# Week six addresses the concept of active filters and oscillators. The low, high, bandpass and notch active filters, their circuits, and operating parameters are covered. The Wein-bridge RC oscillator, its configuration, operation and circuit is discussed.

Part A-Active Filters

Week six addresses the concept of active filters and oscillators. The low, high, bandpass and notch active filters, their circuits, and operating parameters are covered. The Wein-bridge RC oscillator, its configuration, operation and circuit is discussed.

TCO #6:

Given an application requiring active filters calculate, simulate and measure filter characteristic of a low-pass, high-pass, band-pass, and notch filter.

A. Identify the gain-versus-frequency response of basic filters.

a. Draw the frequency response of a low-pass, high-pass, bandpass, and a notch filter. Label each axis, the critical frequency, bandwidth and the 3-dB point.

Low High

Description: Axis Description: Axis

Band Notch

Description: Axis Description: Axis

b. What is the “order” of a filter? How is an order created?

B. Simulate the frequency response of an active low-pass filter such as a Butterworth filter.

a. Given the following circuit, calculate the critical frequency and the closed-loop gain.

= _________ = _________

Description: Lowpass1

b. Download “ECT246_Week_6_Low1.ms” from Doc Sharing, week 6. Run the simulation. Use the Bode Plotter to find the -3-dB point and determine the critical frequency.

= _________ @ -3 dB

c. Compare the calculated value to the simulated results.

C. Prototype the active single-pole low-pass filter and measure and sketch its frequency response, compare and contrast the simulated and measured results.

a. Prototype the active single-pole low-pass filter in the diagram above using a LM741 op-amp.

b. Connect a frequency generator to the input of the filter. Set the frequency generator’s output to a sine wave at 100 mV peak and as close to 0 Hz as possible.

c. Connect channel 1 of an oscilloscope to the input and channel 2 to the output of the filter.

d. Vary the input frequency according to the chart below and record the input and output voltage in the table. Calculate the other values.

Frequency (Hz)

Input Voltage (peak)

Output Voltage (peak)

Gain

Gain dB

1

10

100

1k

10k

100k

500k

e. Using the Graph Paper.doc file located in Doc Sharing, week 6, plot the frequency response of the circuit.

f. Locate for the filter. Compare its value to the calculated and simulate results.

D. Simulate the frequency response of an active single-pole high-pass filter such as a Butterworth filter.

a. Given the following circuit, calculate the critical frequency and the closed-loop gain.

= _________ = _________

Description: High1

b. Download “ECT246_Week_6_High1.ms” from Doc Sharing, week 6. Run the simulation. Use the Bode Plotter to find the -3-dB point and determine the critical frequency.

= _________ @ -3 dB

c. Compare the calculated value to the simulated results.

E. Prototype the active single-pole high-pass filter. Measures and sketch its frequency response. Compare and contrast the simulated and measured results.

a. Prototype the active single-pole high-pass filter in the diagram above using a LM741 op-amp.

b. Connect a frequency generator to the input of the filter. Set the frequency generator’s output to a sine wave at 100 mV peak and as close to 0 Hz as possible.

c. Connect channel 1 of an oscilloscope to the input and channel 2 to the output of the filter.

d. Vary the input frequency according to the chart below and record the input and output voltage in the table. Calculate the other values.

Frequency (Hz)

Input Voltage (peak)

Output Voltage (peak)

Gain

Gain dB

1

10

100

1k

10k

100k

500k

e. Using the Graph Paper.doc file located in Doc Sharing, week 6, plot the frequency response of the circuit.

f. Locate for the filter. Compare its value to the calculated and simulate results.

F. The active two-pole band-pass filter below consists of a low-pass and a high-pass filter. Predict the critical frequencies.

a. Given the following circuit, calculate the critical frequencies and the closed-loop gain.

= _________ = _________ = _________

b. Download ECT246_Week_6_Band1.ms, and run the simulation. Use the Bode Plotter to find the -3-dB point and determine the critical frequencies.

= _________ @ -3 dB; = _________ @-3dB

c. Compare the calculated value to the simulated results.

Description: Band1

G. Prototype the active two-pole band-pass filter consisting of a low-pass and a high-pass filter and predict the critical frequencies. Compare the results with the simulated values.

a. Prototype the active single-pole band-pass filter in the diagram above using a LM741 op-amp.

b. Connect a frequency generator to the input of the filter. Set the frequency generator’s output to a sine wave at 100 mV peak and as close to 0 Hz as possible.

c. Connect channel 1 of an oscilloscope to the input and channel 2 to the output of the filter.

d. Vary the input frequency according to the chart below and record the input and output voltage in the table. Calculate the other values.

Frequency (Hz)

Input Voltage (peak)

Output Voltage (peak)

Gain

Gain dB

1

10

100

1k

10k

100k

500k

e. Using the Graph Paper.doc file located in Doc Sharing, week 6, plot the frequency response of the circuit.

f. Determine, , and the bandwidth for the filter. Compare its value to the calculated and simulate results.

H. Explain the operation of a notch filter.

a. Explain the operation of a notch filter.

b. Draw the frequency response curve for a notch filter.

Description: Axis

Part B- Oscillators

TCO#7:

Given an oscillator application, such as a tone generator, determine the operating parameters and calculate and measure the oscillator’s voltage, frequency and relative stability.

A. Relate the principles of an oscillator using a block diagram and explain Barkhausen criteria.

a. Draw a block diagram of an oscillator.

b. Explain the relationship between feedback, phase shift and oscillation

c. What is Barkhausen criterion?

d. What three conditions must be met for oscillation to occur?

B. Calculate and analyze the operation of an oscillator using RC feedback, such as a Wien-bridge oscillator.

a. Given the Wien-bridge oscillator in the diagram below, calculate the resonant frequency. Assume = 0.

= ________

b. Determine the closed-loop gain.

= _________

C. Simulate an RC feedback oscillator, such as a Wien-bridge. Record the oscillation frequency and compare the results with the calculated values.

a. Download “ECT246_Week_6_Wien1.ms” from Doc Sharing, week 6. Run the simulation. Use the oscilloscope to determine the resonant frequency. Adjust if necessary to obtain oscillation.

= ________

b. Compare the calculated value to the simulated results.

Description: Wien1

D. Prototype the RC feedback oscillator, such as a Wien-bridge. Measure the frequency of oscillation and compare the results with the calculated and simulated results.

a. Prototype the Wien-bridge oscillator in the diagram above using a LM741 op-amp.

b. Connect an oscilloscope to the output. Apply power and measure the resonant frequency. Adjust as necessary to obtain oscillation.

c. Compare the measured values to the calculated and simulated results.

d. Connect a speaker to the output and observe the results.

e. Substitute different values for and/or and observer the results.

Part C-Filters and Oscillator Simulations

1. The circuit below is a two-pole high-pass Butterworth filter. The upper critical frequency should be 1.12kHz. Find the required value for R4 and the closed loop gain. Verify the circuit operation in Multisim. Make the necessary changes to the circuit and verify its operations. The Multisim file (ECT246_Week_6_Two_high_trouble.ms) can be found in Doc Sharing, week 7.

The R4 value should be _______; Acl = ___________