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# Activity: Low Pass and High Pass Filters

## Objective:

The objective of this Lab activity is to study the characteristics of passive filters by obtaining the frequency response of low pass RC filter and high pass RL filter.

## Background:

The impedance of an inductor is proportional to frequency and the impedance of a capacitor is inversely proportional to frequency. These characteristics can be used to select or reject certain frequencies of an input signal. This selection and rejection of frequencies is called filtering, and a circuit which does this is called a filter.

Figure 1: Low Pass RC filter.

Figure 2: High Pass RL filter.

If a filter passes high frequencies and rejects low frequencies, then it is a high-pass filter. Conversely, if it passes low frequencies and rejects high ones, it is a low-pass filter. Filters, like most things, aren't perfect. They don't absolutely pass some frequencies and absolutely reject others. A frequency is considered passed if its magnitude (voltage amplitude) is within 70% or 1/sqrt(2) of the maximum amplitude passed and rejected otherwise. The 70% frequency is called corner frequency, roll-off frequency or half-power frequency.

The corner frequencies for RC filter and RL filter are as follows:

For RC filters:

(1)

For RL filters:

(2)

Frequency Response: It is a graph of magnitude of the output voltage of the filter as a function of the frequency. It is generally used to characterize the range of frequencies in which the filter is designed to operate within.

Figure 3: Frequency Response of a typical Low Pass Filter with a cut-off frequency fc

## Materials:

Solder-less breadboard, and jumper wire kit
1 1 KΩ resistor
1 1 µF capacitor
1 12 mH inductor

## Questions:

Calculate the Cut-off frequencies for the RC low pass and RL high pass filter using equations (1) and (2). Compare the computed theoretical values to the ones obtained from the experimental measurements and provide a suitable explanation for any differences.

Lab Resources:

• Fritzing files:
• LTSpice files: