8.17.2021 -- The purpose of this lab is to introduce students to the different devices, such as the oscilloscope, function generator, and the DC power supply, that will be used for the rest of the lab periods.
"So what level are you guys at for RF lab?"
- Dr. Wentworth during the first lab lecture
1.0 Oscilloscope Basics
os•cil•lo•scope ə-sĭl′ə-skōp″ An electronic measuring instrument which provides a visual representation of the time variation of electrical quantities, such as voltage or current. It may be used to measure the shape of a voltage pulse or the frequency of an oscillating voltage. It can also be used to measure properties of other physical variables, such as sound or light intensity, if they can be translated into electrical voltage or current.
definition from https://www.wordnik.com/words/oscilloscope
Oscilloscopes are used to measure the incoming voltage or current signal with respect to time. In this lab, we are only measuring the voltage signal. You can adjust the display of the signals by using the voltage scale knob and the timescale knob. The voltage scale controls the vertical display of the wave and the time scale controls the horizontal display of the wave. There is also a center knob that'll adjust where the center of the wave is displayed.
1.0a Triggering
One important part of using the oscilloscope is know how to stabilize the incoming signal on the oscilloscope display, which is known as triggering. The trigger level basically tells the oscilloscope to start measuring the wave. If the trigger level isn't properly set, the wave on the oscilloscope display won't be very clear and start shaking like that one kindergardener who found the candy bowl during recess.
1.1 Simple Sine Waves
The function generator is used to generate sinusiodal voltage signals. You can use the side buttons on the oscilloscope to modify the amplitude, frequency, and other settings. For this part of the lab, the function generator is connected to the oscilloscope via BNC-to-BNC cable.
The first signal coming from the function generator is continuous sine wave with an offset of 0V, an amplitude of 2Vpp, and a frequency of 10kHz. The voltage scale and the timescale knobs are used to fit this wave on the oscilloscope screen, and the trigger knob is used to stabilize the wave. The trigger level was set around +0.5V to +0.75V on the oscilloscope graph. The autoscale button on the oscilloscope can also be used to fit the signal on the screen, but it can adjust other settings and mess up the display of any other incoming signals.
Some questions from the lab manual
timescale for 10kHz = 10ms
DMM voltage reading reading = 1.66V, The multimeter appears to read the RMS voltage of an AC signal.
Figure 1.1 A simple sinusoidal signal on the oscilloscope
Unfortunately, you cannot upload those figures directly from the oscilloscope to your phone (in my case, via lightning cable to iPhone), so a flash drive will be used to save figures from the oscilloscope as a .png file. However, if a flash drive is available to save figures from the oscilloscope, figures can be renamed with one of the Multipurpose a scroll wheel.
1.2 Amplitude Modulation
amplitude modulation the encoding of a carrier wave by varying its amplitude in accordance with an input signal.
defintion from https://www.wordnik.com/words/amplitude%20modulation
AM radio works by recieving the intelligence signal (bascially the audio signal of voices or music) on a carrier signal. The intelligence signal will modulate the amplitude of the carrier wave. The frequency spectrum of the AM signal can also be displayed on the oscilloscope.
Question from the lab report: Since you want to see the intelligence signal of the modulated waveform, what would be a good setting for the timescale of a signal with a 1 kHz intelligence frequency and a 1 MHz carrierfrequency? Time Scale: 1000 s/div
timescale = (intelligence frequency) / (carrier frequency)
1.2a A Simple AM Signal
The carrier signal and the intelligence signal can be generated from the function generator to create an amplitude modulated signal. The frequency of the carrier signal is set to 10kHz. First, the carrier signal settings are set on the main screen like it was set in section 1.1. For this part of the lab, the Mod button then AM Frequency are selected to set the internal modulation of the signal. The frequency of the intelligence signal is set to 1kHz, and its depth is set to 50% modulation. Below is the signal produced by the function generator on the oscilloscope in Figure 1.2.
Figure 1.2 Simple AM Signal
1.2b AM Signal Frequency Spectrum
After generating the AM signal, the frequency specturm can be observed on the oscilloscope as well. The carrier signal generated will have a frequency of 1230 kHz, an amplitude of 100 mVpp, and an offset of 0 V. The intelligence signal will have a 1kHz frequency and a modulation depth of 50%. The voltage amplitude must be below 1V or else the spectrum analyzer will be damaged. The Function generator is connected to the RF input of the oscilloscope to observe the frequency generator. Figure 1.3 displays the spectrum of the described signal, and Table 1.1 shows the spectrum data of this wave.
The following formula was dervived from the formulas on page 7 of the lab 1 manual.
Vamp = √[ 10^(((dB-30)/10)+2) ]
Figure 1.3 Frequency Spectrum of AM signal, 50% modulation depth
frequency | dBm level | voltage amplitude | |
lower sideband | 1.229 MHz | -33.4 dBm | 6.761mV |
carrier | 1.230 MHz | -21.4 dBm | 26.915mV |
upper sideband | 1.231 MHz | -33.4 dBm | 6.761mV |
Table 1.1 Spectrum Data from AM signal of ~50% depth modulation
Figure 1.4 displays the spectrum of the described signal but with a 100% modulation depth, and Table 1.2 shows the spectrum data.
Figure 1.4 Frequency Spectrum of AM signal, 100% modulation depth
| frequency | dBm level | voltage amplitude |
lower sideband | 1.229 MHz | -27.4 dBm | 13.49mV |
carrier | 1.230 MHz | -21.4 dBm | 26.915m |
upper sideband | 1.231 MHz | -27.4 dBm | 13.49mV |
Table 1.2 Spectrum Data from AM signal of ~100% depth modulation
1.3 AM Signal Frequency Spectrum (FFT method)
The oscilloscope has a Fourier transform setting on it to observe the frequency spectrum. The lab manual decribes a Fourier transform as a signal that "decomposes a function of time into the frequencies that make it up." To observe this, press the red M ("math") button and then select the "FFT" setting at the bottom of the oscilloscope display. The input signal from the function generator is set to output a 50 kHz, 100mVpp carrier sine wave with a 1kHz intelligence signal with a depth of 50%. Figures 1.5 and 1.6 show the frequency spectrums for the AM signals: one with a 50% modulation depth and one with a 100% modulation depth. Tables 1.3 and 1.4 show the spectrum data for both of these cases.
The following formula was dervived from the formulas on page 8 of the lab 1 manual.
Vamp = √(2) * 10^(dB/20)
Figure 1.5 Fourier Transform to observe the frequency spectrum (~50% modulation)
| frequency | dB level | voltage amplitude |
lower sideband | 49 kHz | -50.59 dB | 4.178mV |
carrier | 50 kHz | -35.97 dB | 22.491mV |
upper sideband | 51 kHz | -50.65 dB | 4.15mV |
Table 1.3 Spectrum information with FFT (~50% modulation)
Figure 1.6 Fourier Transform to observe the frequency spectrum (~100% modulation)
| frequency | dB level | voltage amplitude |
lower sideband | 49 kHz | -44.64 dB | 8.289mV |
carrier | 50 kHz | -35.89 dB | 22.699mV |
upper sideband | 51 kHz | -44.69 dB | 8.242mV |
Table 1.3 Spectrum information with FFT (~100% modulation)
Conculsion
In the ELEC 3030 RF lab, the machines used for the rest of the labs are the function generator, oscilloscope, and the DC power supply. The DC supply wasn't used in this lab, but if it were connected to the oscilloscope, the display would look like a flat line. Osciloscopes graph the voltage values as a function of time and will be used to observe the input and output signals of the circuits. It can also be used to observe the frequency spectrum of a signal. The process of using an oscilloscope is farily easy, but this lab taught me that the experiment (no matter how simple) will never work right the first time.
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