1.
NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY
Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite
EXPERIMENT # 1
Passive LowPass and HighPass Filter
Balane, Maycen June 28, 2011
Signal Spectra and Signal Processing/ BSECE 41A1 Score:
Eng’r. Grace Ramones
Instructor
2.
OBJECTIVES
1. Plot the gain frequency response of a firstorder (onepole) RC lowpass filter.
2. Determine the cutoff frequency and rolloff of an RC firstorder (onepole)
lowpass filter.
3. Plot the phasefrequency of a firstorder (onepole) lowpass filter.
4. Determine how the value of R and C affects the cutoff frequency of an RC low
pass filter.
5. Plot the gainfrequency response of a firstorder (onepole) RC high pass
filter.
6. Determine the cutoff frequency and rolloff of a firstorder (onepole) RC
high pass filter.
7. Plot the phasefrequency response of a firstorder (onepole) highpass filter.
8. Determine how the value of R and C affects the cutoff frequency of an RC
high pass filter.
3.
COMPUTATION
(Step 4)
−0.01 dB = 20log A
−0.01 dB
20
=
20log A
20
−0.05 × 10−3
= logA
10−0.05×10−3
= 10log A
10−0.05×10−3
= A
A = 0.99988 ≅ 1
(Step 6)
𝑓𝐶 =
1
2𝜋𝑅𝐶
=
1
2𝜋(0.02µ𝐹)(1𝑘Ω)
= 7.957747𝑘𝐻𝑧 ≅ 7.958𝑘𝐻𝑧
(Question – Step 6)
%𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 = 
(7.958𝑘𝐻𝑧 − 7.935𝐾𝐻𝑧
7.958𝑘𝐻𝑧

× 100% = 0.29%
Question – (Step 7 )
(−2.998dB) –(20.108 dB) = 17.11dB
(Step 15)
0 dB = 20log A
0 dB
20
=
20log A
20
0 = logA
100
= 10log A
100
= A
A = 1
(Step 17)
𝑓𝐶 =
1
2𝜋𝑅𝐶
=
1
2𝜋(0.02µ𝐹)(1𝑘Ω)
= 7.957747𝑘𝐻𝑧 ≅ 7.958𝑘𝐻𝑧
Question (Step 17)
%𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 = 
(7.958𝑘𝐻𝑧 − 7.935𝐾𝐻𝑧
7.958𝑘𝐻𝑧

× 100% = 0.29%
Question (Step 18 )
20.159 dB− (−2.998dB) = 18.161dB
4.
DATA SHEET
MATERIALS
One function generator
One dualtrace oscilloscope
Capacitors: 0.02 µF, 0.04µF
Resistors: 1 kΩ, 2 kΩ
THEORY
In electronic communication systems, it is often necessary to separate a specific
range of frequencies from the total frequency spectrum. This is normally
accomplished with filters. A filter is a circuit that passes a specific range of
frequencies while rejecting other frequencies. A passive filter consists of passive
circuit elements, such as capacitors, inductors, and resistors. There are four basic
types of filters, lowpass, highpass, bandpass, and bandstop. A lowpass filter is
designed to pass all frequencies below the cutoff frequency and reject all frequencies
above the cutoff frequency. A highpass is designed to pass all frequencies above the
cutoff frequency and reject all frequencies below the cutoff frequency. A bandpass
filter passes all frequencies within a band of frequencies and rejects all other
frequencies outside the band. A bandstop filter rejects all frequencies within a band
of frequencies and passes all other frequencies outside the band. A bandstop filter
rejects all frequencies within a band of frequencies and passes all other frequencies
outside the band. A bandstop filter is often is often referred to as a notch filter. In
this experiment, you will study lowpass and highpass filters.
The most common way to describe the frequency response characteristics of a filter
is to plot the filter voltage gain (Vo/Vi) in dB as a function of frequency (f). The
frequency at which the output power gain drops to 50% of the maximum value is called
the cutoff frequency (fC). When the output power gain drops to 50%, the voltage gain
drops 3 dB (0.707 of the maximum value). When the filter dB voltage gain is plotted
as a function of frequency on a semi log graph using straight lines to approximate the
actual frequency response, it is called a Bode plot. A bode plot is an ideal plot of filter
frequency response because it assumes that the voltage gain remains constant in the
5.
passband until the cutoff frequency is reached, and then drops in a straight line. The
filter network voltage in dB is calculated from the actual voltage gain (A) using the
equation
AdB = 20 log A
where A = Vo/Vi
A lowpass RC filter is shown in Figure 11. At frequencies well below the cutoff
frequency, the capacitive reactance of capacitor C is much higher than the resistance
of resistor R, causing the output voltage to be practically equal to the input voltage
(A=1) and constant with the variations in frequency. At frequencies well above the
cutoff frequency, the capacitive reactance of capacitor C is much lower than the res
istance of resistor R and decreases with an increase in frequency, causing the output
voltage to decrease 20 dB per decade increase in frequency. At the cutoff frequency,
the capacitive reactance of capacitor C is equal to the resistance of resistor R,
causing the output voltage to be 0.707 times the input voltage (3dB). The expected
cutoff frequency (fC) of the lowpass filter in Figure 11, based on the circuit
component value, can be calculated from
XC = R
1
2𝜋𝑓𝐶 𝐶
= 𝑅
Solving for fC produces the equation
1
2𝜋𝑅𝐶
= 𝑓𝐶
A highpass RC filter is shown in figure 12. At frequencies well above the cutoff
frequency, the capacitive reactance of capacitor C is much lower than the resistance
of resistor R causing the output voltage to be practically equal to the input voltage
(A=1) and constant with the variations in frequency. At frequencies well below the cut
off frequency, the capacitive reactance of capacitor C is much higher than the
resistance of resistor R and increases with a decrease in frequency, causing the
output voltage to decrease 20 dB per decade decrease in frequency. At the cutoff
frequency, the capacitive reactance of capacitor C is equal to the resistance of
resistor R, causing the output voltage to be 0.707 times the input voltage (3dB). The
expected cutoff frequency (fC) of the highpass filter in Figure 12, based on the
circuit component value, can be calculated from
1
2𝜋𝑅𝐶
= 𝑓𝐶
6.
Fig 11 LowPass RC Filter
When the frequency at the input of a lowpass filter increases above the cutoff
frequency, the filter output drops at a constant rate. When the frequency at the
input of a highpass filter decreases below the cutoff frequency, the filter output
voltage also drops at a constant rate. The constant drop in filter output voltage per
decade increase (x10), or decrease (÷10), in frequency is called rolloff. An ideal low
pass or highpass filter would have an instantaneous drop at the cutoff frequency
(fC), with full signal level on one side of the cutoff frequency and no signal level on the
other side of the cutoff frequency. Although the ideal is not achievable, actual filters
rolloff at 20dB/decade per pole (RC circuit). A onepole filter has one RC circuit
tuned to the cutoff frequency and rolls off at 20dB/decade. At twopole filter has
two RC circuits tuned to the same cutoff frequency and rolls off at 40dB/decade.
Each additional pole (RC circuit) will cause the filter to rolloff an additional 
20dB/decade. Therefore, an RC filter with more poles (RC circuits) more closely
approaches an ideal filter.
In a pole filter, as shown the Figure 11 and 12 the phase (θ) between the input and
the output will change by 90 degrees and over the frequency range and be 45 degrees
at the cutoff frequency. In a twopole filter, the phase (θ) will change by 180 degrees
over the frequency range and be 90 degrees at the cutoff frequency.
7.
Fig 12 HighPass RC Filter
PROCEDURE
LowPass Filter
Step 1 Open circuit file FIG 11. Make sure that the following Bode plotter
settings are selected: Magnitude, Vertical (Log, F=0 dB, I=–40dB), Horizontal (Log,
F=1 MHz, I=100 Hz)
Step 2 Run the simulation. Notice that the voltage gain in dB has been plotted
between the frequencies 200 Hz and 1 MHz by the Bode plotter. Sketch the curve
plot in the space provided.
f
AdB
8.
Question: Is the frequency response curve that of a lowpass filter? Explain why.
=Yes it is the frequency response curve that of a lowpass filter, Lowpass blocks the
frequencies above the cutoff frequency and allows the frequencies below cutoff
frequency.
Step 3 Move the cursor to a flat part of the curve at a frequency of
approximately 100 Hz. Record the voltage gain in dB on the curve plot.
AdB = 0.001 dB
Step 4 Calculate the actual voltage gain (A) from the dB voltage gain (AdB)
A = 0.99988 ≅1
Question: Was the voltage gain on the flat part of the frequency response curve what
you expected for the circuit in Fig 11? Explain why.
= Yes, the voltage gain on the flat part of the frequency response curve what you
expected for the circuit in Fig 11 , because at below cutoff frequency, the VI is
approximately equal to Vo making the voltage gain approximately equal to 1.
Step 5 Move the cursor as close as possible to a point on the curve that is 3dB
down from the dB at 100 Hz. Record the frequency (cutoff frequency, fC) on the
curve plot.
fC = 7.935 kHz
Step 6 Calculate the expected cutoff frequency (fC) based on the circuit
component values in Figure 11.
fC = 7.958 kHz
Question: How did the calculated value for the cutoff frequency compare with the
measured value recorded on the curve plot?
= the calculated value for the cutoff frequency compare with the measured value
recorded on the curve plot is almost equal and has 0.29% difference.
9.
Step 7 Move the cursor to a point on the curve that is as close as possible to ten
times fC. Record the dB gain and frequency (f2) on the curve plot.
AdB = 20.108 dB
Question: How much did the dB gain decrease for a one decade increase (x10) in
frequency? Was it what you expected for a singlepole (single RC) lowpass filter?
= The dB gain decrease for a one decade increase (x10) in frequency is 17.11 dB
decrease per decade increase in frequency.
It is expected for a singlepole (single RC) lowpass filter because above frequency
the output voltage decreases 20dB/decade increase in frequency; 17.11 dB is almost
20 dB per decade.
Step 8 Click “Phase” on the Bode plotter to plot the phase curve. Make sure that
the vertical axis initial value (1) is 90 and the final value (F) is 0. Run the simulation
again. You are looking at the phase difference (θ) between the filter input and output
as a function of frequency (f). Sketch the curve plot in the space provided.
Step 9 Move the cursor to approximately 100 Hz and 1 MHz and record the
phase (θ) in degrees on the curve plot for each frequency (f). Next, move the cursor
f
θ
10.
as close as possible on the curve to the cutoff frequency (fC) and phase (θ) on the
curve plot.
@100 Hz: θ = –0.72o
@1MHz: θ = –89.544o
@fC: θ = –44.917o
Question: Was the phase at the cutoff frequency what you expected for a singles
pole (single RC) lowpass filter? Did the phase change with frequency? Is this
expected for an RC lowpass filter?
= The phase at the cutoff frequency is expected for a singlespole (single RC) low
pass filter. The phase changes between the input and output. It is expected for RC
lowpass filter because the input and the output change 88.824 degrees or 90 degrees
on the frequency range and 44.917 degrees or 45 degrees.
Step 10 Change the value of resistor R to 2 kΩ in Fig 11. Click “Magnitude” on
the Bode plotter. Run the simulation. Measure the cutoff frequency (fC) and record
your answer.
fC = 4.049 kHz
Question: Did the cutoff frequency changes? Did the dB per decade rolloff changes?
Explain.
= The cutoff changes, it decreases as the resistance increases. The dB per decade
rolloff did not change. The single pole’s rolloff will always approach 20 dB per
decade in the limit of high frequency even if the resistance changes.
Step 11 Change the value of capacitor C is 0.04 µF in Figure 11. Run the
simulation. Measure the new cutoff frequency (fC) and record your answer.
fC = 4.049 kHz
11.
Question: Did the cutoff frequency change? Did the dB per decade rolloff change?
Explain.
= The cutoff changes, it decreases as the capacitance increases. On the other hand
the dB per decade rolloff did not change. The single pole’s rolloff will always
approach 20 dB per decade in the limit of high frequency even if the capacitance
changes.
HighPass Filter
Step 12 Open circuit file FIG 12. Make sure that the following Bode plotter
settings are selected: Magnitude, Vertical (Log, F=0 dB, I=–40dB), Horizontal (Log,
F=1 MHz, I=100 Hz)
Step 13 Run the simulation. Notice that the gain in dB has been plotted between
the frequencies of 100Hz and 1 MHz by the Bode plotter. Sketch the curve plot in the
space provided.
Question: Is the frequency response curve that of a highpass filter? Explain why.
= It is the frequency response curve that of a highpass filter. This filter passes the
high frequency and blocks the low frequency depending on the cutoff frequency.
f
AdB
12.
Step 14 Move the cursor to a flat part of the curve at a frequency of
approximately 1 MHz Record the voltage gain in dB on the curve plot.
AdB = 0 dB
Step 15 Calculate the actual voltage gain (A) from the dB voltage gain (AdB).
A = 1
Question: Was the voltage gain on the flat part of the frequency response curve what
you expected for the circuit in Figure 12? Explain why.
= Yes, the voltage gain on the flat part of the frequency response curve is expected
for the circuit in Figure 12 voltage gain on the flat part of the frequency response
curve what I expected frequencies well above the cutoff frequency VO equal to Vi so
the voltage gain A equals 1
Step 16 Move the cursor as close as possible to the point on the curve that is 3dB
down from the dB gain at 1MHz. Record the frequency (cutoff frequency, fC) on the
curve plot.
fC = 7.935 kHz
Step 17 Calculate the expected cut of frequency (fC) based on the circuit
component value in Figure 12
fC = 7.958 kHz
Question: How did the calculated value of the cutoff frequency compare with the
measured value recorded on the curve plot?
= The calculated value of the cutoff frequency compare with the measured value
recorded on the curve plot is almost equal with 0.29% difference.
Step 18 Move the cursor to a point on the curve that is as close as possible to
onetenth fC. Record the dB gain and frequency (f2) on the curve plot.
AdB = 20.159 dB
Question: How much did the dB gain decrease for a onedecade decrease (÷) in
frequency? Was it what you expected for a singlepole (single RC) highpass filter?
13.
= The dB gain decreases 18.161 dB per decrease in frequency. It is expected for a
singlepole (single RC) highpass filter because the frequencies below the cutoff
frequency have output voltage almost decrease 20dB/decade decrease in frequency.
Step 19 Click “Phase” on the Bode plotter to plot the phase curve. Make sure that
the vertical axis initial value (I) is 0o
and the final value (f) is 90o
. Run the simulation
again. You are looking at the phase difference (θ) between the filter input and output
as a function of frequency (f). Sketch the curve plot in the space provided
Step 20 Move the cursor to approximately 100 Hz and 1 MHz and record the
phase (θ) in degrees on the curve plot for each frequency (f). Next, move the cursor
as close as possible on the curve to the cutoff frequency (fC). Record the frequency
(fC) and phase (θ).
@ 100 Hz: θ = 89.28
@ 1MHz: θ = 0.456o
@ fC(7.935 kHz): θ = 44.738o
Question: Was the phase at the cutoff frequency (fC) what you expected for a single
pole (single RC) high pass filter?
= Yes, the phase at the cutoff frequency (fC) what I expected for a singlepole (single
RC) high pass filter the input and the output change 89.824 degrees almost 90
degrees on the frequency range and 44.738 degrees almost degrees.
f
θ
14.
Did the phase change with frequency? Is this expected for an RC high pass filter?
= Yes the phase between the input and output changes. It is expected in RC high
pass filter.
Step 21 Change the value of resistor R to 2 kΩ in Figure 12. Click “Magnitude”
on the Bode plotter. Run the simulation. Measure the cutoff frequency (fC) and record
your answer.
fC = 4.049 kHz
Question: Did the cutoff frequency change? Did the dB per decade rolloff change?
Explain.
= The cutoff changes it decreases as the resistance increases. The dB per decade
rolloff did not change. The single pole’s rolloff will always approach 20 dB per
decade in the limit of low frequency although if the resistance changes.
Step 22 Change the value of the capacitor C to 0.04µF in Figure 12. Run the
simulation/ measure the cutoff frequency (fC) and record you answer.
fC = 4.049 kHz
Question: Did the cutoff frequency change? Did the dB per decade rolloff change?
Explain.
= The cutoff changes it decreases as the resistance increases. The dB per decade
rolloff did not change. The single pole’s rolloff will always approach 20 dB per
decade in the limit of low frequency although if the resistance changes.
15.
CONCLUSION
I therefore conclude that: a simple passive Low Pass Filter and High Pass Filter, can
be easily made by connecting together in series a single resistor with a single
capacitor.
For LowPass Filter:
Low pass filter pass the signals with low frequency and attenuates the signals
with high frequency.
The voltage gain at well below the cutoff frequency is approximately equal to 1;
Vo = Vi.
Frequencies at well above cutoff the dB per decade rolloff decreases by 20 dB
per decade increase in frequency
The phase of lowpass between the input and the output changes 90 degrees
over the frequency range and 45 degrees at the cutoff frequency.
If the resistance or capacitance changes, the cutoff frequency also changes.
Cutoff is inversely proportional to the resistance and capacitance.
Rolloff not remain constant even the resistance and capacitance changes.
For HighPass Filter
A high pass filter pass high frequency signals and attenuates low frequency
signals..
Voltage gain at well above the cutoff frequency is 1
HighPass Filter frequencies below fC the dB per decade rolloff decreases by
20 dB per decade decrease in frequency.
In addition, the phase of highpass between the input and the output changes
90 degrees over the frequency range and 45 degrees at the cutoff frequency.
If the resistance or capacitance changes, the cutoff frequency also changes.
Cutoff is inversely proportional to the resistance and capacitance.
Rolloff remain constant even the resistance and capacitance changes.
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