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AM Detector
Objectives:
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The objective of the AM Detector lab is to breadboard and analyze various forms of AM detector circuits. Following, the goal was to choose a detector circuit and include it in a chosen audio amplifier for testing.
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The various AM Detectors tested in this lab are the:
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Simple diode detector circuit
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Biased diode detector circuit
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Complementary feedback pair
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The pre-lab:
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The main focus this pre-lab was to gain an understanding of a simple diode detector circuit, biased diode detector circuit, and analyze a complementary feedback pair all with the assistance of LTspice.
Constructing AM Detectors
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This lab has one primary objective, which is to build and test the given forms of AM Detectors in order to pick one to be used with a chosen Audio Amplifier.
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The goal involved:
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Testing the Simple Detector circuit.
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Testing the Biased diode detector circuit.
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Testing the CFP detector circuit.
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Chosing an AM detector circuit to be used with a chosen Audio Amplifier.
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The first AM Detector to be constructed was the Simple detector circuit. This circuit consists of a diode rectifier circuit which was examined in Digital Electronics. The circuit consisted of an RC time constant fast enough to follow the audio signal, yet too slow to follow the carrier. The first requirement was to feed an AM signal to the detector circuit. To do this, lab 1 had to be revisited to remember the precise steps. The dual channel feature on the oscilloscope was used to observe the generator signal and the rectifier output signal simultaneously. Unfortunately in this case, there was no output to be seen using the Simple AM Detector circuit. Precise reasoning is unclear. The results are seen below:
Figure 1: Simple detector circuit which the output being measured.
Figure 2: Simple detector circuit input and output measurements.
Following the simple detector circuit; the biased diode detector circuit was breadboarded. This was easily done by connecting two resistors between the 47uf capacitor connected to the input signal and the diode. The 100kΩ resistor was attached to the 9V supply and the 10KΩ resistor was attached to ground. This detector circuit works well in the case where the simple detector circuit does not. The signal coming into the simple detector circuit is usually weak, causing the output to suffer as a result of the diodes not turning on. The biased detector circuit works well to give an amazingly strong output signal in the case of which it was used. The output of the biased diode detector is viewed in the same way as that of the simple detector, which is to read across the second 47uf capacitor to ground (as shown in figure ). An image of the attached resistors to the biased diode circuit can be seen below:
Figure 3: Biased detector circuit
Figure 4: Input and output of the Biased Detector circuit
The last AM detector circuit tested was the Complementary Feedback Pair (CFP). This detector is achieved by modifying the modified common-collector amplifier. In addition to the common-collector based AM detector, the CFP adds a pnp transistor as well as a collector resistor to the npn. The emitter capacitor charges when the input signal is high, causing both transistors to turn on. When the input signal becomes low, the emitter capacitor discharges through the emitter resistor; in addition, the pnp transistor provides more current for charging the emitter capacitor. The CFP detector works well to extract weak AM signals. However in this case, the signals presented by the CFP proved rather messy and noisey. The results are seen below:
Figure 5: Complementary Feedback Pair circuit, shown with output being measured.
Figure 6: Complementary Feedback Pair circuit with input (2V amplitude) and output being shown.
Figure 7: Complementary Feedback Pair circuit with input (450mV amplitude) and output being shown.
As realized through the results of the experiments; in this case, the biased diode detector circuit can be determined to have the strongest output signal in terms of tracking and extracting the signal. This is even the case for low input signals. As for the CFP, the results were not as expected and the signals were not clean at 2V amplitude nor low voltage amplitude. The simple detector failed to produce an output all together. As a result, the biased diode detector circuit is chosen to be paired with the audio amplifier.
In chosing an audio amplifier, the two stage Op-Amp - AB Push-Pull amplifier and the LM386 were up for discussion. However, the results after testing the sound and output signal of the LM386 proved to fail in comparison to that of that Op-Amp - AB Push-Pull amplifier.
Figure 8: Biased diode detector paired with Op-Amp - AB Push-Pull audio amplifier.
Though difficult to see, the only circuit connected in circuitry is the biased diode detector which is connected to the Op-Amp - AB Push-Pull audio amplifier. This combination produced the best sound and output in comparison to others. As a result, this combination will be used for the further part of this radio lab.
This lab mainly revealed the truth about engineering. Things will not always go as planned and sometimes designs that are expected to be better will prove not to be. Theoretical outcomes are not always what will happen in real life scenerios. This lab relates to many courses in the sense of reality. The further parts of this radio lab will reveal many more scenerios in which actual results differ from theoretical results.
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