B&K Precision 2530B Instruction Manual download pdf (Page 3)

the electron beam is turned on and the time base oscillator applies the voltage ramp to the

horizontal axis. The input voltage is applied to the vertical axis. The signal is swept across

the screen from left to right at a constant speed. This takes a time P. At the same time

the vertical deﬂection of the signal is proportional to the input voltage. A spot of light is

observed to move across the screen if the sweep speed is slow enough. This spot of light

traces the input voltage for a time P. If the sweep speed is fast the trace will be too faint

to see unless many sweeps executing the same trace are made. When the ramp reaches V

the electron beam and the light spot reach the right side of the screen. The sweep voltage

is then very quickly returned to V

. During this process the beam is shut oﬀ or “blanked”

so that a return trace is not seen. For a stationary trace to be properly observed the trigger

pulses must all occur at similar points of the input voltage waveform. Those similar points

are taken to be the same voltage and the same slope of the input voltage.

We’re interested in only one of these signals which is the voltage waveform you want to

examine.

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 is to occur.

We call this voltage V

. This can be a positive or negative voltage, but for the trigger to

occur V

, 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. 3, which shows a sinusoidal input voltage and the horizontal sweep

voltage as a function of time. The trigger voltage V

has been chosen as positive. The

triggering slope has also been chosen as positive. Fig. 3 shows that whenever the input

voltage has a positive slope and reaches the value V

, 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/DIV. In Fig. 3 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. 3, 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 time/div sweep speed is decreased, more of the input waveform

or more cycles will be displayed.

• If the trigger voltage V

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

is kept constant but the trigger slope is changed, the trace will

be shifted left or right. For example, Fig. 3 is drawn for triggering to occur on a positive

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B&K Precision 2530B Instruction Manual Page 3

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