OSCILLOSCOPE
Oscilloscopes Applications:
1. Consumer electronics repair
2. Digital systems troubleshooting
3. Control system design
4. Physics laboratories
Its ability:
1. Measure the time, frequency and voltage
level of a signal
2. View rapidly changing waveforms
3. Determine if an output signal is distorted
Classification of an Oscilloscope
1. Analog Oscilloscopes - directly apply the
voltage being measured to an electron beam
moving across the oscilloscope screen. This
voltage deflects the beam up, down, and
across, thus tracing the waveform on the
screen.
Characteristics:
It can display high-frequency varying
signals in “real time”
Classification of an Oscilloscope
2.
Digital Oscilloscopes sample the input
waveform and then use an analog-to-digital
converter (ADC) to change the voltage being
measured into digital information. The digital
information is then used to reconstruct the
waveform to be displayed on the screen.
Characteristics:
Allows you to capture and store
information that can be accessed at a later
time or be interfaced to a computer.
DUAL-TRACE OSCILLOSCOPES
It has the ability to measure two input
signals at the same time. It has two separate
vertical amplifiers and an electronic switching
circuit. It is then possible to observe two timerelated waveforms simultaneously at different
points in an electric circuit.
OPERATING CONTROLS OF AN OSCILLOSCOPE
INTENSITY.
This control sets the level of
brightness or intensity of the light trace on the
CRT. Rotation in a clockwise (CW) direction
increases the brightness. Too high an intensity can
damage the phosphorous coating on the inside of
the CRT screen.
FOCUS. This control is adjusted in conjunction
with the intensity control to give the sharpest trace
on the screen. There is interaction between these
two controls, so adjustment of one may require
readjustment of the other.
OPERATING CONTROLS OF AN OSCILLOSCOPE
ASTIGMATISM. This is another beam-focusing
control found on older oscilloscopes that operates in
conjunction with the focus control for the sharpest
trace. This control is sometimes a screwdriver
adjustment rather than a manual
control
OPERATING CONTROLS OF AN OSCILLOSCOPE
HORIZONTAL AND VERTICAL POSITIONING
OR CENTERING. These are trace-positioning
controls. They are adjusted so that the trace is
positioned or centered both vertically and
horizontally on the screen. In front of the CRT
screen is a faceplate called the graticule, on which
is etched a grid of horizontal and vertical lines.
Calibration markings are sometimes placed on the
center vertical and horizontal lines on this
faceplate.
OPERATING CONTROLS OF AN OSCILLOSCOPE
VOLTS/DIV. This control attenuates the vertical
input signal waveform that is to be viewed on the
screen. This is frequently a click-stop control that
provides step adjustment of vertical sensitivity. A
separate Volts/Div. control is available for each
channel of a dual-trace scope. Some scopes mark
this control Volts/cm.
OPERATING CONTROLS OF AN OSCILLOSCOPE
VARIABLE. In some scopes this is a concentric
control in the center of the Volts/Div. control. In
other scopes this is a separately located control. In
either case, the functions are similar. The variable
control works with the Volts/Div. control to provide
a more sensitive control of the vertical height of the
waveform on the screen.
OPERATING CONTROLS OF AN OSCILLOSCOPE
The variable control also has a Calibrated Position
(CAL) either at the extreme counterclockwise or
clockwise position. In the CAL position the
Volts/Div. control is calibrated at some set value—
for example, 5 mV/Div., 10 mV/Div., or 2 V/Div.
This allows the scope to be used for peak-to-peak
voltage measurements of the vertical input signal.
Dual-trace scopes have a separate variable control
for each channel.
OPERATING CONTROLS OF AN OSCILLOSCOPE
INPUT COUPLING AC-GND-DC SWITCHES. This
three-position switch selects the method of coupling the
input signal into the vertical system.
a. AC - The input signal is capacitively coupled to the
vertical amplifier. The dc component of the input
signal is blocked.
b. GND - The vertical amplifier’s input is grounded to
provide a zero-volt (ground) reference point. It does
not ground the input signal.
c. DC - This direct-coupled input position allows all
signals (ac, dc, or ac-dc combinations) to be applied
directly to the vertical system’s input.
OPERATING CONTROLS OF AN OSCILLOSCOPE
VERTICAL MODE SWITCHES. These switches select
the mode of operation for the vertical amplifier system.
a. CH1 - Selects only the Ch. 1 input signal for display.
b. CH2 - Selects only the Ch. 2 input signal for display.
c. Both - Selects both Channel 1 and Channel 2 input
signals for display. When in this position, ALT,
CHOP, or ADD operations are enabled.
d. ALT - Alternatively displays Ch. 1 and Ch. 2 input
signals. Each input is completely traced before the
next input is traced. Effectively used at sweep
speeds of 0.2 ms per division or faster.
OPERATING CONTROLS OF AN OSCILLOSCOPE
e. CHOP - During the sweep the display switches
between Ch. 1 and Ch. 2 input signals. The
switching rate is approximately at 500 kHz.
This is useful for viewing two waveforms at slow
sweep speeds of 0.5 ms per division or slower.
f. ADD - This mode algebraically sums the Ch. 1
and Ch. 2 input signals.
g. INVERT - This switch inverts Ch. 2 (or Ch. 1 on
some
scopes) to enable
a differential
measurement when in the ADD mode.
OPERATING CONTROLS OF AN OSCILLOSCOPE
TIME/DIV. This is usually two concentric controls that
affect the timing of the horizontal sweep or time-base
generator. The outer control is a click-stop switch that
provides step selection of the sweep rate. The center
control provides a more sensitive adjustment of the
sweep rate on a continuous basis. In its extreme
clockwise position, usually marked CAL, the sweep rate
is calibrated. Each step of the outer control is therefore
equal to an exact time unit per scale division. Thus, the
time it takes the trace to move horizontally across one
division of the screen graticule is known. Dual-trace
scopes generally have one Time/Div. control. Some
scopes mark this control Time/cm.
OSCILLOSCOPE PROBE
Types of Probe:
1. Direct probe - is just a shielded wire without any
isolating resistor. A shielded cable is necessary to
prevent any pickup of interfering signals, especially
with the high resistance at the vertical input
terminals of the oscilloscope.
It has relatively high capacitance. Typically 90
pF for 3 ft (0.9 m) of 50-Ω coaxial cable plus the vertical
input terminals shunt capacitance of about 40 pF total
of 130 pF. This much capacitance can have a big effect
on the circuit being tested.
Effects: it could detune a resonant circuit & nonsinusoidal wave shapes are distorted.
Advantage: it does not divide down the amount of input
signal, since there is no series-isolating
resistance.
OSCILLOSCOPE PROBE
2. Low-capacitance probe (LCP) with a seriesisolating resistor
LCP usually has a switch to short out the
isolating resistor so that the same probe can be
used either as a direct lead or with low
capacitance.
Schematic Diagram of a Low
Capacitance Probe (LCP) for
Oscilloscopes
An oscilloscope voltage probe by
Tektronix, Inc. A Low
Capacitance Probe (LCP)
OSCILLOSCOPE PROBE
With an LCP, the input capacitance of the probe is
only about 10 pF.
When to use LCP:
1. The signal frequency is above audio frequencies.
2. The circuit being tested has R higher than about
50 kΩ.
3. The wave shape is non sinusoidal, especially
with square waves and sharp pulses.
The observed waveform can be distorted
without LCP
Areas of application for specific frequency bands
Areas of application for specific frequency bands
OSCILLOSCOPE PROBE
THE 1:10 VOLTAGE DIVISION OF THE LCP
Current Measurement:
x
𝑉 = Vertical sensitivity, in
`
𝑉
𝑉 = Vertical deflection, in div
x
𝐻
𝐻 = Horizontal sensitivity, in
𝐻 = Horizontal deflection, in
Sinewave
Horizontal & Vertical Graticule
Determining Vpk, tp, prt, prf, and % duty cycle from
the rectangular wave displayed on the scope graticule
𝑉
𝑉
Horizontal
Sensetivity, 𝐻 =
`
Horizontal & Vertical Graticule
Sine and Cosine waveform
For Hs: (1-, 0.5-, 0.2-, 0.1-, 50 m-, 20 m-, 10 m-, 5 m-, 2 m-, 1 m-, 0.5 m-, 0.2 m-, 0.1 m-, 50 µ-,
20 µ-, 10 µ-, 5 µ-, 2 µ-, 1 µ-, 0.5 µ-, 0.2 µ-, 0.1 µ-, 50 n-, 20 n-, 10 n-, 5 n-, 2 n- and 1 n-) sec/div.