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Band-Pass Filter

Objectives:
  • The Bandpass Filter Lab is designed for the following purposes:​
    • ​To introduce the tank circuit as a bandpass filter.
    • To familiarize individuals with bandwidth and Q of a resonant circuit.
    • Breadboard and test filters.

The Tank Circuit

  • This lab introduces the concepts of filters. Active filters are those made using Op Amps and passive filters are those made using inductors and capacitors. The four categories of filters are low pass, high pass, band pass, and band stop. This lab is based on the bandpass filter, which is termed a tank circuit.

  • The circuit is termed "tank circuit" because the parallel inductor and capacitor combination stores energy at a resonance frequency. 

    • Also, the impedance at the resonance frequency will be equal to the load resistance.

    • The impedance drops at values away from the resonance frequency.

  • The term "Q" or "resonator Q" in resonant circuits define the quality of the resonant Circuit.

The first task in the in-lab procedure was to construct the circuit.

  • L = 10 µH

  • C = 0.069 µF

  • R(Load) = 100 Ω

  • R(Source) = 100 Ω

 

By using the information achieved by using LTspice during pre-lab, the variable frequency input signal was found to be approximately 190 kHz; therefore, the frequency generator was set to this frequency to achieve the actual resonant frequency with the assistance of the oscilloscope. The dual trace feature of the oscilloscope was used to observe the input and output signals simultaneously. Measurements such as the resonant frequency, bandwidth (using - 3 dB frequencies), and resonant Q  were then recorded. Comparitively, the load and source resistances were changed to 1 kΩ and measurements were again recorded. These measurements can be seen below:

Table 1: Measurement Data

L_fl

10 µH

10 µH

C_fl

0.069 µF

0.069 µF

R(load), R(source)

100 Ω 

1 kΩ 

resonant frequency

194 kHz

228 kHz

f (-3 dB, low)

168 kHz

204 kHz

f (-3 dB, high)

278 kHz

262 kHz

BW

110 kHz

58 kHz

3.93

1.76

Q = res_freq / BW

Figure 1: Breadboarded tank circuit  100 Ω (source and load)

Figure 2: Measuring resonant frequency on oscilloscope by first finding highest peak.

The bandpass filter lab was informative in the sense of gaining an understanding of filters and how they work. Filters were briefly introduced in Circuit Analysis and similar circuitry was seen in applications in Linear Signals and Systems. This lab gives a detailed view at how these filters perform and how they are used to achieve desired parameters for a given project.

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