Measurement of the Frequency Response

You will now measure the frequency response of the lowpass filter in Figure experimentally.

1. Obtain a 100 resistor and a 47 nF mylar capacitor from the parts bin. Measure (and record) the exact capacitance with the capacitance meter in the lab, as well as the resistance using the ohmmeter. After using the capacitance meter, always turn off its power or the battery will die.
2. Connect the function generator to the series combination of the resistor and capacitor. Reset the function generator (Turn it off. Wait a few seconds and then turn it on again.) so that it can be set manually. Set the amplitude to 1.0 volt peak-to-peak (on the front display panel). Set the frequency to the values given in Table and record the AC voltage measured across the capacitor using the multimeter set to measure AC voltage.
3. The maximum response should occur at DC (f=0 Hz) but cannot be measured with the function generator as the source because it is incapable of generating 0 Hz. However, the value of the response you measure at f = 10 Hz should be nearly identical to the DC response. Using the value of the response you measured at 10 Hz, adjust the frequency of the function generator until the response drops to times the response at 10 Hz. Record that frequency as the measured cutoff (half-power) frequency .

  
Table: Lowpass filter experimental frequency response

You will now measure the frequency response of the lowpass filter circuit using the frequency response VI in LabVIEW.

1. Set the High frequency to 10 MHz and Low frequency to 1 Hz.
2. Set the Number of Steps to 20.
3. Select Run in the Operate menu.
4. Observe the function generator stepping through different frequencies from 1 Hz to 10 MHz and the multimeter measuring the corresponding responses.
5. When the simulation is complete, record the value of the DC response. Divide this value by to determine the response at the half-power frequency.
6. Run the VI several times, each time narrowing the frequency range to focus on the frequencies near the cutoff frequency. Increase the number of steps to obtain smoother plots (This will slow down the VI, so stay below 50). Use the graph cursors on a smooth plot with a fairly narrow frequency range to determine .