PART 1: TEST AND MEASUREMENT
INTRODUCTION
The following lab is designed to familiarize the student with the lab equipment that will be used in this course. The goals for this lab are to to generate and measure sine waves. As well as, to become familiar with amplitude modulation, and generate and measure an AM signal. This will be completed using the following lab equipment: AFG1062 Arbitrary Function Generator, Tektronix MDO 3032 oscilloscope, Tenma DC power supply, and Omega HHM90 digital multimeter (DMM).
1.1 SIMPLE SIGN WAVES
To begin, the student turned on both the MDO 3032 oscilloscope and AFG 1062 function generator. Next, a 10 kHz sinewave with a 1V amplitude was generated with the function generator. The peak-to-peak voltage was set to 2 Vpp, the minimum voltage to -1V, and the Offset to 0V. To view the wave on the oscilloscope, a BNC-to-BNC cable was connected from the function generator to channel 1 of the oscilloscope. For the input signal, the oscilloscope was set to 1V and a time scale of 40µs. Using the Trigger Level knob on the function generator, the signal was adjusted until the sine wave appeared, and was sharpened using the Acquire button. Next, the student began to practice using the cursors on the oscilloscope by pressing the cursors button and using the multipurpose a and b knobs. The vertical cursors were adjusted to find frequency and period for the waveform. Given a 10kHz signal, 40µs was the best setting for the time scale, shown below in Figure 1.1.
FIGURE 1.1: Simple Sine Wave
1.2 AMPLITUDE MODULATION
For the next part of the lab, the student looked at amplitude modulated AM signals. The student then learned about intelligence signals modulating the amplitude of the carrier (Figure 1.4), the spectrum of the AM signal (Figure 1.5), and the spectrum of intelligence extracted by an AM detector (Figure 1.6).
The student started the simulation by using the AFG (Arbitrary Function Generator) to create a 10 kHz AM signal with 50% modulation and 1 kHz intelligence frequency. This was done by using the AFG to select mod, then type, then AM Frequency on the display. 1 kHz was entered on the number pad, the depth of 50 was set as well. After adjusting the trigger level on the oscilloscope, the student the student got an amplitude modulated signal shown below in Figure 1.2.
FIGURE 1.2: An amplitude modulated wave with 50% modulation
For the second part of the Amplitude Modulation section of this lab, the student again used the AFG and oscilloscope, but this time to get the AM Signal Frequency Spectrum. To do this, the present wiring was unplugged and the equipment is paused so the BNC can be plugged into the spectrum analyzer of the oscilloscope. Next, the student set the function generator to a frequency of 1230 kHz and an amplitude of 100mV peak to peak, leaving the intelligence frequency unchanged. The RF button was pressed to show the modulation. The frequency was set span to 50 kHz, and the reference label to 0 dBm. Then, the BW button to was used to set the RBW to 200 Hz. After that, the student used the cursors of the oscilloscope to record data (Table 1) for 50% modulation, shown in Figure 1.3. These steps were then repeated for 100% modulation, shown in Table 2. The spectrum analyzer could also be used to view optical light waves.
FIGURE 1.3: An waveform spectrum with 50% modulation
1.3 AM SIGNAL FREQUENCY SPECTRUM (FFT METHOD)
For the final part of this lab, the student again viewed an AM signal frequency spectrum, but used the FFT method on the oscilloscope. The student changed the oscilloscope input to channel 1. The frequency was set to 50 kHz with a 100 mV amplitude on the function generator. The student checked to see if the internal intelligence frequency was 1 kHz, the modulation on the ARB was 50%, the trigger to the horizontal scale to 400µs. For the FFT method, the student pressed the M (Math) button on the oscilloscope. The signal is shown below in Figure 1.7. The cursors were used again to read the peak of the carrier frequency. After that, the frequency and dB of the carrier and sideband waveforms for both 50% and 100% modulations are recorded. The values are then converted to voltage amplitudes recorded below in Tables 3 and 4. The results were similar to the Spectrum Analyzer, but not the same. This could be because with the Spectrum Analyzer, you to view the signal in the time domain, and with the FFT you to view the signal in the frequency domain.
FIGURE 1.7: AM Signal Frequency Spectrum (FFT method)
CONCLUSION
In this lab, the student learned very important skills in the lab that will not only be helpful in this class, but in other classes and in the workforce as well. This lab was very helpful in getting the student to thoroughly understand some abilities of the oscilloscope and function generator.