CEEN 3250 Communication Systems
Dr. Jang

LAB REPORT

EXPERIMENT #11:
Amplitude Modulation (AM) using an Operational
Transconductance Amplifier

Rance Fredericksen

LAB Partners
Whouji Lo
Ryan Leudders

Date: April 3, 2007

University of Nebraska – Lincoln
Peter Kiewit Institute
OBJECTIVE

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.

PRELAB

None.

PROCEDURE

First, we built the circuit shown here:

Figure 11.1. AM Circuit Schematic.

In order for the non-inverting input of the Operational Amplifier to be exactly ZERO VOLTS, the variable resistor (shown on the left-hand side of Figure 11.1) was set to 5.6k/4.4k. The switch shown on the bottom left corner of the diagram was set initially to position 1 (ground).

Once this circuit was constructed, we applied a 60mVp-p sine wave at the input. At the output (Vo), we observed a sinusoid, and we again adjusted the variable resistor to obtain a ZERO VOLT DC OFFSET.

We then moved the switch to POSITION 2. Once in position 2, we took measurements of amplifier gain while adjusting the variable resistor (on the right-hand side of Figure 11.1) between its extremes in 2v increments, as shown here in Table 11.1:

Table 11.1. Adjustable gain values on OTA.

This shows that directing a variable input at this location will cause a variation in gain, or modulate the amplitude of the output signal!

As eluded to previously, we shall now vary this input by changing the switch position to a variable input (waveform generator) labeled vintelligence. This will allow us to modulate the information signal over the carrier frequency, and essentially create an amplitude modulated signal. 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)

UNDERMODULATION
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.

OVERMODULATION
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 11.7.

Figure 11.7. Simple Peak-Detector Circuit.

Using the component values shown in Figure 11.7, we were able to recreate the original vintelligence. Next we varied the value of the capacitance and noticed the following: When a 2.2nF capacitor was used in lieu of the 220nF capacitor, it caused sever peak detection errors via distortion; when a 2.2uF capacitor was used, it caused severe signal attenuation and a phase offset of π/2.

REPORT / QUESTIONS

1. Draw a graph of Table 11.1.

Figure 11.8. Graph of OTA gain.

2. Using Emin and Emax of 0.8v and 2.4v, the modulation percentage is:

(2.4 – 0.8)(2.4 + 0.8) * 100% = (1.6/3.2) = 50%

3.

4. The OTA causes the gain of the output signal to vary due to the value of the signal input (vintelligence); henceforth, the output signal oscillates with a sinusoidal intelligence input. The peak detector simply follows the voltage peaks of the AM signal, causing an enveloping effect, and catching the original signal.

5. As the information signal vintelligence varies from negative to positive, the oscilloscope shows the outline of the triangle, as the AM signal is also varying at the same frequency. The content inside the trapezoid/triangle comes from the variation of the carrier inside the envelope.

6. When the capacitor storing the envelope charge is too small, the voltage would discharge from the capacitor too quickly, causing a distorted signal output. When the capacitance was too large, it would cause the envelope detector’s output to vary too slowly, not encompassing the value of the AM signal.