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Page 1
CAL POLY POMONA
Nguyen Jason, Andrew Co
EXPERIMENT #7
RESONANT CIRCUITS
EXPERIMENT PERFORMED
22nd February 2012
EXPERIMENT DUE DATE28th February 2012
ECE 231L
WEDSNDAY 8:00-10:50 AM
PROFESSOR RICHARD F. SMITH
WINTER 2012
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Lab
Experiment #7
Resonant Circuits
OBJECTIVE
- The main objective of this experiment is to observe and calculate the response of a
resonant circuit; while gaining experience in plotting series and parallel circuit response and
experience in varying the bandwidth of a resonant circuit.
BACKGROUND
Resonant circuits are used in many applications such as computer circuits, high voltage
generators, and communications devices such as radios. In this laboratory experiment you will
construct and measure the performance of both a series and parallel resonant circuits.
For a series resonant circuit the voltage of the voltage across the resistor will be the same as the
source voltage; however, the voltage across the inductors L and capacitor C will be
considerably higher depending upon the quality factor Q of the circuit. The series circuit is often
called a bandpass circuit. It usually provides voltage gain.
For a parallel circuit, just the opposite is true. The voltage across the inductor and capacitor
will equal the source voltage and the voltage across the resistor will approach zero. This type of
circuit is often called a notch filter. They are often used to drive induction heaters and welders.
This circuit usually provides current gain.
Use the inductor as the output load for the series circuit. If you use the capacitor and you are
able to build a circuit with a very high Q you could damage the capacitor. The series circuit you
will construct is shown in Figure 1. It has a resonant frequency of about 5 KHz.
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PROCEDURE
The following series and parallel circuits shown were built and connected to our signal generator
to the input. We chose the input to be 5 volts peak and swept the frequency from about two
decades below the resonant frequency to decades above the resonant frequency. We then
observed the output on the oscilloscope and recorded the phase angle and output voltage.
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LAB MEASUREMENTS AND CALCULATIONS
Table 1. Series Circuit
Frequency
F out (kHz)
Phase
Degrees
o
Output Voltage
Vout
3.5 60.8 2.875 V
4.0 309.2 3.750 V
4.5 30.0 4.625 V
5.0 0 5.062 V
5.5 28.84 4.812 V
6.0 45.54 4.188 V
6.5 57.6 3.562 V
Table 2. Parallel Circuit
Frequency
F out (kHz)
Phase
Degrees o
Output Voltage
Vout
3.5 83.0 1.700 V
4.0 -275.0 1.181 V
4.5 -265.0 600 mV
5.0 108.0 118mV
5.5 -78.0 515mV
6.0 78.8 921mV
6.5 76.7 1.328V
7.0 -75.0 1.7V
7.5 -72.0 2 V
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LAB MEASUREMENTS AND CALCULATIONS CONTINUED
R=100 ohms , L= 0.01 H, C= 100 nF
( )()
Vomax occurs at o (center frequency ) = where XL= Xc = 2 f
kRad = 5.032 kHz
Cutoff frequency (1) ( ) ( ) (27,015/2PI) = 4.299 kHzCutoff frequency (2) = ( ) ( ) (37,015/2PI) = 5.891 kHzWhere = kHzThe bandwidth is defined at the frequencies where Vout drops to 0.707 Vsource or3dB.
That is bandwidth ccR/L
Quality factor =
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CONCLUSION:
Overall, the experiment was a great success where the measurements were close to the
theoretical value. Experimental error did play a factor in the difference between the
measurements and theoretical value because we were unable to take precise readings as the
numbers jumped from a range of values.