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Twin T Notch Filter

Twin T Notch Filter. Noise. White noise E qual intensity at all frequencies. 60Hz noise is a frequent signal picked up in electronic circuits, most noticeably in audio systems as a low frequency hum that is located between A # (58.27Hz) and B (61.74 Hz).

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Twin T Notch Filter

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  1. Twin T Notch Filter

  2. Noise • White noise • Equal intensity at all frequencies. • 60Hz noise is a frequent signal picked up in electronic circuits, most noticeably in audio systems as a low frequency hum that is located between A# (58.27Hz) and B (61.74 Hz). • Such applications require a high Q/narrow BW notch filter that will remove the noise without impacting the strength of the signal.

  3. Twin T Notch Filter • Design an adjustable Twin T notch filter to remove the 60 Hz signal. • Application note • http://www.national.com/ms/LB/LB-5.pdf • As shown in passive filters experiment, the parasitic resistance of an inductor limits the bandwidth of an RLC notch filter.

  4. Twin T Notch Filter • Operates by phase shifting the signals in the different legs and adding them at the output. • At the notch frequency, the signals passing through each leg are 180 degrees out of phase and cancel out to provides a complete null of the signal. • Components are required to have values that are very close to the nominal values to achieve a high Q notch at the design frequency.

  5. Twin T Circuit

  6. Redrawn to highlight T’s Zo1 io io1 Zo2 io2

  7. At the center frequency • Ideally, io1 - io2 = 180 and magnitude of Zo1 = Zo2 at the center frequency of the Twin-T filter. • This will force Vo = 0V at f = fo.

  8. Adjustable Twin T Notch Filter

  9. Transfer Function where s = jw, w = 1/CR, and a is the fraction of the trim pot resistance R4 that is connected to the input terminal of U2.

  10. Twin T Filter • Use an LM 324 op amp chip. • V+ should be +9V, V- should be -9V. • Either a 10 kW trim pot or a set of resistors that add up to 10 kW may be used for R4.

  11. PSpice • Use two resistors instead of a trim pot. • Perform three simulations of the Twin T notch filter. • Set the resistors value to 9k/1k, 5k/5k, and 1 k/9 kW • Plot the power as a function of frequency • Macro in Trace/Add Trace is DB() • Determine center frequency, , and bandwidth, for each value of R • Bandwidth is the difference in frequency between the -3dB points of the output signal. • Calculate Q where

  12. Measurements • When measuring the characteristics of the Twin T filter • Use the Velleman function generator as Vs of the notch filter. • Set the voltage so that it does not cause the output of the operational amplifiers in the filter to saturate. • Use the Bode Plot at a high resolution to measure the performance of the notch filter around 60 Hz. • Measure center frequency and bandwidth of the notch when the trim pot resistance is approximately 1k, 5k, and 9 kW • Calculate the Q of the filter

  13. Velleman Function Generator • To create a arbitrary waveshape using MATLAB, you must first install the support package for Velleman PCSGU250. This is available on the MathWorks website. • Follow the instructions posted on the Week 9 module to download and install this package.

  14. MATLAB code • In the example file – makeSampleLibs.m, which is linked in Week 11 module, lines 20-26 create a library file called AMWave that can be used by the Velleman scope. • The program at the moment creates a wave shape called AMWave that is sin(f)*sin(20f), where f is the frequency that you set on the Velleman function generator. • The file is saved under c://Velleman/PCSGU250_DLL/lib

  15. Waveform for this experiment • Modify the makeSampleLibs.m file so that a wave shape created is a sum of three sinusoids with equal amplitudes: sin[(2p55Hz)t]+sin[(2p60Hz)t]+sin[(2p65Hz)t] • Write the code so that 60 Hz is the frequency that should be set on the arbitrary function generator.

  16. To Output the Waveform • Click on MORE FUNC. • A pop-up window open. • Click on LIB • Find the correct library file name (AMWave, if you didn’t change it) in directory: c://Velleman/PCSGU250_DLL/lib • Set the frequency to 60 Hz. • Set the amplitude to obtain accurate voltage measurements.

  17. Bode Plot • To see the notch, you will have to set the Frequency Step Size, located under Options, to a small percentage of the total range when the bandwidth of the notch is small. • Note: You can pause the Bode Plot measurement, change the frequency step size, and then unclick the Pause button to vary the speed at which the data is collected.

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