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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.