Leader LBO-516 Instruction Manual - page 26
and measured. The ground terminals from the two probes or
cables are simply connected to the chassis or ground bus of the
circuit under observation. Figure 2-16b shows the connection
technique needed for low-level signals in a noisy environment
(strong AC fields). Using a separate ground connection and
not
connecting the probe or cable shields to the circuit under test
avoids ground loops and EMI pickup. This setup allows fullest
utilization of the CMRR (common-mode rejection ratio) of the
LBO-516's differential facility.
2-4-3 Time Interval Measurements
The second major measurement function of the triggered-
sweep oscilloscope is the measurement of time interval. This is
possible because the calibrated timebase results in each division
of the CRT screen representing a known time interval.
Basic Technique. The basic technique for measuring time
interval is described in the following steps. This same technique
applies to the more specific procedures and variations that follow.
1. Set up the LBO-516 as described in
2-3-2 Single-trace
Operation.
2. Set the A TIME/DIV switch (24) so that the interval you
wish to measure is totally on screen and as large as possible.
Make certain the A VARIABLE control (26) is rotated fully
clockwise and detented in its CAL'D position. If the UNCAL
lamp (27) is lit, any time interval measurements made under
this position will be inaccurate.
3. Use the vertical POSITION control (17) or (18) to position
the trace so the central horizontal graticule line passes
through the points on the waveform between which you want
to make the measurement.
4. Use the horizontal POSITION controls (29) and (30) to set
the left-most measurement point on a nearby vertical
graticule line.
5. Count the number of horizontal graticule divisions between
the Step 4 graticule line and the second measurement point.
Measure to a tenth of a major division. Note that each minor
division on the central horizontal graticule line is 0.2 major
division.
6. To determine the time interval between the two measurement
points, multiply the number of horizontal divisions counted
in Step 5 by the setting of the A TIME/DIV switch. If the A
VARIABLE control (26) is pulled out (X10 MAG), be
certain to divide the TIME/DIV switch setting by 10.
Period, Pulse Width, and Duty Cycle. The basic technique
described in the preceding paragraph can be used to determine
pulse parameters such as period, pulse width, duty cycle,etc.
The period of a pulse or any other waveform is the time it
takes for one full cycle of the signal. In Figure 2-17, the distance
between points (A) and (C) represent one cycle; the time interval
of this distance is the period. The time scale for the CRT display
of Figure 2-17a is 10 mS/div, so the period is 70 milliseconds in
this example.
Pulse width is the distance between points (A) and (B). In
our example it is conveniently 1.5 divisions, so the pulse
width is 15 milliseconds. However, 1.5 divisions is a rather small
distance for accurate measurements, so it is advisable to use a
faster sweep speed for this particular measurement. Increasing the
sweep speed to 2 mS/div as in Figure 2-17b gives a large display,
allowing more accurate measurement. An alternative technique
useful for pulses less than a division wide is to pull the A
VARIABLE (X10 MAG) control (26) outward, and reposition the
pulse on screen with the horizontal POSITION control (29). Pulse
width is also called
"on" time in some applications. The distance
between points (B) and (C) is then called
off time. This can be
measured in the same manner as pulse width.
When pulse width and period are known, duty cycle can be
calculated. Duty cycle is the percentage of the period (or total of
on and ofttimes) represented by the pulse width (on time).
PW(100) A >
>
>
> B(100)
Duty cycle (%) = Period = A >
>
>
> C
1
5mS X 100
Duty cycle of example = 70 mS = 21.4%
Lead and Lag Time. When two signals have the same fre-
quency, but not the same phase, one signal is said to be
leading,
and the other
lagging. To measure this lead/lag time, proceed as
follows:
1. Set up the LBO-516 as described in
2-3-5 Dual-trace Op-
eration, connecting one signal to the CH-1 IN connector (14)
and the other to the CH-2 IN connector (15).
NOTE: At high frequencies use identical and correctly
compensated probes, or equal lengths of the same type
of coaxial cable to ensure equal delay times.
2. Position the trigger SOURCE selector (34) to the channel
with the leading signal (CH- 1 in the Figure 2-18 example).
3. Use the A TIME/DIV switch (24) to display the time
difference as large as possible (Figure 2-18b).
4. Use the CH- l vertical POSITION control (17) to drop the
bottom of the channel 1 trace a little below the central hori-
zontal graticule line, and the CH-2 vertical POSITION
control (18) to raise the top of the channel 2 trace a little
above the line.
5. Use the horizontal POSITION controls (29) and (30) to align
the left-most trace edge (of channel I in this case) with a
nearby vertical graticule line. The horizontal distance
between this line and the point at which the leading edge of
the other trace crosses the central horizontal graticule line
represents the time difference between the two signals. The
channel 1 signal may be said to he leading the channel 2
trace, or the channel 2 trace may be said to be lagging the
channel 1 trace, depending on the point of view.
6. Make sure the A VARIABLE timebase control (26) is ro-
tated fully clockwise and detented in its CAL'D position.
Then, count the number of horizontal divisions between the
leading edges of the traces and multiply this number by the
setting of the A TIME/DIV switch to determine the time
difference. For example, the time difference in Figure 2-18b
is l0 microseconds (5.0 div X 2/PS).
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