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Narrow Band Pass Filter

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I. Introduction:

This lab is intended for designers that have time to check filter theory as shown in the college communications textbook. The pre-engineers are going to build a Narrow Band Pass Filter to compare theory with designed circuit behavior. By doing this the students will be able to better understand the theory based calculations and build hard wired circuits. When designing a band-pass filter, the parameters of interest are the gain at the mid frequency and the quality factor which represents the quality of the band-pass filter. A band-pass filter is one that allows a narrow range of frequencies around a center frequency to pass with minimum attenuation but rejects frequencies above or below this range. The students built a narrow band-pass filter which is a fairly respected filter because it holds a narrow bandwidth.

II. Materials:

* Capacitors: (2) .001µF
* Resistors: (2) 390Ω, (1) 7.3kΩ (1) 20Ω
* Amplifier: 741 Op Amp
* LPS-151 DC Tracking Power Supply
* 2532 Digital Storage Oscilloscope
* Fluke Digital Multi-meter
* Leader LFG 1300S Functional Generator

III. Procedure:

The engineering team built the circuit shown in figure 1. Keep in mind; use the minimal amount of wiring to avoid stray capacitance which generally happens at high frequency. We used a LN-741 with two dc power supplies and one ac power supply. The dc power supplies consisted of a positive power supply and a negative power supply set at 12V. Once the circuit were built the positive power supply was connected to the number 7 pin of the op-amp. The negative power supply was connected to the number 4 pin of the op-amp. It is very important that the circuit be wired correctly. We had our instructor verify the circuit was wired correctly. The ac power supply was set to approximately 10V and connected to the R1 resister. Secondly, we adjust the signal generator to 19kHz. Once the circuit was fully connected and working properly, we verified at which frequency the bandwidth picks up and drops out. In order to know that the circuit is working properly, the output voltage should read approximately 6 to 8 volts. The voltage (sine wave) should read the entire oscilloscope screen, using the *1 setting on the oscilloscope prongs.

VI. Conclusion:

The pre-engineering team built the band-pass filter shown in figure 1 on page 2. There were some wiring issues at the beginning of the lab, but with a little trial and error the team was able to receive an output that somewhat resembled the characteristics of a narrow width band-pass filter. The data sheet which showed the gain to frequency ratio was very similar to the gain to frequency ratio the team received in the data they found using the 741 op amp. Figure 3 is not as steep of slope or cut-off point as figure. Figure 3 shows output voltage compared to frequency and Figure 2 compares gain and frequency. Keeping in mind Figure 2 is the ideal graph, the op-amp used has certain percent of error which does make some calculations differences and errors.

The resonant frequency is where the circuit worked the best. A good example of this is in figure 4. The resonant frequency was approximately 7.8khz. This frequency was half of the expected resonant frequency. The resonant frequency was intendted to be 19kHz. The reason the frequency was half of the intended frequency is because we could not adjust the input voltage any higher to the ac amplitude. Furthermore the oscilloscope would not adjustable to read the exact output voltage causing there to be problems in calculating the correct voltage gain. However the results were not as exactly as intense pated, but the engineering team was able to get a response with the lab equipment offered.

Jr. Frenzel, Louis. Principles of Electronic Communications: Chapter 2. New York: Culverwell, 2008. Print

Sabin, William E. Narrow Band-Pass Filters for HF: Band-pass filters can be critical components in competitive stations. This setup may help put your station on the map. WYOMING:QEX, 2000. print .

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