6.3.5 Timebase circuit
The purpose of a timebase is to apply
a voltage to the horizontal deflector plates such that the horizontal position
of the spot is proportional to time. This voltage, in the form of a ramp known
as a sweep waveform, must be applied repetitively, such that the motion of the
spot across the screen appears as a straight line when a d.c. level is applied
to the input channel. Furthermore, this timebase voltage must be synchronized
with the input signal in the general case of a time-varying signal, such that a
steady picture is obtained on the oscilloscope screen. The length of time taken
for the spot to traverse the screen is controlled by a time/div switch, which
sets the length of time taken by the spot to travel between two marked
divisions on the screen, thereby allowing signals at a wide range of
frequencies to be measured.
Each cycle of the sweep waveform is
initiated by a pulse from a pulse generator. The input to the pulse generator
is a sinusoidal signal known as a triggering signal, with a pulse being
generated every time the triggering signal crosses a preselected slope and
voltage level condition. This condition is defined by the trigger level and
trigger slope switches. The former selects the voltage level on the trigger
signal, commonly zero, at which a pulse is generated, whilst the latter selects
whether pulsing occurs on a positive- or negative-going part of the triggering
waveform.
Synchronization of the sweep waveform
with the measured signal is most easily achieved by deriving the trigger signal
from the measured signal, a procedure that is known as internal triggering.
Alternatively, external triggering can be applied if the frequencies of the
triggering signal and measured signals are related by an integer constant such
that the display is stationary. External triggering is necessary when the
amplitude of the measured signal is too small to drive the pulse generator, and
it is also used in applications where there is a requirement to measure the
phase difference between two sinusoidal signals of the same frequency. It is
very convenient to use the 50 Hz line voltage for external triggering when
measuring signals at mains frequency, and this is often given the name line
triggering.
6.3.6 Vertical sensitivity control
This consists of a series of
attenuators and pre-amplifiers at the input to the oscilloscope. These
condition the measured signal to the optimum magnitude for input to the main amplifier
and vertical deflection plates, thus enabling the instrument to measure a very
wide range of different signal magnitudes. Selection of the appropriate input amplifier/attenuator
is made by setting a volts/div control associated with each oscilloscope
channel. This defines the magnitude of the input signal that will cause a
deflection of one division on the screen.
6.3.7 Display position control
This allows the position at which a
signal is displayed on the screen to be controlled in two ways. The horizontal
position is adjusted by a horizontal position knob on the oscilloscope front
panel and similarly a vertical position knob controls the vertical position.
These controls adjust the position of the display by biasing the measured
signal with d.c. voltage levels.
6.4 Digital storage oscilloscopes
Digital storage oscilloscopes consist
of a conventional analogue cathode ray oscillo[1]scope
with the added facility that the measured analogue signal can be converted to
digital format and stored in computer memory within the instrument. This stored
data can then be reconverted to analogue form at the frequency necessary to
refresh the analogue display on the screen. This produces a non-fading display
of the signal on the screen.
The signal displayed by a digital
oscilloscope consists of a sequence of individual dots rather than a continuous
line as displayed by an analogue oscilloscope. However, as the density of dots
increases, the display becomes closer and closer to a continuous line, and the
best instruments have displays that look very much like continuous traces. The
density of the dots is entirely dependent upon the sampling rate at which the
analogue signal is digitized and the rate at which the memory contents are read
to reconstruct the original signal. Inevitably, the speed of sampling etc. is a
function of cost, and the most expensive instruments give the best performance
in terms of dot density and the accuracy with which the analogue signal is
recorded and represented.
Besides their ability to display the
magnitude of voltage signals and other parameters such as signal phase and
frequency, some digital oscilloscopes can also compute signal parameters such
as peak values, mean values and r.m.s. values. They are also ideally suited to
capturing transient signals when set to single-sweep mode. This avoids the
problem of the very careful synchronization that is necessary to capture such
signals on an analogue oscilloscope. In addition, digital oscilloscopes often
have facilities to output analogue signals to devices like chart recorders and
output digital signals in a form that is compatible with standard interfaces
like IEEE488 and RS232. Some now even have floppy disk drives to extend their
storage ability. Fuller details on digital oscilloscopes can be found elsewhere
Hickman, (1997).
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