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The Oscilloscope: Operation and
Applications
1.
The Oscilloscope
Oscilloscope Operation (X vs Y mode)
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
“Amp” block in figure 1).
Figure 1
.
X vs. Y Deflection Block Diagram of the CRT
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.
-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 Inputs
The input of the oscilloscope can usually be modelled as a resistance and a parallel capacitance
(see figure 3). The resistance is usually 1M
6
but it and the capacitance can vary greatly. The
total or effective capacitance includes the oscilloscope circuitry (approx. 30 pF), cables (approx.
30 pF/m) and stray capacitance. The resistance will draw current from the circuit while the
capacitance will add an RC time constant with its associated time delay, frequency response and
distortion of some waveforms.
The common connection (black lead or shield) at the input of the oscilloscope goes to the metal
case as the symbol by the input connector shows. Because of this, the common input can only
be connected to a 0V point in the circuit. Since the common inputs for both the A and B
channels are connected to the case, they are effectively shorted together.
-2-
Figure 3. DC-coupled Oscilloscope Input Circuit
and Frequency Response
Frequency response is calculated or measured by applying a pure sinusoidal waveform to a
circuit. The circuit response is the output voltage divided by the input voltage. This is a complex
number that can also be expressed as a magnitude (gain) and phase.
Due to limitations in the amplifiers, the oscilloscope's frequency response is limited. The
manufacturer simply lists the half-power point for the oscilloscope without any external effects.
Half power is also called the -3dB point. At this point, the voltage has decreased to 70.7% of its
maximum. This means that only one-half of the maximum power would be dissipated in a
resistive load. Keep in mind that an oscilloscope that is rated at 20 MHz is usually only accurate
to 4 MHz for non-sinusoidal waveforms before distortion becomes a problem.
With ac coupling (figure 4), an oscilloscope has another series RC circuit. It acts like a high
pass filter (HPF). If you are viewing low frequency signals when ac coupled, not only will you
not be able to measure any dc offset, but you will also be removing some low frequency
information.
Figure 4
.
AC-coupled Oscilloscope Input Circuit and
Frequency Response
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 as shown in figure 5. 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).
To obtain a stable picture on the CRT screen, the ramp waveform has to be in phase with the
signal that you want to observe. This is done with a triggering circuit. The triggering circuit
allows the oscilloscope to draw repeatedly the same waveform over and over by identifying the
same point on a repetitive waveform.
-3-
Figure 5
.
A Ramp-driven X-axis input
The triggering circuit allows you to select a voltage (an analog value) and an edge or slope
(positive or negative) for the triggering circuit to compare to the input waveform. When the
two are equal, the circuit puts out a pulse. This pulse triggers the ramp waveform generator to
do
one cycle
of its rising and falling edges. Once the ramp has started a cycle of increasing
voltage, it can not be retriggered until it has completed the full ramp and returned to 0V. This is
illustrated in figure 6 for a single cycle and in figure 7 for multiple cycles.
Figure 6. A Triggering Example
Figure 7. Several Triggering Cycles
Not only do you have control over the starting point of the ramp, but the amount of time that
the ramp takes to reach its maximum voltage (the right hand side of the CRT screen) can be
adjusted with the timebase control. In essence, you have a “window”. You can move the
window to any point on a waveform with the triggering circuit and you can change the size of
the window with the timebase.
The time-base control allows you to set the time / division that the beams takes to scan across
-4-
 the screen. Just like the voltage selector, there is a calibration knob in the middle of the control.
Unless the vernier (calibration knob) is 'clicked' in to its most clockwise position, the time per
division is unknown.
When set to AUTO (automatic) triggering, the oscilloscope will always show a trace. However,
when you use a manual triggering mode (DC, AC), many strange things can happen. For
example, if the triggering voltage or level is set to +10V and the waveform never exceeds +5V,
the triggering circuit will never trigger and the screen will stay blank.
You may think that in a condition of no triggering, you would still have a bright dot on the
screen because the electron beam would go to its 'home' or undeflected position. Since the
oscilloscope is designed to work with a moving electron beam, a stationary beam can very
quickly 'burn' a hole in the phosphorous coating of the screen. To prevent this, there is a '
blanking' circuit which turns off the electron beam. Blanking occurs when there is no triggering
or when the electron beam is sweeping from the right edge back to the left side of the screen.
Time measurements are done the same way as voltage measurements. As long as the timebase is
calibrated you multiply the number of divisions by the number of seconds per division to get the
total time difference. Phase measurements are done by comparing the measured time to the
period of the waveform.
Oscilloscope Two Channel Operation
You can view two voltage waveforms at once by using two Y-axis (vertical) input channels.
The individual channels are sometimes labelled as '1' and '2' or as 'A' and 'B'. Since there is only
one electron beam, you have to share its drawing time between both waveforms. This may be
accomplished using either the
chop
or
alternate
modes.
When in the chop mode (figure 8), the oscilloscope displays a little bit of channel A, then a little
bit of B, then A, then B ....during a single sweep of the electron beam. If you increase the
timebase to about 1µ s/division, you can start to see the individual pieces as it chops between one
channel and the other channel.
Figure 8
.
Chop Mode
In the alternate mode (figure 9), the oscilloscope will sweep the electron beam twice across the
screen. The first time it will draw the signal from channel A and the next time from channel B.
At very low timebase settings, you can see it draw one channel and then the other in successive
passes.
Note: When you use the alternate function, the two waveforms that you see are from different
points in time and the triggering circuit has to trigger twice.
-5-
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