CEEN 3250 Communication Systems
Amplitude Modulation (AM) using an Operational
Date: April 3, 2007
University of Nebraska – Lincoln
Peter Kiewit Institute
1. To become familiar with the 3080E operational transconductance amplifier (OTA) and how its variable gain feature makes it ideam for use as an AM modulator.
2. To observe typical AM waveforms that exhibit undermodulation, overmodulation, and 100% modulation.
3. To understand how a peak detector functions to extract the original intelligence from an AM waveform.
The output signal is shown in Figure 11.2 below.
Figure 11.2. Output waveform sketch (using MSPaint®).
Using the trapezoidal display to identify the purity of the waveform yielded a scope output shown in Figure 11.3 below.
Figure 11.3. Trapezoidal Display sketch at 100%
modulation (using MSPaint®).
We then analyzed the output waveform with a spectrum analyzer, yielding the diagram shown in Figure 11.4.
Figure 11.4 Spectral Digram of AM signal
(50kHz carrier, 200Hz information signal)
Next, we reduced the modulation index by reducing the amplitude of the information signal vintelligence such that the amplitude was half of its original value. The AM signal then looked like the one shown in Figure 11.5 in both linear and trapezoidal graphs here :
Figure 11.5. AM waveform at 50% modulation (undermodulation).
At 50% modulation, we noticed that the frequency components at fc +/- 200Hz, when measured on the spectrum analyzer, appeared at 5dB less power than at 100% modulation.
Lastly, we made an attempt to overmodulate the information signal by increasing the amplitude until it was larger than that of the carrier offset. The signal then looked like the one shown in Figure 11.6.
Figure 11.6 Overmodulated AM Signal and associated scope, trapezoid, and spectrum outputs.
Now that we had built and implemented an AM transmission network, we move on to AM demodulation, or AM receiver circuitry. We built a simple peak detector as shown here in Figure...