Bandpass Filters
Filters are circuits that let a signal that has a certain frequency range pass through circuit, while blocking other frequencies. The unwanted frequencies can cause noise in the output, or throughout the circuit, and causes the circuit to dissipate more power amplifying a wider range of frequencies. Thus, a filter is sometimes required to decrease the noise in the circuit, lower the power dissipated, or both.
There is no filter that works in any circuit or any frequency, depending on the application and operating frequency the type of filter and magnitude of components differ. For instance, a pi filter, not be confused with a pie filter, which does not exist (yet), the pi filter is used in rectifier circuits to rid the signal of ripple voltage when converting from AC to DC.
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For the radio design, a bandpass filter is used to filter out signals above and below a certain frequency range. The difference between the higher and lower frequency is called the bandwidth of the filter, the bandwidth, upper and lower frequencies change depending on the components in the circuit. Which is to say, a bandpass filter can be designed with a certain frequency range and can be a wide range of frequencies, or a smaller range to pick a specific frequency.
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Tank Circuit
The bandpass filter discussed here utilizes the tank circuit, the tank circuit stores energy at resonant in the active elements. The tank circuit tested is shown in figure 1, the circuit is tested for two different variations in resistance, the first is the shown 100Ω and the second being 1kΩ. The resonant frequency is important to know because it is where the circuit has the highest voltage output, the further the frequency is, the lower the voltage is.
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Figure 1: Tank circuit tested
The resonant frequency of the circuit is then found by analyzing the circuit, as shown in figure 2, the resonant frequency of the circuit is 212kHz. The details of the circuit can be seen in table 1. It is evident from the data that the second circuit has a higher resonator Q, which defines the quality of the resonant frequency, which makes it a better filter compared to the other circuit. However, they differ slightly in their resonant frequency, which should not be a problem for the radio.
Figure 2: Input (yellow) and output (blue) of the circuit.
Table 1: Comparison between tank circuits with different resistances