Lab 3: Low Pass and High Pass Filters

Purpose
The purpose of this lab is to introduce you to Low Pass and High Pass Filters. As part of
performing this lab you will
• Determine the amplitude and frequency response characteristics of Low and High
Pass filters,
• Compare passive and active LPF and HPF filter configurations, and
• Compare theoretical models with actual filters.

Introduction
In this experiment you will design, build1, and test four filters. The configurations you will build
are
• Passive Low-Pass Filter,
• Active Low-Pass Filter,
• Passive High-Pass Filter, and
• Active High-Pass Filter.
For each of the configurations you will
1. Design the filter for a specified cut-off frequency,
2. Model the filter in MatLab,
3. Simulate the design with PSpice2, and
4. Test the design in the Lab.
Steps 1, 2, and 3 are Prelab activities. Step 4 will be performed during the lab period.
In your report you will compare the theoretical and actual amplitude and phase plots for each of
the four configurations.
If you are rusty on transfer functions or op amp usage you may want to review those subjects.

1
UTD will supply integrated circuits, the student is expected to purchase resistors and capacitors. See
Introduction to Communications System Labs, page 4.
2
PSpice is available on the Communications Lab PCs. A free demo version may be obtained from MicroSim by
calling (800) 245-3022. A limited number of copies are available for check out from Steve Gibbs, EC 4.216.

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Prelab 3. LPF and HPF

Prelab 3.1 Introduction to Filtering.

1. Draw graphs showing the frequency responses of an ideal low-pass filter (LPF) and an
ideal high-pass (HPF) filter.
2. Generate a signal in MatLab composed of two sine waves using the following
parameters:
a) f1 = 2000 Hz
b) f2 = 40000 Hz
c) 0 ≤ t ≤ 2 milliseconds
d) Ts = 5 microseconds
e) y(t) = sin ( 2 π f1 t ) + 0.25 cos ( 2 π f2 t )
3. Plot the time domain and the frequency domain representations of the signal.
4. "Filter" the wave form by "zeroing out" the high frequency components of the FFT wave
form:
There are two ways in MatLab to "filter" a wave form. One technique is to "zero out" the
higher frequency elements of the FFT. The other is to use the transfer function of a low
pass filter. In the steps below you will zero out the high frequency elements of the FFT.
a) % create a new vector containing only the low
b) % frequency components of Y
c) L = length(Y);
d) NewY = zeros(L,1);
e) % define the number of data points to copy from the original file
f) NewY(1:b)=Y(1:b);
As an option, instead of replacing the values of higher frequency region with zeros, it is also
feasible to replace the original values with very small values, such as 0.001, which
represent out of band noise.
Although at this point you are not actually filtering the signal, you are performing the same kind
of function that a Low Pass Filter performs. The performance of this filtering method
depends on the number of vector elements you replace with zeros in the high frequency
components of the FFT.
5. After filtering in the frequency domain, reconstruct the signal using "ifft".
In your report, compare the reconstructed signal with the original signal in the time domain.
6. Repeat this procedure for the lower frequency region of the FFT and observe the output.
This method introduces the concept of filtering. This filtering is considered ideal filtering in
terms of the performance of the filtering. However this filtering requires additional
procedure for reconstruction of the signal. In the ‘real world’ the goal is to design filters
that perform as well as this simulation.

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7. Use Convolution to demonstrate filtering. (This will use the convolution technique you
used in Lab 2.)
a) Determine the impulse response of the filter.
b) Convolve the impulse response of the filter with the signal that you are going to
filter.
Pre3_13.m simulates LPF filtering using this filtering method in MatLab. In your Lab
Report, comment on the performance of the filter.

Prelab 3.2 MicroSim Simulation

1. Build the following passive low-pass filter in MicroSim and determine its transfer function
and cut-off frequency:

2. Build the following passive high-pass filter in MicroSim and determine its transfer function
and cut-off frequency:

Figure 2. RC High Pass Filter

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3. Build the following active low-pass filter in MicroSim and determine its transfer function and
cut-off frequency:

Figure 3. Sallen-Key Equal Component Active Low Pass Filter

4. Build the following active high-pass filter in MicroSim and determine its transfer function
and cut-off frequency:

Figure 4. Sallen-Key Equal Component Active High Pass Filter

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Lab Period – Low Pass and High Pass Filters

Parts required for this lab:

Resistors:
Determined by circuit
Capacitors:
Determined by circuit
Integrated circuits:
2 – Op-amp uA741

Lab 3.1 Frequency response of a RC low pass filter

In this section you will determine the frequency response of an RC Low Pass Filter. Also, you
will verify the filtering process by applying the sum of two sinusoids at the input and
measuring the attenuation caused by the filter on the amplitudes of the two signals.
Hint: If you decide to print the graphs from LabVIEW during the lab period, you may want to
zoom the display to improve the resolution of the plots before you print them.
1. Build the circuit shown in Figure 1. Manually determine its cut-off frequency. Recall that
voltage at the cut-off frequency is 0.707 (- 3 dB) times the input voltage to the filter.
Connect channel 2 of the oscilloscope to the input and channel 1 to the output.
2. Use RESPONSE.VI to obtain the frequency response of this filter. Enter the parameters for
the sweep. Use Lowfreq = 100 Hz, Upperfreq = 100000 Hz, Amplitude = 10 V,
points/decade = 10. Note the GPIB address of the signal generator that you are using.
Select an appropriate filename to save the data in the "Data file" box. Remember to use a
different filename each time that you run the VI.
3. Notice that the frequency axis in the graph has a logarithmic scale. Report the cut-off
frequency by finding the -3 dB point (i.e. the point which is 3 dB below the maximum
amplitude). Also, find the amplitudes at f1 = 10 kHz and f2 = 100 kHz.
Cut-off frequency (fc) =
Amplitude at f1 (m1) =
Amplitude at f2 (m2) =
4. From the last two coordinates, estimate the slope of the magnitude response. Determine the
phase shift at the cut-off frequency. Derive a function to determine the phase shift of this
circuit and calculate the phase shift at the theoretical cut-off frequency. Compare it with the
measured value.
Slope (dB/decade) =
Phase shift at cut-off frequency =
Theoretical =
Measured =

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Hint: To make the measurements easier, zoom in on the graph by changing the lower and upper
values shown for each axis. Select by clicking on the parameter to be changed and enter the
new value.
1. Disconnect the signal generator from the circuit. Connect the ground clips of the two signal
generators to the circuit ground, and the red test leads to the circuit's input. Set both signal
generators to produce 10 VPP sine waves. Set one of the function generators to produce a
10 kHz sine wave, and the other function generator to 100 kHz. Connect channel 1 of the
oscilloscope to the same point as the red clips. Connect oscilloscope channel 2 to ground.

Red Test Lead

Red Test Lead

Function
Generator 1

Function Black Test Lead

Generator 2
Black Test Lead
Figure 5. Function Generator Connection for LPF and HPF Filters

2. Using WAVEFFT.VI, obtain the frequency spectrum of the input signal. Enter ts = 8E-3 as
the time span in the control panel of the VI. Measure the amplitude of the main peaks of the
spectrum.
Amplitude at f1 (pi1) =
Amplitude at f2 (pi2) =
3. Apply the signal to the circuit by connecting the leads as shown in Figure 5. Move the
channel 1 probe to the output of the RC Low Pass Filter. Run WAVEFFT.VI using the
same time span, remember to save the data with a different filename. Measure the amplitude
of the main peaks in the displayed spectrum.
Amplitude at f1 (po1) =
Amplitude at f2 (po2) =
4. What is the relationship between m1, pi1 and po1? Verify your conclusion with m2, pi2 and
po2.

Lab 3.2 Frequency response of an RC High Pass Filter

1. Construct an RC High Pass Filter as shown in Figure 2.
2. Repeat the procedure described in LAB 3.1 to analyze the frequency response of the RC
High Pass Filter. Use f1=100 Hz, f2 =1000 Hz, and ts = 0.5 (i.e. 500E-3).

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Lab 3.3 Frequency response of an active low-pass active
1. Construct the active Low Pass Filter shown in Figure 3 using the following parameters:.
Equipment Settings:
VCC 18 V (power supply voltages)
VSS -18 V
f1 3,000 Hz
f2 30,000 Hz
Amplitude 5 Vpp
LabVIEW Settings:
Lowfreq 100 Hz
Upperfreq 30 kHz
points/decade 10
ts 20E-3
2. Analyze the response of the filter by following the steps of Lab 3.1.

Lab 3.4 Frequency response of a high-pass active filter.

1. Construct the active High Pass Filter shown in Figure 4 using the following parameters:
Equipment Settings:
VCC 18 V
VSS -18 V
f1 50 Hz
f2 500 Hz
Amplitude 5 VPP
LabVIEW Settings:
Lowfreq 50 Hz
Upperfreq 50 kHz
points/decade 10
ts 20E-3.
2. Analyze the response of the filter by following the steps of Lab 3.1.
Do not disassemble the active filters, they will be used in the Lab 4.
Lab Report
During this Lab you modeled and built several different low pass and high pass filters. In your
report,
• Compare the frequency and phase responses of all the Low Pass Filters.
• Compare the frequency and phase responses of all the High Pass Filters.
• Explain any discrepancies you noted between the models and the actual circuits.
• Note any deviations you made from the assigned procedure.
• The filters designed in this lab were limited to first or second order filters. Design a
4th order LPF with a cutoff frequency of 3500 Hz. With your design, specify the
transfer function and the formula to determine the cutoff frequency.

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