EMEC 2434- Measurements and Instruments
Experiment: Low Pass Filter
Name: Ronit Kotwani
Lab Section: 2434L
Group: 9
Date of Experiment: 10/2/25
Date of Submission:17/2/25
Objective: A Low Pass Filter (LPF) is a circuit designed to retain low-frequency components
while eliminating all undesired high-frequency elements of an electrical signal. There are
two types of LPFs: active filters and passive filters.
Theory:
Passive Low Pass Filter:
Passive filters are constructed using passive components such as resistors, capacitors, and
inductors, and they do not include any amplifying elements like transistors or operational
amplifiers (Op Amps). As a result, they do not provide signal gain, and the output signal
magnitude is always lower than the input. A simple passive RC low pass filter can be easily
created by connecting a single resistor in series with a single capacitor, as shown in Fig. 3.1. In
this configuration, the input signal is applied across the series combination of the resistor and
capacitor, while the output signal is measured solely across the capacitor. This filter type is
commonly called a “first-order filter” or “one-pole filter” because it contains only one reactive
component, the capacitor, in the circuit. The low pass filter allows low-frequency components
ranging from 0 Hz to its cutoff frequency fc (in Hz) to pass through while attenuating frequencies
above fc .The cutoff frequency can be calculated using Equation (3.1):-
Figure 3.2 depicts the circuit diagram of a second-order low-pass filter. It is constructed by
connecting or "cascading" two passive first-order low-pass filters together, forming a secondorder or two-pole filter network. This demonstrates that a first-order low-pass filter can be
transformed into a second-order filter by simply adding an additional RC network. Consequently,
the more RC stages added, the higher the order of the filter becomes.
The cut-off frequency fc (Hz) for the second-order Low Pass Filter can be calculated by using
Equation (3.2):-
The Figure 3.3 displays the frequency response, indicating that all input signals are transmitted
directly to the output up to the cutoff frequency.
Active Low Pass Filters:
A first-order active low pass filter combines a passive RC filter with a non-inverting Op-Amp
configured as a voltage follower, providing unity DC gain (Av = +1). Unlike passive filters, this
design ensures minimal loading effects and stable cutoff frequency due to the Op Amp's high
input and low output impedance. However, its voltage gain is limited to unity, though it offers
high power gain. Figure 3.4 shows the circuit of a first-order active LPF.
There are numerous ways to design second-order active low-pass filters. Figure 3.5 provides an
example of such a circuit, utilizing a single Op-Amp chip.
How to Use a Breadboard:Breadboards contain small "holes" where component leads can be inserted. The top and bottom
rows (highlighted in blue) are typically used for connecting the (+) and (-) power supply
terminals. These rows run horizontally across the breadboard, while the holes for components are
arranged vertically. Each horizontal or vertical set of connected holes forms a single node. A
node is a point in a circuit where two or more components are connected. To create connections
between components, their leads are inserted into a common node. On the breadboard, a node
consists of a row of holes linked by a metal strip underneath. The long top and bottom rows are
generally reserved for power supply connections. Figure 3.6 demonstrates the basic breadboard
layout, while Figure 3.7 shows an example of a microcontroller connected to a breadboard.
Procedure:➢ Circuit Implementation: Assemble the four specified circuits on the breadboard. Ensure
proper connections and placement of components.
➢ Op Amp Configuration: Carefully connect the Op Amp (MCP 601) as per the pin
diagram provided in Figure 3.8. Pay special attention to the single-supply voltage range
of 2.7V to 6.0V for CMOS Op Amps.
➢ Resistor Value Identification: Use the resistor color code chart in Figure 3.9 to
determine the resistor values. The first band represents the 1st digit, the second band
represents the 2nd digit, and the third band indicates the multiplier.
➢ Frequency Adjustment: Adjust the frequency on the function generator to identify the
cutoff frequency range for the circuit
➢ Cutoff Frequency Selection: From the determined range, select a specific value to use
as the cutoff frequency.
➢ Capacitor Calculation: Calculate the required capacitor value using Equations (3.1) and
(3.2) based on the selected cutoff frequency.
Results: Data:-
Circuit 1:Table-1-
Graph-1:
Output Voltage vs. Frequency : Circuit 1
120
100
Output voltage (mV)
80
60
40
20
0
0
1000
2000
3000
4000
5000
6000
Frequency (Hz)
7000
8000
9000
10000
Circuit 2- Table 2:-
Graph 2:-
Output voltage vs. Frequency: Circuit 2
140
120
Output Voltage (mV)
100
80
60
40
20
0
0
2000
4000
6000
Frequency (Hz)
8000
10000
12000
Circuit 3- Table 3:-
Graph 3-
Output Voltage vs. Frequency: Circuit 3
140
120
Output Voltage(mV)
100
80
60
40
20
0
0
2000
4000
6000
Frequency (Hz)
8000
10000
12000
Circuit 4- Table 4:-
Output Voltage vs. Frequency : Circuit 4
140
120
Output voltage (mV)
100
80
60
40
20
0
0
2000
4000
6000
Frequency (Hz)
8000
10000
12000
Q1) What is the purpose of filtering a signal?
The purpose of filtering a signal is to remove unwanted frequency components while preserving
the desired frequencies. A low-pass filter (LPF) specifically allows low-frequency signals to pass
while attenuating higher-frequency signals, which is useful for noise reduction, signal
conditioning, and preventing aliasing in signal processing applications.
Q2) What is the difference between an active filter and a passive filter?
1. A passive filter consists of only passive components such as resistors, capacitors, and
inductors. It does not require an external power source and has no signal gain, meaning
the output signal's magnitude is always lower than the input.
2. An active filter includes active components such as operational amplifiers (Op-Amps) in
addition to passive components. It requires an external power source and can provide
signal amplification, resulting in a higher output voltage than passive filters while
maintaining stability and preventing signal loss.
Q3) Determine the cutoff frequency of these passive and active LPFs.
➢ The cutoff frequency fc for a first-order passive LPF is given by:
…….. Eqn 3.1
➢ For a second-order passive LPF, the cutoff frequency is:
……. Eqn 3.2
Similarly, for active LPFs, the cutoff frequency can be calculated using the same equations. The
actual cutoff frequency. Even though the cutoff frequency values for first-order and secondorder filters appear the same in this case, the second-order filter provides better filtering
performance by more aggressively attenuating high frequencies. For Active 1st and Second
order the answer calculated was the same in this particular case fc= 159.15 Hz.
Q4) By using those cutoff frequencies, determine the value of capacitors.
Similar to Q3 since the equations for 1st-order Active and Passive filters are the same and
even for 2nd-order the capacitor value will also be the same in this particular case.
Q5) Compare the performance between a first order LPF and a second order LPF.
Features
First-Order LPF
Second-Order LPF
Cutoff Frequency
Same for both (depends on RR
and CC)
Same as first-order but with
sharper transition
Attenuation Rate
-20 dB/decade
-40 dB/decade
Filtering
Effectiveness
Weaker filtering of high
frequencies
More effective at blocking high
frequencies
Phase Shift
Maximum -90°
Maximum -180°
Response
Sharpness
Gradual transition from passband
to stopband
Steeper transition
Circuit Complexity Simple (1 resistor + 1 capacitor)
More complex (2 resistors + 2
capacitors)
Noise Rejection
Higher noise
Moderate
1. First-Order LPF:
➢ Simpler to design and implement.
➢ Attenuates high-frequency signals, but not as sharply.
➢ Useful for basic filtering applications.
1. Second-Order LPF:
➢ Provides better attenuation of unwanted high frequencies.
➢ Has a steeper frequency response, meaning a more defined cutoff.
➢ More effective for applications needing strong noise suppression.
If better filtering performance is needed, a second-order LPF is preferred despite its
slightly increased complexity.
Q6) Comparison of Characteristics: Passive LPF vs. Active LPF
Feature
Passive Low-Pass Filter (LPF)
Active Low-Pass Filter (LPF)
Components
Used
Resistors (R) and Capacitors (C)
Resistors (R), Capacitors (C), and
an Op-Amp
Signal Gain
Always less than 1 (attenuates the
signal)
Can provide voltage gain
(depends on Op-Amp
configuration)
Power
Requirement
No external power needed
Requires external power for the
Op-Amp
Impedance
Output impedance depends on circuit
components
Low output impedance (due to
Op-Amp buffer)
Cutoff Slope
-20 dB/decade for first-order, -40
dB/decade for second-order
Similar cutoff slopes, but with
better performance due to
amplification
Filtering
Effectiveness
Limited attenuation and buffering
capability
Better filtering due to
amplification and improved
impedance matching
Load
Dependence
Affected by connected loads (can
cause signal degradation)
Not affected by load changes due
to Op-Amp buffering
Complexity
Simple design, easy to implement
More complex due to active
components
Frequency
Range
Works well at high frequencies but
may not work efficiently at very low
frequencies
More effective at lower
frequencies due to Op-Amp
amplification
Cost
Lower cost due to fewer components
Higher cost due to the Op-Amp
and power supply requirement
If amplification and better filtering are needed, an active LPF is preferred. If simplicity and
cost-saving are priorities, a passive LPF is sufficient.
Sources of Errors:
1. Measurement Accuracy: Inaccurate readings from instruments.
2. Breadboard Connections: Loose or poor connections may distort the signal.
3. Op-Amp Issues: Incorrect operation due to insufficient power or malfunctioning
components.
4. Environmental Factors: Temperature changes can impact component behavior.
5. Human Error: Mistakes in component values, connections, or measurements.
Conclusion:
This experiment compared passive and active low-pass filters. Passive LPFs, made with resistors
and capacitors, are simple and cost-effective but provide no amplification and are affected by
load impedance. Active LPFs, using operational amplifiers, offer better performance with voltage
gain, improved filtering, and lower output impedance, though they require external power and
are more complex. While both filters have the same cutoff frequency, second-order LPFs provide
sharper attenuation and more effective noise suppression than first-order filters. Ultimately,
passive filters are suitable for simple applications, while active filters are preferred for better
performance and signal amplification all these technicalities were understood by the students in
this lab.
Reference:
All the information used in this lab report were taken from the Lab Manual of EMEC2434L of Lakehead University,
and
Alan S Morris. And Reza Langari (2015). Measurement and Instrumentation: Theory and
Application (3rd ed.). Elsevier.