PART 4: AM Detector
INTRODUCTION
In this lab, the student will be working with AM Detectors. The goals of this lab are to understand simple, and biased diode detector circuits, breadboard and analyze them. The student will also learn how to create the circuit on a breadboard and analyze a complementary feedback pair (CFP) detector. Looking at different AM detectors, the student will be able to decide which one will improve the quality of their radio. This will be done by seeing how the detectors extract audio signals from an AM waveform.
4.5 Constructing AM Detectors
To begin, the student built a simple detector circuit from Figure 4.1, except that the voltage source will be the function generator. An AM signal was fed to the detector circuit with a carrier frequency fc = 200 kHz, internal modulation for intelligence frequency fi = 1 kHz, and 50% modulation. A low carrier amplitude of 1Vpp was used to start because a weak signal best represents the signal entering the input from the antenna. The dual channel feature of the scope was used to observe the generator signal input and the rectifier output signal at the same time, shown in Figure 4.2. The amplitude was changed to 100mVpp and the student noticed a change in the output voltage, but the input voltage remained too weak, shown in Figure 4.3.
FIGURE 4.1: Simple Diode Detector Circuit
FIGURE 4.2: Simple Diode Detector Signals for 1Vpp Amplitude
FIGURE 4.3: Simple Diode Detector Signals for 100mVpp Amplitude
Next, the student faced some difficulties with their workstation and had to move locations. During this time, the circuit started to create issues and the TA stepped in to help. The student ended up using the circuit provided by the TA because they could not figure out what was wrong with her circuit. Once this was settles, the student continued to adjust the signal amplitude until the steady input and output signals became clear and easy to read. Then R1 was changed to 10kohm where the signals, shown in Figure 4.4 below seem to be close to that of Figure 4.5 from the lab manual. R1 was changed once again to 100kohms, with the results shown below in Figure 4.6. These results seemed to be a lot clearer and more precise than before.
FIGURE 4.4: Simple Diode Detector Signals with 10kohm Resistor
FIGURE 4.5: Simple Diode Detector LTspice Simulation
FIGURE 4.6: Simple Diode Detector Signals with 100kohm Resistor
For the last part if this section, the student constructed the biased diode detector circuit from Figure 4.7, shown in Figure 4.8. A suitable AM signal was fed to this detector circuit like previously done above. The dual channel feature of the scope was used to examine the results as it was before. The AM signal level was varied from 1Vpp (Figure 4.9) and 350mVpp (Figure 4.10) which are shown below.
FIGURE 4.7: Biased Diode Detector Circuit
FIGURE 4.8: Breadboarded Biased Diode Detector Circuit
FIGURE 4.9: Biased Diode Detector Circuit with 1Vpp
FIGURE 4.10: Biased Diode Detector Circuit with 350mVpp
4.7 Add AM Detector to Your Radio
To conclude this lab, the student added the AM detector of choice to their radio. Based on the above measurements, and the example in Figure 4.11, the student chose the biased diode detector because it be the best choice for her radio. The student then build and tested the radio, shown below in Figure 4.12, to make sure that it worked. This was a success, and the radio did indeed work for the student.
FIGURE 4.11: Biased diode detector circuit added to the CE-CC
FIGURE 4.12: Breadboarded AM Detector Added to Radio
CONCLUSION
In conclusion to this lab, the student was able to successfully work with AM Detectors. The goals of this lab, to understand simple, and biased diode detector circuits, breadboard and analyze them, were achieved. The student also learned how to create the circuit on a breadboard and analyze a complementary feedback pair (CFP) detector. Looking at different AM detectors, the student was able to decide that the biased diode detector would improve the quality of her radio. Though the student did have issues with her workstation in this lab, she feels like a lot of beneficial knowledge was gain in building the ideal radio.