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Test and Measurement
Objectives:
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This lab is designed to help students become familiar with lab test equipment.
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A part of being comfortable with the equipment means being able to use and generate desired signals such as sine waves.
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Another asset of this lab is that it introduces amplitude modulation and ways of adjusting the modulation.
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Lastly, this lab provides information on how to generate and measure an AM signal.
Equipment:
HP 3311A function generator
BNC-to-banana adaptor
Pomona 20 dB attenuator
BK Precision 4040 function generator
Tektronix TDS2022 oscilloscope
Tenma DC power supply
Omega HHM90 digital multimeter(DMM)
Simple Sine Waves
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The first section of the lab introduces "Simple Sine Waves". In lab, we used the 3311A function generator along with the TDS2022 oscilloscope to produce a 1 V amplitude 10 kHz sine wave.
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Following, we repeated using the BK Precision 4040 function generator.
The steps to achieving the desired goals were relatively simple and understood. Starting by turning on the systems; I ensured that the strength of the input was minimized by turning the amplitude on the function generator completely counterclockwise. The function generator was adjusted accordingly by selecting "10k" range and "sine wave function." This was the input, however, the input signal had to be fed to the output system by some means; this was achieved by using a BNC-to-banana adaptor and coaxial cable. This adaptor cable combination allowed the input signal to become the output signal; thus feeding one of the channels on the oscilloscope with the information needed to produce and output a sine wave!
The oscilloscope was ready to deliver the sine, however there were some adjustments to be made. The oscilloscope had to average the data in order for the signal to viewed in full detail, this was done by using the "Acquire" button followed by selecting "Average" from the menu. Once this was done, the amplitude on the function generator was adjusted to achieve 1 V and the signal frequency was adjusted to get approximately a 10 kHz reading. Finally, the cursors were selected and used to measure the peak to peak voltage on the oscilloscope.
This part of the lab was repeated using the BK Precision 4040 function generator. This was done to show that there are different yet similar ways of achieving the same goal. In this case, the BK Precision 4040 function generator has more functions and features associated with it. This function generator has a digital monitor which reads the frequency; another key feature of this function generator is the "-20 dB button" which adjust the output amplitude range, enabling the user to reach desired signals on the oscilloscope. There are also both course and fine tune adjustment knobs to assist in achieving a desired frequency; however the knobs are extremely sensitive.
Lastly, in each case, the function generators were disconnected from the oscilloscope and their outputs were measured using a Digital Multimeter (DMM). In both cases, the voltages were the same. The value recorded is also known as the Vrms (Voltage root mean squared).
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DMM Voltage = 0.67 V
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Vrms = Vpeak/sqrt2
This part of lab was very simple, yet reminiscent upon days during Co-op terms. There was much work done with ocsilloscopes to interpret signals from circuits in our products. This portion of the lab also grazed a small portion of the class of Linear Signals and Systems; the generation and understanding of waveforms was an important aspect of that course.
Below are a couple of pictures from this portion of the experiment:
Shown above in this figure is the BK Precision 4040 function generator along with the ocsilloscope with the generated sine wave.
This figure is showing the use of the DMM to measure the Vrms from the 3311A function generator.
Amplitude Modulation
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The next portion of lab introduces Amplitude Modulation.
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AM radio signals are Amplitude modulated. This meaning that the intelligence signal, such as a voice, modulates the strength (amplitude) of the carrier frequency.
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To realize an amplitude modulationed signal, the BK Precision 4040 function generator was used along with the oscilloscope of course.
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The 3311A function generator was used as an external modulation source.
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Once modulation was received, the functions on the oscilloscope were used to show the AM Signal Spectrum Frequency
Starting off, the BK Precision 4040 function generator was used to generate a 1 MHz frequency and the amplitude was set to 4 volts pk-pk. Unfortunately this generator is limited to a 1 kHz internal amplitude modulated signal; however the way to get around this will be discussed later. This internal amplitude modulated signal was activated. Also, the percent modulation knob on the function generator was adjusted to receive the 50% modulation. On the scope, the Sec/Div knob had to be adjusted to about 350us. This is because the frequency is moving so fast that only way to get a clear view is to speed up the time in which the signals are generated. Another important aspect of getting the desired signal was to adjust the trigger level; the trigger could not be too high or too low because this would cause distortion. However, if the trigger was set on the modulated signal, a clear output would be recieved.
50% modulation on amplitude modulated wave
As mentioned above, the internal modulation on the BK Precision 4040 function generator is limited to a 1 kHz signal. The way to get around this is by using an external modulated source, in this case, the 3311A function generator. This function generator was used to obtain a 2 kHz, 2 V amplitude. In order to realize the newly desired output, the two function generators had to be connected using a system. This was done by using the VCG/Mod input on the BK Precision 4040. The output of the 3311A was connected to the input of the 4040 which was connected to the oscilloscope. Because the 4040 was being fed the new signal, it had to be set to External Modulation. The Sec/Div knob was adjusted to about 100us and the trigger was again adjusted because the signal on the scope changed. lastly, the amplitude of the 3311A was adjusted to receive approximately 50% modulation.
External modulated signal, 2 kHz, 2 V amplitude
The last portion of this lab involved looking at the AM signal in more detail. This was done by using some of the built in features of the oscilloscope. One of the features utilized is the fast Fourier transform (FFT) that allows viewing the spectrum of the signal. To begin, the output of the BK Precision 4040 function generator was again connected to scope channel 1. The generator was set to output a 50 kHz sine wave with minimum amplitude. The function generator was set to internal modulation since the 3311A function generator was not to be used in this portion of lab. The modulation knob and the Sec/Div knob were adjusted accordingly to achieve the desired output. The desired modulation was 50% and the Sec/Div knob was set to 500us. Again, because the output is occuring so rapidly, the oscilloscope has to be set at a rate where the user can have a clear picture of the signal being portrayed.
Once the scope had been adjusted accordingly, the cursors were then used to measure maximum and minimum amplitudes by adjusting the generator output and the percent modulation to a 100 mV maximum, 50 mV minimum which is 50% modulation.
On the scope, the "Math" menu was selected. This is where the FFT function was chosen. From here there were a few adjustments made. The time base knob was adjusted to 12.5 kHz, the horizontal position was set to center the main peak of the spectrum and the FFT Zoom was changed to "X 10" to get a scaled version of the spectrum. The cursor type was changed to read the frequency of the carrier and the two AM sidebands. Following, the cursors were set to measure the magnitudes of the carrier and the two AM sidebands in dBs. The magnitude value was then converted to voltage amplitude (Vamp) using the following formulas.
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dB = 20 log(Vrms / 1)
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Vrms = 10^(dB/20)
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Vamp = sqrt2 * Vrms
These steps were completed again to achieve 100% modulation. This meaning that the maximum voltage was set to 100 mV and the minimum voltage was set to 0 mV.
Spectrum for 50% Modulation
AM signal 50% Modulation
Information for the Spectrum at 50% modulation:
Frequency
dB level Voltage amplitude
Lower Sideband
Carrier
Upper Sideband
49 kHz
-41.8 dB
11.5 mV
72.5 mV
11.5 mV
50 kHz
-25.8 dB
-41.8 dB
51 kHz
AM signal 100% Modulation
Spectrum for 100% Modulation
Information for the Spectrum at 100% modulation:
Frequency
dB level Voltage Amplitude
Lower Sideband
Carrier
Upper Sideband
49 kHz
-35 dB
25.15 mV
45.76 mV
25.15 mV
50 kHz
-29.8 dB
-35 dB
51 kHz
This lab was great in the sense of becoming familiar with actual lab equipment. This is a more hands on approach rather than simulations on a computer. This lab also reminds me of some of the work I do while Co-oping in terms of scopes and function generators; it also reminds me of Linear Signals and Systems, with the sine waves and especially in relations with the spectrums in the later parts of this lab. Great lab overall! ! !
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