Presentation on theme: "Twin T Notch Filter. Noise White noise – Equal intensity at all frequencies. 60Hz noise is a frequent signal picked up in electronic circuits, most noticeably."— Presentation transcript:
Twin T Notch Filter
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.
Twin T Notch Filter Design an adjustable Twin T notch filter to remove the 60 Hz signal. Application note – – As shown in passive filters experiment, the parasitic resistance of an inductor limits the bandwidth of an RLC notch filter.
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.
Twin T Circuit
Redrawn to highlight T’s i o1 i o2 ioio Z o1 Z o2
At the center frequency Ideally, i o1 - i o2 = 180 and magnitude of Z o1 = Z o2 at the center frequency of the Twin-T filter. – This will force V o = 0V at f = f o.
Adjustable Twin T Notch Filter
Transfer Function where s = j , w = 1/CR, and a is the fraction of the trim pot resistance R 4 that is connected to the input terminal of U2.
Twin T Filter – Use an LM 324 op amp chip. V+ should be +9V, V- should be -9V. – Either a 10 k trim pot or a set of resistors that add up to 10 k may be used for R 4.
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 k – 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
Measurements When measuring the characteristics of the Twin T filter Use the Velleman function generator as V s 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 k – Calculate the Q of the filter
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.
MATLAB code In the example file – makeSampleLibs.m, which is linked in Week 11 module, lines 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
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[(2 55Hz)t]+sin[(2 60Hz)t]+sin[(2 65Hz)t] – Write the code so that 60 Hz is the frequency that should be set on the arbitrary function generator.
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.
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.