3.2 Measuring Frequency Response Of Low-Pass Filter :

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5 2.5 Filter Band-Width : In ideal Band-Pass Filters, the band-width is the frequency range in Hz where the magnitude response is at is maximum (or the attenuation is at its minimum) and constant and equal to : BW = f u f 1 In real band-pass filters, the lower and upper cut-off frequencies are defined as the frequencies where the power at the output falls to half its value at the pass-band range or where the voltage is at 1/ 2 of its value at the pass-band range. 3. Configuration and Measurements : 3.1 Equipment required Carrier Frequency Transmitter (this board contains the low-pass and the high-pass filters) Oscilloscope Function Generator Frequency Counter Storage Oscilloscope (for the supplementary experiment to measure the bandwidth of the band-pass filter) 3.2 Measuring Frequency Response Of Low-Pass Filter : Connect the sinusoidal output of the function generator to the input of the low-pass filters and adjust its level to 1 V peak (2 Vpp). Do not connect to the spectrum analyzer yet) Connect the output of function generator to the input of Low-Pass Filter as shown in diagram Connect the output of Low-Pass Filter to the oscilloscope Obtain the frequency response point-by-point by measuring the output amplitude V o of the filter as a function of the frequency f Enter your measured values in the following table. Proceed in frequency intervals of 300 Hz up to about 6 khz.

6 N F khz V o G LP g LP db With G LP = Gain of Low-Pass Filter = V o / Input g LP = Logarithmic gain of Low-Pass filter = log 10 G LP

7 3.3 Measuring Frequency response of band-pass filter : Repeat steps and after replacing the low pass-filter by the band-pass filter. Adjust the frequency of function generator to start from 9 khz. Proceed in increasing steps of 0.5 khz up to 18 khz. 3.4 Graphical measurement of the band-width with the aid of Spectrum Analyzer In addition to spectrum representation of harmonics and discrete signals, spectrum analyzers (refer to diagram II) can be used to represent continuous frequency response, such as that of filters, as follows : a. Connect the input of the band-pass filter to the VCO output of the spectrum analyzer. Refer to diagram II. b. Connect the output of the filter to the input of the spectrum analyzer. c. Connect the X outlet of the spectrum analyzer to the X input of the storage oscilloscope and the output of the spectrum analyzer to the y input of oscilloscope. d. Adjust the spectrum analyzer parameter as follows : B = 50 Hz F r = 20 khz t =10 s e. Span : approx. 8 khz to 20 khz ; adjust using spectrum analyzer while looking the frequency to the function generator. f Adjust vertical and horizontal button of oscilloscope to get a nice dot. g Press auto store of oscilloscope and press run of spectrum analyzer. After displaying a stored graph of the frequency response of the band-pass filter on storage oscilloscope obtain the band-width of the filter (use the cursor line to determine the lower and upper cut-off frequencies, 3 db drop)

8 4. Report 4.1 Graph the magnitude frequency response for the low-pass filter. 4.2 Graph the logarithmic magnitude frequency response (Bode diagram) of the low-pass filter (both axis, i.e gain and frequency in logarithmic scale) 4.3 Graph the amplitude frequency response for the band-pass filter. 4.4 Graph the logarithmic magnitude frequency response for the band-pass filter. 4.5 Determine the 3 db cut-off frequencies of the band-pass filter and obtain from them the band-width. Compare this with the one obtained under What is the expected voltage output if the gain falls by 6 db, 12 db and 20 db.

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