Lab 1 - Test and Measurement
When you don't read the lab manual
https://www.youtube.com/watch?v=hp97GjuULX8
This lab is the first of the semester, and the goal of this lab is to get students acquainted with the equipment. Students are asked to generate and measure sine waves using the function generator and oscilloscope, get familiar with amplitude modulation, and learn about the spectrum analyzer.
​
1.1 Simple Sine Waves
​
The first task is to learn how to generate a sine wave on the function generator and properly display it on the O-Scope. Students will learn how to scale the x and y axis properly and set an appropriate trigger so the waveform is sable. The first task is to generate and display a 2 Vpp, 10 kHz sine wave. We are shown multiple was to set the amplitude and frequency to accomplish this. We are also taught how to set up addition measurements with the cursors, as well as how to sharpen the signal using the "acquire" button menus. The only issues that came up was that the measured peak voltage is twice what was expected, but this is something the TA mentioned may happen. My results for this exercise are shown in Figure 1.
​
In the manual, there are two questions to answer about this section.
​
1. Given a 10 kHz signal, what would be a good setting for timescale?
​
A signal with frequency 10 kHz has a period of 100 micro second, so a time scale around 50 microseconds is appropriate.
​
2. View the sine wave on the Omega DMM. Is the voltage what you expected? Do you think the DMM is measuring amplitude or RMS?
​
I measured a voltage of 1.6 V on the DMM. I believe that the DMM is measuring RMS values, so while 1.6 V is a little high it is around what I would expect. This slight error is most likely do to poor probing of the BNC cable on my part.
​
1.2 Amplitude Modulation
​
Next, the student is introduced to the concept of Amplitude Modulation (AM). The basics of AM is that an intelligence wave is carried along by a carrier wave. The carrier wave's amplitude is modulated by the intelligence wave. The amount of modulation is given by a modulation index, m, which is a ratio of the intelligence and carrier amplitudes.
​
The student is then asked, what would be a good timescale to view the intelligence given a 1 kHz intelligence frequency and a 1 MHz carrier frequency.
​
Since the intelligence is what we are interested in, we should consider the period of the intelligence, which is 1 millisecond. An appropriate timescale for this period would be around 500 microseconds.
​
Now the student is asked to generate an AM signal with a carrier frequency of 10 kHz and a carrier frequency of 1 kHz with 50% modulation index. My results for this are shown in figure 2.
​
1.2b AM Signal Frequency Spectrum
​
Now the student is to use the O-Scope spectrum analyzer to view the AM signal. For this case, a carrier frequency of 1230 kHz and intelligence frequency of 1 kHz is used as well as a carrier amplitude of 100 mVpp. Modulation depth is set to 50%. It is very important to make sure that the amplitude of the signal is below 1 V, otherwise it may damage the oscilloscope. After this, it is time to set up to spectrum analyzer display. The center frequency is set to 1.25 MHz, and the span 50 kHz. Next, the reference amplitude is set to 0 dB and the resolution bandwidth is 200 Hz. The spectrum can now be seen, and the student is then asked to repeat this for a modulation depth of 100% and fill in the table below. Results are shown in figures 3 and 4.
Frequency | dBm Level | Voltage Amplitude
Lower Sideband| 1.229 MHz| -33.8 dBm | 0.129 mV
Carrier | 1.230 MHz| -21.7 dBm | 0.52 mV
Upper Sideband|1.232 MHz| -33.8 dBm | 0.129 mV
50% Modulation
Figure 1: 2 Vpp 10 kHz Sine Wave
Figure 2: AM signal w/ Fc 10 kHz and Fi 1 kHz
Figure 3: AM signal spectrum 50% modulation
Frequency | dB Level | Voltage Amplitude
Lower Sideband| 1.229 MHz| -5.78 dB | 0.258 mV
Carrier | 1.230 MHz| -5.17 dB | 0.52 mV
Upper Sideband|1.232 MHz| -5.78 dB | 0.258 mV
100% Modulation
Figure 4: AM signal spectrum 100% modulation
1.3 AM Signal Frequency Spectrum (FFT Method)
​
Lastly, the student will now look at the AM signal using the fast fourier transform feature of the oscilloscope. The carrier wave is set to a frequency of 50 kHz and a 100 mV peak to peak value. The intelligence will have a frequency of 1 kHz and a modulation index of 50%. They should make sure to scale the oscilloscope properly for these values. Also, they should change the acquire mode to High Res. Once again, the student is asked to fill out a table of various values at both 50% and 100 % modulation. If the values they get differ from their spectrum analyzer experiment, they should explain why. Results of the experiments are included in figures 5 and 6.
Frequency | dB Level | Voltage Amplitude
Lower Sideband| 48.9 kHz| -25.6 dB | 74.2 mV
Carrier | 50 kHz| -105.2 dB | 7.77 uV
Upper Sideband| 48.9 kHz| -25.6 dB | 74.2 mV
50% Modulation
Figure 5: AM signal FFT 50% modulation
Frequency | dB Level | Voltage Amplitude
Lower Sideband| 48.9 kHz| -25.6 dB | 74.2 mV
Carrier | 50 kHz| -105.2 dB | 7.77 uV
Upper Sideband| 48.9 kHz| -25.6 dB | 74.2 mV
100% Modulation
A few things to note about these results. First, they are not the same as what I got for the spectrum analyzer. This may be due to the difference in carrier frequency, but I think the bigger reason was error on my part. The amplitude i got for the carrier wave just doesn't make sense so I'm not sure what went wrong. I think if a screenshot of an example FFT was included in the lab manual this could have been avoided.
​
In conclusion, I feel that this lab did a good job exposing me to the instruments in the lab, and how to operate them. I feel confident I can properly set up the function generator and oscilloscope in future labs based on the experience I gained this week. However, one thing I am less confident about is the FFT function, and is something I will need some more practice with to make sure I do not repeat my mistakes from this week. Overall, this lab was a success and accomplished its goal of familiarizing me with the lab equipment.
