Lab IX Frequency Selective RC and RL circuits

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University of Maryland, Baltimore County *

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306

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Electrical Engineering

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Dec 6, 2023

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CMPE 306 Lab IX: Frequency Selective RC and RL Circuits Created by: EFC LaBerge based on 2008 lab by Dr. L. Yan and Ryan Helinski April 2014 Updated for Keysight Material November 2023 EFC LaBerge

1 1. Purpose and Introduction The purpose of this lab is to study simple frequency-selective first-order circuits consisting of resistors and capacitors (RC) or resistors and inductors (RL). These circuits are frequency-selective because the outputs due to input sinusoidal signals at certain frequencies have larger amplitude than the outputs due to input signals at other frequencies. Thus the circuit selects certain frequencies (the high amplitude ones) in preference to other frequencies (the low amplitude ones). Such frequency selective circuits are the most common use for RC and RL designs. For the purpose of this lab, we concentrate solely on low pass and high pass filters. To get other sorts of filters, we need a second order circuit, which we will do in the Lab X. In a low pass filter, input signals at lower frequencies result in higher amplitude output signals than input signals at higher frequencies. Thus, we say the filter “passes” lower frequencies and attenuates higher frequencies. Similarly, in a low pass filter, input signals at higher frequencies result in higher amplitude output signals than input signals at lower frequencies. Thus, we say the filter “passes” higher frequencies and attenuates lower frequencies. By the end of this labs session, students will be able to perform the following tasks: 1. Simulate and analyze prototype RC and RL circuits. 2. Predict circuit performance using complex impedances. 3. Construct low pass and high pass filters in simple RC and RL forms. 4. Collect and analyze data illustrating that the circuits have the desired frequency selective characteristics.

2 2. Pre-Lab Figure 1 LTSPICE Circuit #1 Showing AC Voltage Source and AC Sweep Commad 1. Create the simple circuits shown in Figure 1 . The voltage source is set to be an AC source (subtly different from a SINE source), with amplitude 5V and DC offset 0 volts. Selecting an AC source permits us to use the .ac directive in LTSPICE, as shown in the figure. The .ac command increments the frequency of the source with 20 steps per decade 1 (“dec 20”) from 100 Hz to 300 kH. Run the simulation. 2. From plotting window (which should come up immediately), select the Add Traces option. Add the trace for V(n001) and V(vout). You should see a plot that looks like Figure 2 . Collaborate with your lab partner to answer the following questions. Notice that the x-axis (the frequency scale) is logarithmic in nature, and the y-axis (the amplitude scale) is in decibels (dB). 2 Why is the green curve (V(n001)) constant? The solid blue curve is the output voltage. What is the impact of increasing the frequency of the sine wave? Does this make sense, given the DC characteristics of the capacitor? What does this suggest about the high frequency characteristics of the capacitor? The dotted blue curve is the phase. We’ll come back to that late in this course. Figure 3 shows the same plot with the y-axis now in volts (not decibels) and a logarithmic x axis. You should recognize this as a semilog-x plot. 1 A decade corresponds to an increase in frequency by a factor of 10, e.g., from 100 Hz to 1 kHz, or from 1 kHz to 10 khz. This sweep has slightly more than three decades (100 Hz – 1 kHz, 1 kHz – 10 kHz, 10 kHz – 100 kHz). 2 A decibel (dB) is also a logarithmic unit. For voltages, , where is the value of the amplitude of the voltage given in decibels, and is some reference voltage. For the case of AC 5 0 V1 R1 2kohm C1 100nF Vout .ac dec 20 100 300e3 v [ ] dB = 20log 10 v v ref ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ v ⎡ ⎣ ⎤ ⎦ dB v v ref

3 Figure 2: Log-Log (dB vs log(frequency)) Plot of Vout From Circuit of Figure 1 Figure 3 Semi-log Plot of Vout vs. Frequency 100Hz 1KHz 10KHz 100KHz B d 0 4 - B d 5 3 - B d 0 3 - B d 5 2 - B d 0 2 - B d 5 1 - B d 0 1 - B d 5 - B d 0 B d 5 B d 0 1 B d 5 1 ° 0 9 - ° 1 8 - ° 2 7 - ° 3 6 - ° 4 5 - ° 5 4 - ° 6 3 - ° 7 2 - ° 8 1 - ° 9 - ° 0 V(n001) V(vout) 100Hz 1KHz 10KHz 100KHz V 0 . 0 V 5 . 0 V 0 . 1 V 5 . 1 V 0 . 2 V 5 . 2 V 0 . 3 V 5 . 3 V 0 . 4 V 5 . 4 V 0 . 5 V 5 . 5 V 0 . 6 ° 0 9 - ° 1 8 - ° 2 7 - ° 3 6 - ° 4 5 - ° 5 4 - ° 6 3 - ° 7 2 - ° 8 1 - ° 9 - ° 0 V(n001) V(vout)

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