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- 1. NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Building, Aguinaldo Highway Dasmariñas City, Cavite Experiment No. 4 ACTIVE BAND-PASS AND BAND-STOP FILTERSCauan, Sarah Krystelle P. July 21, 2011Signal Spectra and Signal Processing/BSECE 41A1 Score: Engr. Grace Ramones Instructor
- 2. OBJECTIVES1. Plot the gain-frequency response curve and determine the center frequency for an active band-pass filter.2. Determine the quality factor (Q) and bandwidth of an active band-pass filter3. Plot the phase shift between the input and output for a two-pole active band-pass filter.4. Plot the gain-frequency response curve and determine the center frequency for an active band-stop (notch) filter.5. Determine the quality factor (Q) and bandwidth of an active notch filter.
- 3. SAMPLE COMPUTATIONStep 3 (Actual Voltage Gain)Step 4 (Expected voltage gain)Step 4 Question (Percentage Difference)Step 7 (Expected Center frequency)Step 7 Question (Percentage Difference)Step 8 (Measured quality factor)Step 9 (Expected quality factor)
- 4. Step 9 Question (Percentage Difference)Step 14 (Measured Voltage Gain)Step 15 (Expected Voltage Gain)Step 18 (Expected center frequency)Step 18 Question (Percentage Difference)Step 19 (Measure Quality Factor)Step 20 (Expected Quality Factor)Step 20 Question (Percentage Difference)
- 5. DATA SHEETMATERIALSOne function generatorOne dual-trace oscilloscopeTwo LM741 op-ampsCapacitors: two 0.001 µF, two 0.05 µF, one 0.1 µFResistors: one 1 kΩ, two 10 kΩ, one 13 kΩ, one 27 kΩ, two 54 kΩ, and one 100kΩTHEORYIn electronic communications systems, it is often necessary to separate a specific range offrequencies from the total frequency spectrum. This is normally accomplished with filters. Afilter is a circuit that passes a specific range of frequencies while rejecting other frequencies.Active filters use active devices such as op-amps combined with passive elements. Active filtershave several advantages over passive filters. The passive elements provide frequency selectivityand the active devices provide voltage gain, high input impedance, and low output impedance.The voltage gain reduces attenuation of the signal by the filter, the high input impedanceprevents excessive loading of the source, and the low output impedance prevents the filter frombeing affected by the load. Active filters are also easy to adjust over a wide frequency rangewithout altering the desired response. The weakness of active filters is the upper-frequency limitdue to the limited open-loop bandwidth (funity) of op-amps. The filter cutoff frequency cannotexceed the unity-gain frequency (funity) of the op-amp. Therefore, active filters must be used inapplications where the unity-gain frequency (funity) of the op-amp is high enough so that it doesnot fall within the frequency range of the application. For this reason, active filters are mostlyused in low-frequency applications.A band-pass filter passes all frequencies lying within a band of frequencies and rejects all otherfrequencies outside the band. The low cut-off frequency (fC1) and the high-cutoff frequency (fC2)on the gain-frequency plot are the frequencies where the voltage gain has dropped by 3 dB(0.707) from the maximum dB gain. A band-stop filter rejects a band of frequencies and passesall other frequencies outside the band, and of then referred to as a band-reject or notch filter. Thelow-cutoff frequency (fC1) and high-cutoff frequency (fC2) on the gain frequency plot are thefrequencies where the voltage gain has dropped by 3 dB (0.707) from the passband dB gain.The bandwidth (BW) of a band-pass or band-stop filter is the difference between the high-cutofffrequency and the low-cutoff frequency. Therefore, BW = fC2 – fC1
- 6. The center frequency (fo) of the band-pass or a band-stop filter is the geometric mean of the low-cutoff frequency (fC1) and the high-cutoff frequency (fC2). Therefore,The quality factor (Q) of a band-pass or a band-stop filter is the ratio of the center frequency (fO)and the bandwidth (BW), and is an indication of the selectivity of the filter. Therefore,A higher value of Q means a narrower bandwidth and a more selective filter. A filter with a Qless than one is considered to be a wide-band filter and a filter with a Q greater than ten isconsidered to be a narrow-band filter.One way to implement a band-pass filter is to cascade a low-pass and a high-pass filter. As longas the cutoff frequencies are sufficiently separated, the low-pass filter cutoff frequency willdetermine the low-cutoff frequency of the band-pass filter and a high-pass filter cutoff frequencywill determine the high-cutoff frequency of the band-pass filter. Normally this arrangement isused for a wide-band filter (Q 1) because the cutoff frequencies need to be sufficient separated.A multiple-feedback active band-pass filter is shown in Figure 4-1. Components R1 and C1determine the low-cutoff frequency, and R2 and C2 determine the high-cutoff frequency. Thecenter frequency (fo) can be calculated from the component values using the equationWhere C = C1 = C2. The voltage gain (AV) at the center frequency is calculated fromand the quality factor (Q) is calculated from
- 7. Figure 4-1 Multiple-Feedback Band-Pass Filter XBP1 XFG1 IN OUT 10nF C1 100kΩ R2 741 3 Vo 6 Vin 1kΩ 2 10kΩ 10nF R1 RL C2Figure 4-2 shows a second-order (two-pole) Sallen-Key notch filter. The expected centerfrequency (fO) can be calculated fromAt this frequency (fo), the feedback signal returns with the correct amplitude and phase toattenuate the input. This causes the output to be attenuated at the center frequency.The notch filter in Figure 4-2 has a passband voltage gainand a quality factor
- 8. The voltage gain of a Sallen-Key notch filter must be less than 2 and the circuit Q must be lessthan 10 to avoid oscillation.Figure 4-2 Two pole Sallen-Key Notch Filter XBP1 XFG1 IN OUT 27kΩ 27kΩ R52 R/2 50nF 50nF 0.05µF 3 0.05µF C3 C Vin C C 6 2 741 Vo RL 54kΩ 54kΩ 10kΩ 54kΩ 54kΩ R3 R R R 0 R2 100nF 2C R1 10kΩ 13kΩ 0 0
- 9. PROCEDUREActive Band-Pass FilterStep 1 Open circuit file FIG 4-1. Make sure that the following Bode plotter settings are selected. Magnitude, Vertical (Log, F = 40 dB, I = 10 dB), Horizontal (Log, F = 10 kHz, I = 100 Hz)Step 2 Run the simulation. Notice that the voltage gain has been plotted between the frequencies of 100 Hz and 10 kHz. Draw the curve plot in the space provided. Next, move the cursor to the center of the curve. Measure the center frequency (fo) and the voltage gain in dB. Record the dB gain and center frequency (fo) on the curve plot. fo = 1.572 kHz AdB = 33.906 dB AdB 40dB 10 dB f 100 Hz 10 kHzQuestion: Is the frequency response curve that of a band-pass filters? Explain why. Yes, the frequency response is a band-pass filter. The filter only allows the frequencies lying within the band which is from 100.219 Hz to 10 kHz. Moreover, the frequency response shows the highest gain at the center frequency.Step 3 Based on the dB voltage gain at the center frequency, calculate the actual voltage gain (AV) AV = 49.58
- 10. Step 4 Based on the circuit component values, calculate the expected voltage gain (AV) at the center frequency (fo) AV = 50Question: How did the measured voltage gain at the center frequency compare with the voltage gain calculated from the circuit values? There is only a 0.84% difference between the measured and the calculated values of voltage gain. And also, the measured and calculated values have a difference of 0.42.Step 5 Move the cursor as close as possible to a point on the left of the curve that is 3 dB down from the dB gain at the center frequency (fo). Record the frequency (low- cutoff frequency, fC1) on the curve plot. Next, move the cursor as close as possible to a point on the right side of the curve that is 3 dB down from the center frequency (fo). Record the frequency (high-cutoff frequency, fC2) on the curve plot. fC1 = 1.415 kHz fC2 = 1.746 kHzStep 6 Based on the measured values of fC1 and fC2, calculate the bandwidth (BW) of the band-pass filter. BW = 0.331 kHzStep 7 Based on the circuit component values, calculate the expected center frequency (fo) fo = 1.592 kHzQuestion: How did the calculated value of the center frequency compare with the measured value? Their values are close. The percentage difference of the calculated value and the measured center frequency is 1.27%. There is a difference is 0.02.Step 8 Based on the measured center frequency (fo) and the bandwidth (BW), calculate the quality factor (Q) of the band-pass filter. Q = 4.75
- 11. Step 9 Based on the component values, calculate the expected quality factor (Q) of the band-pass filter. Q=5Question: How did your calculated value of Q based on the component values compare with the value of Q determined from the measured fo and BW? The two values are almost alike. The calculated and measured quality factor differs with only 0.25. It is 5.26% difference between the expected and the measured quality factor of the band-pass filter.Step 10 Click Phase on the Bode plotter to plot the phase curve. Change the vertical initial value (I) to -270o and the final value (F) to +270o. Run the simulation again. You are looking at the phase difference (θ) between the filter input and output wave shapes as a function of frequency (f). Draw the curve plot in the space provided. θ o 270 o -270 f 100 Hz 10 kHzStep 11 Move the cursor as close as possible to the curve center frequency (fo), recorded on the curve plot in Step 2. Record the frequency (fo) and the phase (θ) on the phase curve plot. fo = 1.572 kHz θ = 173.987oQuestion: What does this result tell you about the relationship between the filter output and input at the center frequency? The phase shows that the relationship between the filters output is 173.987o or almost 180o out of phase compared to input.
- 12. Active Band-Pass (Notch) FilterStep 12 Open circuit file FIG 4-2. Make sure that the following Bode plotter settings are selected. Magnitude, Vertical (Log, F = 10 dB, I = -20 dB), Horizontal (Log, F = 500 Hz, I = 2 Hz)Step 13 Run the simulation. Notice that the voltage gain has been plotted between the frequencies of 2 Hz and 500 Hz. Draw the curve plot in the space provided. Next, move the cursor to the center of the curve at its center point. Measure the center frequency (fo) and record it on the curve plot. Next, move the cursor to the flat part of the curve in the passband. Measure the voltage gain in dB and record the dB gain on the curve plot. fo = 58.649 Hz AdB AdB = 4. dB 10 dB -20 dB f (Hz) 2 Hz 500 HzQuestion: Is the frequency response curve that of a band-pass filters? Explain why. Yes, the frequency response is that of a band-stop filter. The filter only allows the frequencies outside the band and rejects all frequencies lying within the band which. And also, the center frequency is at the lowest voltage gain.Step 14 Based on the dB voltage gain at the center frequency, calculate the actual voltage gain (AV) AV = 1.77Step 15 Based on the circuit component values, calculate the expected voltage gain in the passband. AV = 1.77
- 13. Question: How did the measured voltage gain in the passband compare with the voltage gain calculated from the circuit values? They have the same values. The measured and expected voltage gain has no difference.Step 16 Move the cursor as close as possible to a point on the left of the curve that is 3 dB down from the dB gain in the bandpass Record the frequency (low-cutoff frequency, fC1) on the curve plot. Next, move the cursor as close as possible to a point on the right side of the curve that is 3 dB down from dB gain in the passband. Record the frequency (high-cutoff frequency, fC2) on the curve plot. fC1 = 46.743 Hz fC2 = 73.588 HzStep 17 Based on the measured values of fC1 and fC2, calculate the bandwidth (BW) of the notch filter. BW = 26.845 HzStep 18 Based on the circuit component values, calculate the expected center frequency (fo) fo = 58.95HzQuestion How did the calculated value of the center frequency compare with the measured value? There is a 0.51% difference between the expected and measured value of the center frequency. They are almost equal.Step 19 Based on the measured center frequency (fo) and bandwidth (BW), calculate the quality factor (Q) of the notch filter. Q = 2.18Step 20 Based on the calculated passband voltage gain (Av), calculate the expected quality factor (Q) of the notch filter. Q = 2.17Question: How did your calculated value of Q based on the passband voltage gain compare with the value of Q determined from the measured fo and BW? The values are almost equal. The difference between the calculated and measure quality factor is 0.01. The calculated only differs 0.46% compared to the measured value.
- 14. CONCLUSION After plotting the gain-frequency response curve of each filter, I conclude that theresponse of active filter is the same as the response of a passive filter. Active band pass filters aresimply filters constructed by using operational amplifiers as active devices combined withpassive elements. Still, active band-pass filter passes frequencies within a certain range andrejects frequencies outside that range. The active band-stop filter is its counterpart which passesthe frequencies outside the band and attenuates the frequencies lying within that band. Furthermore, the center frequency is the geometric mean of the low and high cutoff. Inotice that the center frequency of a band pass filter is the peak of the mountain like responsewhere it achieves its highest gain. On the other hand, the center frequency of a band-stop filter iswhere the filter achieves its lowest gain. The cutoff frequency is where the gain decreased by 3 dB. The bandwidth of the responsecurve is the difference between the high cutoff frequency and the low-cutoff frequency.Moreover, for a two-pole active band-pass filter, the output is 180o out of phase with its input. Lastly, the quality factor indicates the selectivity of the filter. It is inversely proportionalto the bandwidth. If Q is less than one it is considered to be wide-band filter and if Q is greaterthan ten it is considered as narrow-band filter. For Sallen-Key Notch Filter’s quality factorshould be less than 10.

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