Transcript Week 7: Electromagnetic Interference

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
– 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.
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
fo 
1
2R1C1
R1  R2  2 R3
C3
C1  C 2 
2
Redrawn to highlight T’s
Zo1
io1
io2
io
Zo2
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.
Adjustable Twin T Notch Filter
fo 
1
2R1C1
R1  R2  2 R3
C3
C1  C 2 
2
Transfer Function
Vo
s w
H ( ) 
 2
VS s  4w(1  a) s  w 2
2
2
where s = j, w = 1/CR, and a is the fraction of the
trim pot resistance R4 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 kW trim pot or a set of resistors that add
up to 10 kW may be used for R4.
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, f o , and bandwidth,
for each value of R
f
– Bandwidth is the difference in frequency between the -3dB
points of the output signal.
fo
• Calculate Q where Q 
f
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
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 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
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.