Instruction manual

HB 08-18-2010 4
We consider the case where the trigger pulse is derived from the input voltage, which is the
voltage that is to be observed. The operator of the scope, by using a scope control usually
marked “trigger level”, chooses a value of the input voltage at which the trigger pulse is
to occur. We call this voltage V
T
. This can be a positive or negative voltage, but for the
trigger pulse to occur V
T
must be between between the maximum and minimum values of
the input voltage. If it is not, the trigger pulse will not occur. The scope operator can also
choose whether the trigger pulse occurs when the slope of the input voltage (with respect
to time) is positive or negative. Look at Fig. 4, which shows a sinusoidal input voltage
and the horizontal sweep voltage as a function of time. The trigger voltage V
T
has been
chosen as positive. The triggering slope has also been chosen as positive. Fig. 4 shows that
whenever the input voltage has a positive slope and reaches the value V
T
, a trigger pulse is
produced and the ramp voltage produced by the scope sweeps the electron beam horizontally
across the screen at a rate determined by the TIME BASE. In Fig. 4 the time it takes for
the electron beam to go from the left side of the screen to the right side of the screen is
designated by “P.” It does this repetitively, always starting at the “same” point of the input
voltage and always tracing out the same curve. The result is a stationary trace of the input
voltage on the scope screen. In the example shown in Fig. 4, note that somewhat less than
one cycle of the input voltage will be displayed. During the time intervals “N” the electron
beam is blanked and is not being swept across the phosphor. An additional point is that
once a sweep starts it is always completed, even if the trigger conditions are met during the
sweep. This allows the use of a low enough sweep speed so that a number of cycles of the
input waveform can be displayed on the screen.
You should understand the following statements. If you do not, please reread and study
the previous material.
• For a given input signal, if the electron horizontal sweep speed is increased, less of the
input waveform or fewer cycles will be displayed.
• If the electron horizontal sweep speed is decreased, more of the input waveform or more
cycles will be displayed.
• If the trigger voltage V
T
is changed, the trace of the input voltage can be shifted left or
right in a continuous fashion (exclude the square wave).
• If the trigger voltage V
T
is kept constant but the trigger slope is changed, the trace will
be shifted left or right. For example, Fig. 4 is drawn for triggering to occur on a positive
slope of the input voltage. If V
T
is kept the same but triggering were set to occur on
the negative slope of the input signal, at what points of the input signal in Fig. 4 might
triggering occur? How about a negative V
T
and positive slope? A negative V
T
and a
negative slope?
• Fig. 4 shows plots of three ramp or sweep voltages. A given ramp voltage, no matter
which of the three plots it is associated with, corresponds to the same horizontal position
of the electron beam on the screen.
4 BK Controls for Single Trace Use
This section describes the controls (knobs and switches) and connectors (jacks) of the BK
scope for single trace use. In what follows, we will use controls to mean knobs, switches,