Test and Measurements
This lab was designed to familiarize students with the lab equipment, which are: an arbitrary function generator, a DC power supply and a DC power supply.
Starting with the arbitrary function generator, or fun genie, it is a device that outputs a signal depending on the selected mode the generator is set at, such as, sine wave, square wave, etc. The generator also allows the user to change the frequency and amplitude of the output by pressing one of the buttons next to the screen, then using the keypad on the side to input the desired value and pressing one of the buttons next to the screen to select the multiplier for the magnitude (thankfully, there is no touch screen). The generator used in this particular lab can generate two signals using coaxial cable, more precisely, a BNC cable.
Unfortunately, it would not make much sense to do this in alphabetical order. The next device is the oscilloscope, the secret lover of every electrical engineer. The oscilloscope shows a visual representation of the signal it is connected to, it reads the system output signal and shows it on the screen, however, some adjustments might be required to get a better view of the signal. These settings are not covered here due to the time it will take to explain them (if you are here, then you probably know how to use one, right? right?). Similar to the function generator, the oscilloscope requires a coaxial cable connection (BNC).
The DC power supply... supplies power, what else can be said? the power supply provided in this lab can be used by connecting banana plugs, however, extensions are provided that makes it possible to use alligator plugs, which makes it much easier to plug the power supply to the desired circuit.
Part 1: Simple Sine Waves
The task was to use the function generator to generate a sine wave signal with an amplitude of 1V and frequency of 10KHz. Then, connecting a BNC-BNC cable between the function generator and oscilloscope, after fiddling with oscilloscope settings the signal shows in the oscilloscope as shown in figure 1. Then, changing the mode acquisition mode of the to average the images shown in figures 2 and 3 were acquired.
Figure 1: Oscilloscope reading of the 1KHz wave coming from the function generator.
Some other neat features of the oscilloscope are the pair of cursors, which can be used to measure the values shown on the oscilloscope and can be changed to move along the wave horizontally or vertically. Examples shown in figure 4 and figure 5 Something that is important to keep in mind is the timescale of the oscilloscope, for instance, the time scale shown in the figures above is 40 μs. This specific timescale was used because the frequency of the wave is 10 KHz, if a 1 KHz wave is to be selected, then a 400 μs timescale would be recommended. When probing the function generator to measure the voltage coming out, the voltage measured comes out as 1.66V, this voltage the RMS voltage.
Part 2: Amplitude Modulation
Amplitude modulation (AM) involves a carrier and intelligence frequencies, the latter is the signal that the AM signal is set to, while the former is the signal that needs to be transmitted. The carrier signal is typically larger than the intelligence signal. To clarify how the modulation works, the carrier signal is modulated by the intelligence such that the peaks of signal represents the intelligence. For instance, the carrier signal, shown in figure 6, is a 10 KHz signal with a time scale of 400 μs after modulating the signal with a 1 KHz intelligence signal, the resulting signal is that shown in figure 7.
Looking at the signal in figure 7, it can be deduced that the positive peaks match the intelligence signal (1 KHz wave), only difference being that it is shifted vertically upward. Similarly, the negative peaks of the modulated signal match the inverted intelligence signal shifted vertically downwards. It can be said that the intelligence signal envelopes the carrier signal, thus, modulating the signal. It is important to notice that the timescale for viewing the AM signal should match that of the intelligence signal, for instance, the 1 KHz intelligence is shown setting the timescale to 400 μs.
For the following steps, the settings on the function generator were changed tot he following:
Frequency: 1230 KHz
Amplitude: 100 mVpp
After changing the settings of the function generator it was connected to the RF input. After arranging the settings on the oscilloscope, the AM frequency spectrum is found, shown in figure 8. After doing the necessary calculations, table 1 provides the values of the spectrum when modulating the AM signal with a modulation index of 0.5 or 50%, while table two shows the same values for a modulation index of 1 or 100%.
Table 1: Spectrum information with 50% modulation
Figure 8: Spectrum waveform of the signal
Table 2: Spectrum information with 100% modulation
Part 3: AM Signal Frequency Spectrum
For this part, the function generator is set to the following:
Frequency: 50 KHz
Amplitude: 100 mVpp
Offset: 0V
Then, adding a modulation of 1 KHz with an index of 0.5, the resulting signal is shown in figure 9.
Figure 9: AM signal for part 3
After checking the AM signal on the oscilloscope, the Fast Fourier Transform (FFT) feature is turned on the signal shown in figure 10 is observed. Similar to the spectrum waveform shown in figure 8, the carrier and intelligence signals difference is shown. Again, some calculations have been made with the results presented in table 3 for the 0.5 (50%) modulation index and in table 4 are the results for the signal with modulation index of 1 (100%).
Table 3: FFT Spectrum information with 50% modulation
Figure 10: Signal Frequency Spectrum for the modulated 50 KHz signal with 50% index of modulation
Table 4: FFT Spectrum information with 100% modulation