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Download Experiment 2b: TVS, Simulation using Scope
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Salt Lake Community College Electrical Engineering Department EE 1010 AC Time Varying Signals and RC Filters (Simulation) Lab Original 12/31/2009 by Harvey Wilson Latest revision 6/22/2013 by Harvey Wilson Introduction Many signals vary continuously or temporarily verses time. The concepts and tools of the previous experiments need to be adapted to be useful for these changed conditions. The Oscilloscope will be introduced. Two types of time varying signals will be considered: continuous sinusoids and temporary transients. Objectives 1. Gain confidence in simulation. 2. Recognize measurement conditions where an Oscilloscope may be useful. Equipment Multisim circuit simulator Pre-Lab 1. (None) Experiment 1. Continuous Sinusoids a. Continuous Sinusoids are the same shape (average magnitude and frequency) for a long time. Examples of sinusoids include a single note played on a musical instrument (violin), a whistle sound, etc. b. The complex math associated with time varying signals will be introduced in other classes, is beyond the scope of this class, and will not be covered here. Some observations will be noted, but not thoroughly explained in this class. c. Consider a sine wave of 1.41 Volts magnitude and 15.9 k Hertz frequency connected across a 10nF Capacitor in series with a 1k Ohm Resistor. The voltage across each component, Vc and Vr, not calculated here, is measured with an AC meter as 1.0 Volt magnitude. d. Notice that, at the frequency given, the voltage magnitudes are each 1 Volt, but the total voltage magnitude is not 2, but is 1.41 Volts. We will use the Multisim simulator to observe these voltage values. The meters included in Multisim display RMS rather than peak magnitude AC values, but the source (V2) shows peak rather than RMS (that is what the “free” simulator does), so the AC voltage source will be adjusted such that the meter monitoring the source will display the intended 1.41 Volts value. e. Double-click on the figure below to begin a simulation using Multisim. Then do as directed by lab instructor to estimate the result of voltages and resistors and capacitors connected in various ways. Page 1 of 5 Notice that about 1 volt across C1 added to about 1 volt across R2 does NOT add up to about 2 volts measured across the source! XMM1 V1 12V R3 9.1kohm XMM3 R1 1.0kohm C3 C1 1.0nF 10nF V2 2V 15.8kHz 0Deg XMM2 R2 1.0kohm C4 C2 100nF 10nF f. g. Double-click on each of the XMM symbols, then position the meter displays near the respective meters. If needed, change each meter to measure AC voltage. h. Use “Place Text” to add your name and comments on the simulator screen for each required test condition or circuit before you copy resulting image using “Print Screen”, and paste results in a document. i. Try a capacitor and a resistor in series and also connected to a sine wave voltage source, as given, to learn if voltage distribution is affected by frequency. Use 15.8 kHz as a reference. j. Try changing the voltage source frequency up to 158 kHz, then down to 1.58 kHz to observe the result of increasing or decreasing frequency on the voltage across the capacitor. Is the capacitor voltage affected by frequency? 2. Continuous Sinusoids with RC Filters a. The frequency at which the voltage across the capacitor is the same magnitude as the voltage across the in-series resistor is the cross-over or break or ½ power frequency. That is an important frequency measurement for filters. The RC (Resistor Capacitor) circuit just tested is known as a high-pass filter because the voltage magnitude across the output resistor is greater (passes easier) for frequencies higher than the cross-over frequency. b. Try making a low-pass filter by swapping the positions of the capacitance and resistance such that the source voltage passes through a resistor, then capacitor, then to ground. This way the capacitor is the output circuit element. The RC circuit is known as a low-pass filter because the voltage magnitude across the output capacitor is greater (passes easier) for frequencies lower than the cross-over frequency. (Did you try three frequencies?) c. Try two resistors in series (no capacitor) and also connected to a sine wave voltage source to learn if voltage is distributed according to the ratio of resistances and independent of frequency. (Did you try three frequencies?) d. Try making a voltage divider of this ratio: 2:1 using only two capacitors. Is this voltage divider ratio independent of frequency in the range of 1.58 kHz to 158 kHz? Please describe what you observe. e. The next few tests may be skipped if you do not want the challenge. Page 2 of 5 f. Try making a voltage divider of this ratio: 10:1 using the three capacitors. Is this voltage divider ratio independent of frequency in the range of 1.58 kHz to 158 kHz? Please describe what you observe. g. Try making a voltage divider of this ratio: 10:1 using the two capacitor values from the previous experiment in parallel with a set of two resistors you selected in the DC experiments. This voltage divider would be similar to the voltage divider made by anyone who uses a 10X probe with an oscilloscope. Is this voltage divider ratio independent of frequency in the range of 1.58 kHz to 158 kHz? Please describe what you observe. h. End of “may be skipped”. i. The next set of simulations will include a DC source connected in series with the AC source before the revised source is connected to any load resistor(s) or capacitor(s). Refer to the next figure. Notice that the addition of the 12V DC did not change the AC voltage magnitude across any AC meter. XMM1 V1 12V R3 9.1kohm XMM3 R1 1.0kohm C3 C1 1.0nF 10nF V2 2V 15.8kHz 0Deg XMM2 R2 1.0kohm C4 C2 100nF 10nF j. k. The DC voltage does not show up across the meters while measuring AC voltage, but does show up primarily across the series capacitor when measuring DC voltage. This means that the combination of AC with DC voltages is changed to only AC after passing through a capacitor – the capacitor “blocks” the DC voltage from the circuit that follows. This can be a useful effect, or a bother, depending on what the designer wants the circuit to do. l. Ideal AC meters measure AC frequencies from near 0 Hz (almost DC) to way higher than any frequency our lab equipment can produce (about 5megHz maximum). But our lab meters are not that good. So, how good are they – what range of frequencies will each type of our lab meters measure when set to measure AC? We will make these measurements in a later experiment. m. Again, n. would you want to advertise that feature? 3. Continuous Sinusoids Observed on an Oscilloscope a. One measurement instrument which displays graphically signals which vary over time is called an oscilloscope or “scope”. The scope display will assist in understanding and measuring time varying signals. b. The next set of simulations will include a scope. Refer to the next figure while making as many notes as you need to be able to use and understand a scope in your simulation. Double-click on the figure to start the simulation. Next double-click on the Page 3 of 5 XSC1 symbol and each of the XMM symbols, and then position all displays where you want them (or as instructor shows you). Last double-click on the V2 symbol and position as shown. Run and observe a scope display. XSC1 G XMM1 V1 5V A R1 B 100kohm XMM3 V2 XMM2 2V 100Hz 0Deg R2 C1 100kohm 100nF c. d. The above circuit has 5 volts DC source added to a 2 volts peak (4 volts peak to peak) magnitude AC sine wave source. The AC meters only display the AC RMS (not peak) voltages. The scope shows the voltages verses time. Note that the “B” display on the scope (red color) shows the voltage divider effect on the DC source (½ of 5 is 2.5) combined with the low-pass filter effect on the AC source (smaller and delayed). Note that the “A” display on the scope (green color) shows the combination of the DC and AC sources (4 volts peak to peak with a 5 volt offset). e. The other type of time varying signal is the temporary transient. 4. Temporary Transients a. Temporary Transients are irregular in shape or are momentary in duration or both. Examples of transients are thunder and speech. b. Temporary signals are not easily measured or displayed on the DC or AC meters we have use in class up to this point. We will use a scope which can capture and hold signals which wiggle for only a short time. Scopes are complicated, take hours to master, and are the preferred instrument for many of the measurements in EE classes, including EE1010. c. One common temporary transient signal encountered in EE experiments is the exponential wave. d. The next simulation is different from the previous simulation only in the nature of the AC source – it is a rectangular-shape wave (square wave). Run and observe a scope display. How meaningful are the AC meter displays for this circuit? Page 4 of 5 T XSC1 G XM M 1 V1 5V A R1 B T 10 0kohm XM M 3 V3 XM M 2 - 2 V 2V 100 Hz R2 C1 10 0kohm 10 0nF e. f. Try various changes as instructed by lab instructor and keep good notes. Conclusions Write a conclusion in your notebook. Mention any problems that you encountered in this lab and how you overcame them. Page 5 of 5