25-01-2013, 02:50 PM
THE CATHODE RAY OSCILLOSCOPE
[attachment=48477]
INTRODUCTION
The cathode ray oscilloscope is a vital piece of diagnostic lab equipment for observing and measuring
electrical signals at frequencies ranging from dc to GHz. For this experiment, it will be assumed
that you are familiar with the basic operation of the cathode ray tube, which forms the heart of the
oscilloscope. The procedure will be devoted to refining your skills of using the oscilloscope, which
are essential for this course.
PRE-LABORATORY PREPARATION
Review Secs. 3.1 through 3.3.1 and 3.4 of Reference 2, paying special attention to Sec 3.3.1 and
OPERATION OF AN OSCILLOSCOPE
Sinusoidal signals
Parameters of a sinusoidal signal
A sine wave is completely characterized by its frequency, phase, and amplitude. Instead of
amplitude (zero-to-peak value), it is common to encounter two other measures of its size: the peakto-
peak value, and the rms value. The peak-to-peak value is just the vertical spread between a
positive and negative peak, i.e. twice the amplitude. The rms current is that dc current which
would cause the same power dissipation in a resistor R as the actual ac current. For a current
i = I0 sin ωt, the average power dissipation is given by
i2 Rdt = I2
0R
2 .
We define the rms current Irms by I2
rmsR = P, so that Irms = I0/
Voltmeters and ammeters capable of measuring ac signals are almost always calibrated in terms
of rms readings to facilitate comparison with dc meter readings.
Pulse signals
A single pulse can be described by its amplitude,
rise and fall times, and width. One way to define
the rise time is the time interval for the pulse
voltage or current to rise from 10% to 90% of full
amplitude. Similarly, the width is often defined
as the time interval between the 50% points. If
the pulses repeat periodically, we can also specify
the repetition rate as the number of pulses per
second, and the duty cycle as the fraction of time
that the pulse is ”on”. The duty cycle is given by
the pulse width times the repetition rate.
Parameters of a pulse waveform
Operation
There are three basic sets of controls on any scope: the horizontal sweep, the triggering of that
sweep, and the vertical amplifier(s).
Vertical amplifier
Since the signals normally observed with an oscilloscope range from a few volts to a few millivolts,
the scope has an amplifier on each signal input. The gain of the amplifier is selectable and calibrated
in V/division or V/cm. It is the input amplifier that limits the maximum frequency (bandwidth)
and minimum signal size (sensitivity) of the scope. Note that the coupling of the input signal to the
amplifier may be dc or ac. In the ac case, any non-zero average of the signal (the dc component)
is suppressed by the use of an input coupling capacitor. This is useful when you want to observe a
small time-varying signal on a large dc background. Beware of using ac coupling when observing a
very slowly varying signal; it will be badly distorted.
Many scopes have two separate vertical amplifiers, with inputs usually labelled CH1 and CH2, or
A and B. By switching the vertical deflection of the beam between CH1 and CH2 at high frequency
(chopping) or by displaying CH1 and CH2 on successive traces (alternating), the scope appears to
display two signals at the same time. This feature is useful in comparing two signals with respect
to phase or timing.
[attachment=48477]
INTRODUCTION
The cathode ray oscilloscope is a vital piece of diagnostic lab equipment for observing and measuring
electrical signals at frequencies ranging from dc to GHz. For this experiment, it will be assumed
that you are familiar with the basic operation of the cathode ray tube, which forms the heart of the
oscilloscope. The procedure will be devoted to refining your skills of using the oscilloscope, which
are essential for this course.
PRE-LABORATORY PREPARATION
Review Secs. 3.1 through 3.3.1 and 3.4 of Reference 2, paying special attention to Sec 3.3.1 and
OPERATION OF AN OSCILLOSCOPE
Sinusoidal signals
Parameters of a sinusoidal signal
A sine wave is completely characterized by its frequency, phase, and amplitude. Instead of
amplitude (zero-to-peak value), it is common to encounter two other measures of its size: the peakto-
peak value, and the rms value. The peak-to-peak value is just the vertical spread between a
positive and negative peak, i.e. twice the amplitude. The rms current is that dc current which
would cause the same power dissipation in a resistor R as the actual ac current. For a current
i = I0 sin ωt, the average power dissipation is given by
i2 Rdt = I2
0R
2 .
We define the rms current Irms by I2
rmsR = P, so that Irms = I0/
Voltmeters and ammeters capable of measuring ac signals are almost always calibrated in terms
of rms readings to facilitate comparison with dc meter readings.
Pulse signals
A single pulse can be described by its amplitude,
rise and fall times, and width. One way to define
the rise time is the time interval for the pulse
voltage or current to rise from 10% to 90% of full
amplitude. Similarly, the width is often defined
as the time interval between the 50% points. If
the pulses repeat periodically, we can also specify
the repetition rate as the number of pulses per
second, and the duty cycle as the fraction of time
that the pulse is ”on”. The duty cycle is given by
the pulse width times the repetition rate.
Parameters of a pulse waveform
Operation
There are three basic sets of controls on any scope: the horizontal sweep, the triggering of that
sweep, and the vertical amplifier(s).
Vertical amplifier
Since the signals normally observed with an oscilloscope range from a few volts to a few millivolts,
the scope has an amplifier on each signal input. The gain of the amplifier is selectable and calibrated
in V/division or V/cm. It is the input amplifier that limits the maximum frequency (bandwidth)
and minimum signal size (sensitivity) of the scope. Note that the coupling of the input signal to the
amplifier may be dc or ac. In the ac case, any non-zero average of the signal (the dc component)
is suppressed by the use of an input coupling capacitor. This is useful when you want to observe a
small time-varying signal on a large dc background. Beware of using ac coupling when observing a
very slowly varying signal; it will be badly distorted.
Many scopes have two separate vertical amplifiers, with inputs usually labelled CH1 and CH2, or
A and B. By switching the vertical deflection of the beam between CH1 and CH2 at high frequency
(chopping) or by displaying CH1 and CH2 on successive traces (alternating), the scope appears to
display two signals at the same time. This feature is useful in comparing two signals with respect
to phase or timing.