Class: BE209
Group: R5
Members:Scott Barbash, Meoghan MacPherson , Magdalena Jonikas, Justin Reddy , Melody Tsui

Date: December, 2002

Full Text

Introduction

The purpose of this experiment was to build a bandstop filter to block 120Hz AC noise from the output of the power supply.  Such a filter could be modified to block other frequencies depending on its intended use and the noise present.   One possible use would be in an electronic eye, to be able to analyze frequency data while ignoring the background noise frequency.  Another use is in filtering FM signals from frequencies received by wideband antennas.  Because the FM signals are so powerful, they cause massive intermodulation problems.  The proposed bandstop filters address this problem.  A Twin Tee and a Wien Bridge Notch Filters were chosen to accomplish this goal of blocking 120Hz noise.  The Twin Tee circuit, shown in Figure 1, combines a lowpass and highpass filter to block a frequency f0 according to the equation (1)  where   and .  A 741 Op-Amp chip is included to isolate the filter, should it be integrated with another circuit.  The Wien Bridge circuit, shown in Figure 2, filters out a frequency by altering gain with respect to frequency, and having a minimum gain at the given frequency .  The filtered frequency is determined by the equation (2)   where R = R1 = R2 = R3 = R4 = R5 and C = C1 =C2.  Although the reference resistance (Rref) is constant in this circuit, a change in frequency varies the impedance of the capacitors, which in turn changes the input resistance to the chip (Rin).  Since gain = Rref/Rin, the resistor and capacitor values are designed in such a way that Rin=Rref at the frequency to be blocked, and Rin>Rref at any other frequency.  Thus a minimum gain of 1 occurs at the desired frequency, minimizing the response with respect to those at other frequencies.  Although the goal of this project was to block 120Hz noise, the actual frequency of the AC noise from the output of the power supply was measured in order to make sure that this is an appropriate goal.  This was accomplished with a phototransistor in a voltage divider as shown in Figure 3.  Once the proper frequency to be blocked was determined, appropriate capacitor and resistor values were determined for both filter circuits.  The bandstop filters were also used in combination with the voltage divider circuit to demonstrate the usefulness of the filters.