In this week's lab activity, I studied the bandpass filter, a component of the mixer circuit (an additional component that can be added to the AM radio to pickup more stations) that only allows a certain band of frequencies to pass through the circuit. The purpose of the component is to filter out the unwanted radio frequencies so that the AM radio only picks up the desired stations. To achieve this effect, I constructed what is known as the Tank Circuit (Figure 1) and analyzed the response.
Constructing this circuit in lab, I used the values for the components shown in Figure 1. Adjusting the function generator to propagate a variable frequency signal through the circuit, I utilized the network analyzer to automatically sweep frequencies and view the spectrum of the output. Using the oscilloscope to make measurements, I was able to measure critical information such as the resonant frequency and bandwidth. Using this information, I then calculated the quality factor Q (the "Quality Factor" of the filter) of the filter. Next, changing the value of Rs and RL from 100 Ohms to 1 kOhms, I measured the output again and measured the same parameters, recording the results in Table 1.
The addition of the mixer circuit to the AM radio which I have already built will add the functionality of tuning into different radio stations. As evident from the results in Table 2, I can design a bandpass filter to reject unwanted radio frequencies (unwanted stations) and only pass the desired ones to the circuit. Right now, my radio picks up ESPN Radio and I am satisfied with that. So, I do not intend to add the mixer (and bandpass filter) to my current design because I do not particularly care to have a wide range of stations.
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