Lab One: Tests and Measurements
Simple Sine Wave
To begin, the function generator is used to create a 10KHz sine wave that has a \(V_{pp}\) of 2V with a DC offset of 0V. The function generator is connected directly to the oscilloscope to view the signal as shown in Figure 1. In doing so, an appropriate trigger level and time scale were chosen so that the signal will display nicely.
Amplitude Modulation
Now the function generator is used to created an amplitude modulated signal. As is indicated in the name, AM signals carry a signal by modulating the amplitude of a base carrier signal. This carrier frequency of this signal is designated to be 10KHz with a modulated 1KHz signal as shown in Figure 2. The function generator is once again connected directly to the oscilloscope for viewing.
AM Analysis
Now the previously generated signal is analyzed in the frequency domain. This is done by using the Spectrum Analyzer function of the oscilloscope. The spectrum shown in Figure 3 is the frequency spectrum of a 1.23MHz 100mV \(V_{PP}\) signal at 50% modulation. Similarily Figure 4 shows the same signal at 100% modulation. Tables 1 and 2 show the values of the upper, lower and center bands of the two signals.
\begin{array} {|c|c|c|} \hline Frequency (MHz) & dBm Level & Voltage Amplitude (mV)\\ \hline 1.229 & -33.6 & 6.6 \\ 1.23 & -21.6 & 26.3 \\ 1.231 & -33 & 6.6 \\ \hline \end{array}
\begin{array} {|c|c|c|} \hline Frequency (MHz) & dB Level & Voltage Amplitude (mV)\\ \hline 1.229 & -57.6 & 13.18 \\ 1.23 & -51.6 & 26.3 \\ 1.231 & --57.6 & 13.18 \\ \hline \end{array}
Next a similar signal is analyzed using the FFT (Fast Fourier Transform) function of the oscilloscope. The spectrum shown in Figure 5 is the frequency spectrum of a 50KHz 100mV \(V_{PP}\) signal at 50% modulation. Similarily Figure 6 shows the same signal at 100% modulation. Tables 1 and 2 show the values of the upper, lower and center bands of the two signals.
\begin{array} {|c|c|c|} \hline Frequency (kHz) & dB Level & Voltage Amplitude (mV)\\ \hline 49 & -47.36 & 6.06 \\ 50 & -35.61 & 23.44 \\ 51 & -47.36 & 6.06 \\ \hline \end{array}
\begin{array} {|c|c|c|} \hline Frequency (MHz) & dB Level & Voltage Amplitude (mV)\\ \hline 49.01 & -41.7 & 11.62 \\ 50 & -35.37 & 23.04 \\ 50.99 & -41.92 & 11.33 \\ \hline \end{array}
These values are different from the spectrum analyzer as the carrier frequency is much smaller. This is to be expected. These analysis methods can be used to decompose and isolate wanted and unwanted frequencies.