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Second Order VCVS Sallen Key Low Pass Filter Lab

Objective & Procedure:

Implement a second order, voltage-controlled voltage-source (VCVS) Sallen Key Filter with the following conditions.

a) For a Butterworth Response

Figure 1: 2nd Order Butterworth VCVS Sallen Key Filter

b) For a Chebyshev Response with a 3dB ripple width

Figure 2: 2nd Order Chebyshev VCVS Sallen Key Filter

In both configurations utilize the following parameters:

Make fc = 750Hz and the Gain Stage = 2

The student's will be required to derive the transfer function, model the step function response and create Bode plots utilizing Matlab or comparable simulation tools. Make measurements of the response (i.e. Bode plot) and the step response by using a square wave at a low enough frequency to show the response properly.

1. The 2nd Order VCVS Sallen Key filters transfer function was derived for the Butterworth's response followed by Chebyshev response (see Lab Notes, Page 1 to 4).

2. Simulation and Measured results of the filter responses are recorded below (see Lab Notes, Page 5 to 7).

Conclusions & Discoveries:

It was verified that the expected response in each of the Butterworth and Chebyshev low pass filters rolled of accordingly. The Butterworth filter although smooth in the pass band region, rolled off slower then the Chebyshev. However, to obtain the faster roll off in the Chebyshev low pass filter, a 3dB ripple in the pass band was present. Therefore, depending on the requirements of your filter there is a tradeoff of fast roll off vs. pass band ripple.

In closing, the overall analysis of deriving the transfer function and implementing the circuit using simulations and experimental measurements in a laboratory environment allowed one to develop an understanding of the different response and applications for these types of low pass filter configurations.

Pspice and Graphics
Figure 3: 2nd Order Butterworth Sallen Key LPF
Figure 4: 2nd Order Butterworth Sallen Key LPF, Step Response
Figure 5: 2nd Order Chebyshev Sallen Key LPF
Figure 6: 2nd Order Chebyshev Sallen Key LPF, Step Response
Figure 7: PSpice Output 2nd Order Butterworth Sallen Key LPF
Figure 8: PSpice Output 2nd Order Chebyshev Sallen Key LPF
Filter Lab Notes
Lab Notes Page 1
Lab Notes Page 2
Lab Notes Page 3
Lab Notes Page 4
Lab Notes Page 5
Lab Notes Page 6
Lab Notes Page 7

Electrical Engineering lab key words: Voltage controlled voltage source, VCVS, Sallen key, low pass filter, filters, bandwidth, linear systems, filter analysis, Matlab, Pspice, simulation, filter parameters, Butterworth response, Chebyshev response, passband.

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