Introduction

Analog filters are frequency selective circuits that have the role of attenuating or eliminating unwanted spectral components.

The most used types of filters are:

• Lowpass filter (LPF): Allows low-frequency signals to pass through while attenuating higher frequencies.
• Highpass Filter: Allows high-frequency signals to pass through while attenuating lower frequencies.
• Bandpass filter: Passes a specific range of frequencies while attenuating frequencies outside that range.
• Bandstop filter (or notch filter): Attenuates a specific range of frequencies while allowing frequencies outside that range to pass

Main filter parameters are:

• Passband gain;
•  Passband ripple;
•  Stopband rejection;
• Stopband ripple;
• Cutoff frequency;
• Transition band.

Fig. 1: Main parameters of a filter

The lossy integrator

The OA-RC inverting lossy integrator seen in the following figure is used to design the 1st order section of the LPF.

Fig. 2: OA-RC lossy integrator

The transfer function of the OA-RC inverting lossy integrator and its parameters are:

Procedure

1. Build the circuit depicted in Fig.2. The connections should look similar to Fig.3
2. Set the Hantek generator to generate a harmonic signal (sine) with an amplitude of 1V and apply this signal as input. Start with the signal frequency of 100Hz and repeat the steps the following 1kHz, 10kHz, 11kHz, 12kHz, …, 20kHz, 100kHz.
3. Measure the amplitude of the input signal (should be around 1V) and the output signal
4. Fill in the provided table, and plot the frequency response using the gain of the filter.

Fig.3 : Implementing the lossy integrator on the Helpkit board

Table 1: Filter’s gain over frequency results

Fig.4: Frequency characteristic of|H|dB for the lossy integrator

Required Material

• The Home Electronics Laboratory Platform (HELP) board
• Hantek 2D42 Handheld Oscilloscope
• 1 x TL082 OP-Amp
• 2 x 10kΩ resistors
• 1 x 1nF ceramic capacitor