Experiment No.

1
Simple Measurements with the Oscilloscope

Objective: To get familiar with the oscilloscope, observe waveforms generated with a function
generator, and make measurements for a series RC circuit.
Background: The oscilloscope is one of the instruments you will use very extensively in your
Electronics laboratories. It provides an effective way to observe periodic signals. The early
oscilloscopes (the type which is still commonly used today, including the one you will use in
WEL) were all “analog” type; the input voltage is used to continuously sweep an electron beam
across the “screen” with some clever tricks so as to obtain a steady waveform.
The most modern and advanced oscilloscopes generally are the “digital” type in which an
input signal is “sampled” at a frequency much higher than the signal frequency, and the sampled
waveform is displayed. This also makes it possible to observe non-periodic or “transient”
waveforms.
The display unit of an oscilloscope is the cathode ray tube. Its X and Y plates are driven
by suitable voltages generated by the electronic circuits inside the oscilloscope.
Experiment:
1. Frequency measurement: Set the CRO in the single normal trace mode (AC). Apply a
1V peak sine wave of frequency 1 KHz to channel1. Make the trigger settings as internal,
positive slope, CH 1 and adjust the trigger level to get a stable waveform. Set the time/div
knob so as to get a few cycles on the display. Measure the time period, which then gives
the frequency. In general, while measuring the period or amplitude of a signal, always
make proper use of the resolution offered by the CRO. (The error you make in measuring
the period would be smaller if you have only one cycle on the screen instead of many.)
2. DC, AC and GND modes: When the GND setting is used the applied voltage is
decoupled from the Y plates, and the level you observe on the scope can be considered as
the “zero applied voltage” level. All your measurements must be made with this level in
mind. You could move this level vertically with a knob if you like.
In the DC mode, the actual voltage gets applied to the Y plates and what you
observe in the scope is the true value of the applied voltage.
 In the AC mode, the DC component (if any) of the voltage is filtered out and only
the AC part is sent to the Y plates. What you observe on the scope is thus a “shifted”
signal whose average value will turn out to be zero.
You can test these by applying a triangular wave to one of the channels along
with an “offset” DC voltage. The function generator provides the “offset” option, so need
not apply a DC voltage from any other source.
3. Testing of components:
(a) Study the colour code of various resistors given to you. Also test the resistors using
the Digital Multimeter (DMM).
(b) Study the electrolytic & ceramic capacitors provided to you. Note down the value of
capacitors.
(c) Test the diodes given to you. Obtain the cut-in voltage on the DMM.
(d) Study the collector, base & emitter of the transistor given.
Observe the device signatures of all the components on the CRO using the CTC option.
4. A series RC circuit:
(a) Connect the RC circuit shown in Fig.1 on a breadboard with R=1kΩ and C=0.1µF.
Observe the voltage across R and the input voltage (1 V peak Sine wave) simultaneously
using the dual trace mode. At about 1 KHz, you should notice a phase shift between the
two waveforms. Estimate the value of this phase difference by measuring suitable time
intervals. Note that you have to ensure that CH 1 and CH 2 ground levels are the same to
make these measurements conveniently.

AC Mode

+ 0. 1uF +
Vs 1k
Lissajous Fig.
R
VR
1Vpe a k - -

Figure 1
 (Incidentally, you cannot observe the voltage across C directly on the scope, as one end
of the probe must be always connected to the ground.)
(b) Now put the scope in the X-Y mode, and obtain the Lissajous figure shown in Fig.2.
Estimate the phase difference from this measurement. Note that you will have to make
the (0,0) of this figure coincide with the center of the CRO grid. Make the best use of
resolution offered by the CRO, i.e. you should adjust the Volts/ div settings so as to fill
most of the screen with the Lissajous figure.
Since we are only interested in the AC measurements for the RC circuit, make sure you
use the scope in AC mode.
(c) Now, interchange the R and C (see Fig.3). Keep the amplitude of the input voltage
constant, say 1V peak. Vary the frequency and note down the amplitude of VC (say, in the
frequency range 50 Hz to 10 kHz). You may need to check that the input amplitude is
indeed 1V every time. Plot VC as a function of frequency.

1k

+
R +
Vs C
1Vpe a k 0. 1uF VC
- -

Figure 3