744 Appendix-A A. Essential Electronic Measuring Devices
The time can be measured on the horizontal axis by multiplying the divisions by
the current time per division setting. Usually the time per division is set through a
knob on the front panel and is available in a wide range.
It is not always necessary to use the sweep generator. Sometimes it is desired to
plot one signal with respect to another. Depending on the design of the oscilloscope,
this can be done in more than one ways, the most common of which is to connect
the two signal outputs to the two oscilloscope input channels and use the XY mode
of operation.
A.4 TriggerSystem
We just saw that the horizontal sweep of the beam does not start until the sweep
generator gets a trigger signal. This signal is generated by the trigger system, which
is designed in such as way that, at every sweep, it initiates the trigger signal at the
same height of the input signal. This ensures that each trace of a periodic signal
synchronizes with the previous one and the display looks stable.
Most modern oscilloscopes provide the following three distinct ways to generate
trigger.
NORM Mode: In thisnormalmode, the user chooses a threshold level of
the original signal such that whenever the signal crosses this level, a trigger
signal is generated. This trigger generation can be set to be either with rising
or falling edge of the signal, which are generally referred to as SIGNAL+ or
SIGNAL- on the trigger panel of the oscilloscope.AUTO Mode: In this mode, the trigger system automatically generates a
trigger if after a predefined time no trigger signal was generated. This of course
depends on the preset triggering conditions. The advantage of this mode is that
it always displays a signal even if the trigger conditions are not met.: SINGLE Sweep Mode:This triggering mode is very useful for studying
single or non-periodic events, since here the sweep generator sweeps the beam
only once. If the trigger conditions are properly set according to the expected
signal shape, then as soon as a signal trace is found and displayed, the trigger
system goes into an idle state. The user then has the opportunity to study the
signal or save it in memory.Oscilloscopes are also equipped with option of triggering on on an external signal,
which can be connected to the EXT input. It is also possible to trigger on the power
or line voltage, generally represented by LINE on the trigger panel.
A.2.B Digital Oscilloscopes
In digital version of oscilloscopes, the input signal is first sampled and then converted
to digital counts with an A/D converter before being displayed on the screen. The
advantages are the ability to perform complex mathematical operations on the signal
and storage of data in memory. The biggest downside is the dead time associated
with the whole process. For example, it takes some finite time for the ADC to
convert the analog signal, which then has to be converted back to analog signal