to the input Vi. ii. Measure Vi (CH1) and VO (CH2) using the oscilloscope. Plot both signals on Fig. 8.13. (Important: Indicate the zero voltage levels on the plot.) Channel 1 scale: ………… ……./div Channel 2 scale: ………… ……./div Time division: ………… ……./div Vp-p = …………… T = ………………. f = ……………….. Fig.8.11 8-8