Class: BE209
Group: W5
Members: CHEN, ALICIA Y;KEENAN, KATHRYN E; MERCHANT, MAEESHA; SIEMS, ASHLEY L; WALKER, JUSTIN A
Date: December, 2002

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INTRODUCTION

Electronic devices and circuits are subject to measurement interference from outside sources due to noise.  High pass filters combine capacitors and resistors in series to allow frequencies above a certain limit to pass through the circuit, attenuating frequencies that are lower than this defined limit.  Low pass filters switch the order of resistors and capacitors and work in a reverse manner, attenuating frequencies of high magnitude and passing frequencies of low magnitude.  Combinations of low and high pass filters can create band stop and band pass filters.  Band stop filters, also known as Notch filters, attenuate for a specific frequency or range of frequencies.  Wave traps are special types of band stop filters which attenuate a specific frequency instead of a range, and prevent noise originating from external sources from interfering with data collection. (1)
The Wien Bridge circuit consists of two filter networks: a low pass filter and a high pass filter connected in series and grounded. The two filter networks overlap at a single frequency that is attenuated. (This single frequency will be amplified and fed back via positive feedback to cause oscillation using an LM741 chip.) The frequency of oscillation is attenuated to one third of its normal operating voltage by the filter networks.  The R6 resistor (see Figure 3) returns frequency to oscillating level by increasing the gain by at least a factor of 3. (2)
The Twin Tee is also a notch filter, but differs from the Wien Bridge in its design: a tee network is a two port network whose configuration is shaped in a letter T. (3) Also, the Twin Tee makes use of a voltage follower, a special case of a non-inverting amplifier that isolates a voltage source from the application; thus, Vin = Vout.  The voltage follower also isolates the signal source from the output; therefore, additional circuits can be connected to the Twin Tee without affecting the voltage inputs and outputs.
These circuits can be customized to the measured noise by selecting the values of the resistors and capacitors in the circuit.  Both the Twin Tee and the Wien Bridge function with a cut off frequency as follows (4):
  f =1/(2?RC)   Equation 1
After attenuating the frequency of the lights, the values of resistors and capacitors will be varied to view effects of these changes.
While the Wien Bridge and Twin Tee differ in design standards, both circuits require the use of the LM741 operational amplifier (op-amp) chip in a non-inverting configuration.  The op-amp chip is an ideal voltage amplifier of high gain that translates to infinite input impedance and zero output impedance.  Hence, the voltage between the (-) and (+) input terminals goes to zero.  The op-amp chip is the active element that allows for both circuits to function as an oscillator. (5) Because of the use of the operational amplifier chip in both circuits, it is expected that there will be no difference between attenuating efficiency of the Twin Tee and the Wien Bridge filters:
        D0: µWien bridge - µTwin tee=0
Using two filters allows us to compare designs and to test if frequencies are better attenuated using more than one filter.