Lab Session #1
The Oscilloscope:
An oscilloscope, previously called an oscillograph, and informally known as a scope or o-
scope, CRO, or DSO, is a type of electronic test instrument that graphically displays varying
signal voltages, usually as a calibrated two-dimensional plot of one or more signals as a
function of time.
Oscilloscope Working:
An oscilloscope can be used to measure voltage. It does this by measuring the voltage drop
across a resistor and in the process draws a small current. The voltage drop is amplified and
used to deflect an electron beam in either the X (horizontal) or Y (vertical) axis using an
electric field. The electron beam creates a bright dot on the face of the Cathode Ray Tube
(CRT) where it hits the phosphorous. The deflection, due to an applied voltage, can be
measured with the aid of the calibrated lines on the graticule.
First we will consider the circuitry that amplifies and conditions the voltage to be measured
The deflection of the oscilloscope beam is proportional to the input voltage (after ac or dc
coupling). The amount of deflection (Volts/Division) depends upon the setting of the
AMPL/DIV control for that channel (see figure 2).
The input signal can be ac or dc coupled. Ac coupling involves adding a series capacitor. This
has the effect of blocking (removing) the dc bias and low frequency components of a signal.
Dc coupling does not have this problem and therefore allows you to measure voltages right
down to 0 Hz. Ac coupling is useful when you are trying to measure a small ac voltage that is
“on-top” of a large dc voltage. A typical example is trying to measure the noise of a dc power
supply.
Page-1
Figure 2. Amplifier Block Diagram
Amplifier Features
AMPL/DIV - This abbreviated name varies but it is generally some short form of amplitude
per division. The control is a simple voltage divider (attenuator) which is used to change the
sensitivity of the oscilloscope. At a 1 volt/DIV setting, a deflection of one major division on
the graticule represents a one volt change at the oscilloscope input.
Calibrated voltage measurements
The small knob within the AMPL/DIV control must be rotated clockwise into its detente
position for the amplifiers to be calibrated. Otherwise the voltage/division will be some
unknown value greater than what the dial indicates.
INV - There is almost always a control which lets you invert one channel. This can be used
along with the ADD function to subtract two voltages. This is necessary because the common
input (black lead of the oscilloscope cable) can only be connected to a 0V node. If channel A
has V1 + V2 and channel B has voltage V1 then the reading of channel A + (-channel B) = (V1
+ V2) + (-V1) = V2
Position - For each axis there is a control which lets you shift the electron beam. With this you
can set the zero voltage point to anywhere that is convenient for you.
Oscilloscope Operation (Voltage vs Time)
The main function of an oscilloscope is to show voltage vs time. This is done by applying a
ramp (or sawtooth) waveform into the X-axis amplifier. During the rising edge of the ramp,
the electron beam scans across the screen. When the voltage drops back to 0V, the beam is
turned off and quickly goes back to its starting point. This is signified by a thick line when the
beam is on and a thin one when it is off (blanked).
Accuracy
There are many factors affecting the accuracy of oscilloscope measurements. There are errors
due to the input channel voltage divider, timebase control, the use of magnifiers, the accuracy
to which the CRT deflection can be read, beam thickness, temperature etc
Page-2
Function generator basics
Function generators, whether the old analog type or the newer digital type, have a few common
features:
A way to select a waveform type: sine, square, and triangle are most common, but some will
give ramps, pulses, “noise”, or allow you to program a particular arbitrary shape.
A way to select the waveform frequency. Typical frequency ranges are from 0.01 Hz to 10
MHz.
A way to select the waveform amplitude.
At least two outputs. The “main” output, which is where you find the desired waveform, typically
has a maximum voltage of 20 volts peak-to-peak, or ±10 volts range. The most common output
impedance of the main output is 50 ohms, although lower output impedances can sometimes be
found. A second output, sometimes called “sync”, “aux” or “TTL” produces a square wave with
standard 0 and 5 volt digital signal levels. It is used for synchronizing another device (such as an
oscilloscope) to the possibly variable main output signal.
o A wide variety of other features are available on most modern function generators, such
as “frequency sweep”—the ability to automatically vary the frequency between a
minimum and maximum value, “DC offset”—a knob that adds a specified amount of
DC voltage to the time-varying waveform, and extra inputs or outputs that can be used
to control these extra features by other instruments.
Page-3
