Twin T Notch Filter

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Twin T Notch Filter
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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.

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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.

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Twin T Circuit
6
Redrawn to highlight Ts
io
Zo1
io1
Zo2
io2
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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.

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Adjustable Twin T Notch Filter
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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.
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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.

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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

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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

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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.

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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

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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)tsin(2p60Hz)ts
  in(2p65Hz)t
• Write the code so that 60 Hz is the frequency
  that should be set on the arbitrary function
  generator.

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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 didnt change it) in directory
  c//Velleman/PCSGU250_DLL/lib
• Set the frequency to 60 Hz.
• Set the amplitude to obtain accurate voltage
  measurements.

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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.