OSCILLOSCOPE AND FUNCTION GENERATOR OPERATION
Objective:-
To become familiar with the operation and use of the oscilloscope and function generator
Equipment:-
Oscilloscope
Function Generator
DMM
Introduction:-
As we have already learnt the usage of oscilloscope and function generator in 1st semester, but here we
have to ensure our tight grip on these tools to get both the precision and accuracy.
Oscilloscope:-
The oscilloscope is the most widely used general-purpose signal measuring instrument because it allows
us to see a graph of the voltage as a function of time in a circuit. Many circuits have specific timing
requirements or phase relationships that can be measured with a two-channel oscilloscope. One can
measure almost anything with the two-dimensional graph drawn by an oscilloscope like the average
value, rms value, frequency and period of a sinusoidal or a non-sinusoidal signal. The screen is divided
into centimeter divisions in vertical and horizontal directions. Vertical sensitivity is set in volts/cm while
horizontal sensitivity is set in time (sec)/cm.
If a particular signal occupies 6 vertical divisions and vertical sensitivity is set to 5 mV/cm, the magnitude
of the signal can be determined from the following equation:
Signal voltage = voltage sensitivity (V/cm) x deflection (cm)
Vs = (5mV/s) x (6 cm) = 30 mV
If one cycle of the same signal occupies 8 cm on horizontal scale with a horizontal sensitivity of 5µs/cm,
the period and frequency of the signal can be determined by the following equations:
Period of waveform = horizontal sensitivity (s/cm) x deflection (cm)
T = (5 µs/cm) x (8cm) = 40 µs
F = 1/T = 1/40 µs = 25 kHz
The GOS-6112 is a 100MHz, two-channel, dual-sweep, portable oscilloscope for general purpose use. A
microprocessor-based operating system controls most of the functions of the instrument, including
cursor readout and digitized panel setting. On-screen alphanumeric readout and cursor function for
voltage, time, frequency and phase measurement provide extraordinary operational convenience.
The vertical deflection system has two input channels. Each channel has 11 basic deflection factors from
2mV to 5V per division. The horizontal deflection system provides single, dual or delayed sweeps from
0.5s to 50ns per division (delayed sweep, 50ms to 50ns per division). The trigger system provides stable
triggering over the full bandwidth of the vertical deflection system. The details of operation can be
explored by referring to the User Manual.
The instructor will provide a brief description of various sections of the oscilloscope and function
generator. In your own words, describe the function and use of the following.
Focus:
Intensity:
Vertical sensitivity:
Horizontal sensitivity:
Vertical mode selection:
AC-GND-DC switch:
Trigger section:
External trigger input:
Input resistance and capacitance of oscilloscope:
Probe:
Function Generator
Function Generator is a supply that typically provides a sinusoidal, square-wave and triangular
waveform for a range of frequencies and amplitudes. Although the frequency of the function generator
can be set by the dial position and appropriate multiplier, the oscilloscope can be used to precisely set
the output frequency. The scope can also be used to set the amplitude of the function generator.
Setup
Turn on the oscilloscope and adjust the necessary controls to establish a clear, bright, horizontal line
across the centre of the screen.
Connect the function generator to one vertical channel of the oscilloscope and set the output of the
generator to a 1000 Hz sinusoidal waveform.
Set the vertical sensitivity of the scope to 1 V/cm and adjust the amplitude control of the function
generator to establish a 4 V peak to peak (p-p) sinusoidal waveform on the screen.
Horizontal sensitivity
Set the horizontal sensitivity of the scope to 0.2 ms/cm. using the results of above part, calculate and
predict the number of horizontal divisions required to properly display one full cycle of the 1000 Hz
signal
Determine the period of 1000 Hz sinusoidal waveform in ms using the equation T=1/f. Show all your
calculations:
Calculated No. of divisions = _5 divisions__
Using the oscilloscope, measure the number of required divisions and insert below:
Measured No. of divisions = _5 divisions__
Change the horizontal sensitivity of the oscilloscope to 0.5 ms/cm without touching any of the controls
of the function generator. Using the results of part (d) above, how many horizontal divisions will now be
required to display one full cycle of the 1000 Hz signal?
Calculated No. of divisions = _2 divisions__
Using the oscilloscope, measure the number of required divisions and insert below:
Measured No. of divisions = _2 divisions__
Change the horizontal sensitivity of the oscilloscope to 1 ms/cm without touching any of the controls of
the function generator. Using the results of part (d) above, how many horizontal divisions will now be
required to display one full cycle of the 1000 Hz signal?
Calculated No. of divisions = _1 divisions__
Using the oscilloscope, measure the number of required divisions and insert below:
Measured No. of divisions = _1 divisions__
What was the effect on the appearance of the sinusoidal waveform as the horizontal sensitivity was
changed from 0.2 ms/cm to 0.5 ms/cm and finally to 1 ms/cm?
ANSWER:
Effect on the Appearance of the Sinusoidal Waveform When Horizontal Sensitivity is Changed:
When the horizontal sensitivity (time base) is changed, the appearance of the sinusoidal waveform on
the oscilloscope screen is affected in the following ways:
1. At 0.2 ms/cm:
Each horizontal division (or cm on the screen) represents 0.2 milliseconds.
The waveform appears stretched horizontally, meaning more detail of the waveform is
visible. We may only see a small portion or fewer cycles of the sinusoidal wave on the
screen, depending on the frequency of the signal.
2. At 0.5 ms/cm:
Each horizontal division now represents 0.5 milliseconds.
The waveform appears more compressed horizontally compared to 0.2 ms/cm. We have
seen more cycles of the waveform on the screen because each division now covers
more time.
3. At 1 ms/cm:
Each horizontal division represents 1 millisecond.
The waveform becomes even more compressed horizontally. Several cycles of the
waveform may now fit on the screen, and the sinusoidal shape may look tighter.
Did the frequency of the signal on the screen change with each horizontal sensitivity?
ANSWER:
No, the frequency of the signal did not change as you adjusted the horizontal sensitivity. The
signal frequency remains the same, but the visual appearance on the oscilloscope screen changes.
The horizontal sensitivity only affects how much time is displayed per division, so it changes how
many cycles of the waveform you see on the screen at once.
The actual frequency of the signal stays constant, even though the waveform looks more
stretched or compressed.
Give a sequence of steps to calculate frequency of a sinusoidal waveform appearing on the screen of
oscilloscope.
Steps to Calculate the Frequency of a Sinusoidal Waveform on the Oscilloscope
To calculate the frequency of a sinusoidal waveform using an oscilloscope, follow these steps:
Step 1: Set the Time Base
Adjust the horizontal sensitivity (time base) so that at least one full cycle of the waveform is
clearly visible on the screen.
For example, set the time base to a suitable value (e.g., 0.2 ms/div or 1 ms/div) based on the
signal's frequency.
Step 2: Identify the Period (T)
Count the number of horizontal divisions for one full cycle of the sinusoidal waveform. One cycle
is from one peak to the next peak or from one trough to the next trough.
Multiply the number of divisions by the time/div setting (horizontal sensitivity). This will give you
the period T of the signal in seconds (or milliseconds).
T=Number of divisions per cycle × Time per division
Step 3: Calculate the Frequency
Once we know the period T, the frequency f can be calculated using the formula:
f=1/T
Where:
f = frequency in Hertz (Hz)
T = period in seconds
Vertical sensitivity
Do not touch the controls of the function generator, but return the horizontal sensitivity of the scope to
0.2 ms/cm and change the vertical sensitivity to 2 V/cm. Calculate peak to peak value of the waveform
on the screen.
Calculated peak to peak value = _____4v________
Change the vertical sensitivity of the oscilloscope to 0.5 V/cm and repeat part (j).
Calculated peak to peak value = ____4v_________
What was the effect on the appearance of the sinusoidal waveform as the vertical sensitivity was
changed from 2 V /cm to 0.5 V/cm?
ANSWER:
Effect on the Appearance of the Sinusoidal Waveform When Changing Vertical Sensitivity:
At 2 V/cm: The waveform appears compressed vertically because each division represents a
larger voltage (2 V). The waveform fits in fewer divisions.
At 0.5 V/cm: The waveform appears stretched vertically because each division represents a
smaller voltage (0.5 V). The same waveform now spans more divisions, so the peaks and troughs
are more spread out.
Did the peak to peak vale of the signal on the screen change with each horizontal sensitivity?
ANSWER:
No, the peak-to-peak value of the signal on the screen does not change with adjustments to the
horizontal sensitivity. The horizontal sensitivity affects the time scale (how many cycles of the waveform
are displayed) but does not influence the signal’s amplitude (peak-to-peak voltage).
Can the peak or p-p output voltage of a function generator be set without the aid of an auxiliary
instrument like an oscilloscope or a DMM? Explain:
ANSWER:
It is not recommended to set the peak or peak-to-peak output voltage of a function generator without
the aid of an auxiliary instrument such as an oscilloscope or DMM (Digital Multimeter).
Explanation: The voltage reading on a function generator may be inaccurate or vary due to
factors like load impedance or internal signal generation inaccuracies.
An oscilloscope provides a direct visual measurement of the output waveform, ensuring the
correct peak-to-peak voltage is established.
A DMM can also be used to measure the RMS voltage, which allows you to calculate the peak-
to-peak value, but it’s less ideal for real-time waveform monitoring.
Therefore, using an oscilloscope or DMM is essential for precise control of the output voltage.
Exercise
Make all the necessary adjustments to clearly display a 5000 Hz, 6 V p-p sinusoidal signal on the
oscilloscope. Establish the zero volt line at the centre of the screen. Record the chosen sensitivities:
Vertical sensitivity = ____1v_________
Horizontal sensitivity = ____50us_________
Draw the waveform below clearly mentioning the dimensions:
Calculate the period of the waveform on the screen using the resulting number of required horizontal
divisions for a full cycle.
Calculated T = _____200us___________
Repeat part (a) above for a 200 Hz 0.8 Vp-p sinusoidal waveform:
Vertical sensitivity = ___0.1v__________
Horizontal sensitivity = ___0.5ms__________
Calculated T = ____5ms_________
Draw the waveform below clearly mentioning the dimensions:
Repeat part (a) for a 100 kHz 4 Vp-p square waveform:
Vertical sensitivity = __2v___________
Horizontal sensitivity = ___5us__________
Calculated T = ___10us__________
Draw the waveform below clearly mentioning the dimensions:
Effect of DC Levels
Re-establish the 1 kHz 4 Vp-p sinusoidal waveform on the screen. Calculate the effective value of the
sinusoidal waveform.
Calculated Vrms = __2.82v__________
Disconnect the function generator from the scope and measure the effective (rms) value of the output
of the function generator using DMM.
Measured Vrms = __2.71v__________
Determine the magnitude of the percent difference between the calculated and measured levels using
the following equation:
% Difference = [{V (calc) – V (meas)} ÷ V (calc)] x 100%
% Difference = ____3.9%__________
Reconnect the function generator to the scope with the 1 kHz 4 Vp-p signal and switch the AC-GND-DC
coupling switch of the vertical channel to GND. What is the effect?
ANSWER:
When the switch is set to GND (Ground):
The oscilloscope input is disconnected from the signal and grounded.
The waveform will disappear, and the trace will be a flat horizontal line at the 0V reference
level.
Explain? How can this scope function be used?
ANSWER:
Explanation: This setting is used to set the baseline or reference level of the signal. It's useful for
zeroing the display to ensure accurate measurements.
Now move the AC-GND-DC coupling switch to AC position. What is the effect on the screen? Explain.
ANSWER:
When the switch is set to AC:
The oscilloscope blocks any DC component in the signal, only showing the AC signal (the
sinusoidal waveform).
The waveform will still be present on the screen, but if there is any DC offset in the signal, it will
not be displayed.
Explanation: This setting is used to focus on AC signals and ignore any DC offset. It is useful
when measuring small AC variations superimposed on a larger DC signal.
Then move the AC-GND-DC coupling switch to DC position. What is the effect on the screen? Explain.
ANSWER:
When the switch is set to DC:
Both the AC and DC components of the signal are displayed.
If there is a DC offset, the waveform will shift upward or downward from the center of the
screen.
Explanation: This setting is used when you need to observe the full signal, including any DC
offset present.
