Tektronix 585 - Page 26

Applications — Type 585
Sweep Magnification
Fig. 3-4. Measuring the time interval between
events displayed on the oscilloscope screen.
Maximum Accuracy Time Measurements
The maximum accuracy method of time measurement in 
volves the delayed sweep feature of the Type 585 Oscillo 
scope. This method uses the accuracy of only the DELAY 
TIME MULTIPLIER dial, with the possibility of an ultimate
accuracy of ±0.2%. In cases where an accuracy of ±0.2%
is required, the Time Base B calibration should be checked
on the individual TIME/CM range used, using steps 39 and
40 of the Calibration procedure.
After establishing the accuracy of the Time Base B TIME/
CM range to be used, proceed as follows.
1. Complete the time measurement as described in the pre 
vious section on Increased Accuracy Time Measurements.
Be sure the Time Base A STABILITY control is fully clockwise
at free-run.
2. Return the DELAY-TIME MULTIPLIER dial to the beginning
of the interval to be measured. Set the HORIZONTAL DIS 
PLAY switch to 'A ’ DEL'D BY B'. With the DELAY-TIME
MULTIPLIER dial, horizontally position the beginning of the
interval to be measured to the graticule centerline. (The
graticule centerline is now the reference point of the crt dis 
play for the following measurements.) Record the DELAY 
TIME MULTIPLIER dial reading.
3. Set the HORIZONTAL DISPLAY switch to B' INTENSIFIED
BY A', and rotate the DELAY-TIME MULTIPLIER to the end
of the interval being measured. Set the HORIZONTAL DIS 
PLAY switch to A DEL ’ D BY B'. With the DELAY-TIME
MULTIPLIER dial, horizontally position the end of the interval
to the graticule centerline. Record the DELAY-TIME MULTI 
PLIER dial reading.
4. Subtract the first DELAY-TIME MULTIPLIER dial reading
from the second reading and multiply the result by the set 
ting of the Time Base B TIME/CM or DELAY TIME control.
The figure obtained can be within 0.2% of the true time of
the measured interval.
Frequency Measurements
Using one of the three methods described in the previous
section, you can measure the period (time required for one
cycle) of a recurrent waveform. The frequency of the wave 
form can then be easily calculated since frequency is the
reciprocal of the period. For example, if the period of a
recurrent waveform is accurately measured and found to
be 0.2 microsecond, the frequency is the reciprocal of 0.2
microsecond, or 5 m c .
At any given oscilloscope sweep rate, the number of
cycles of the input waveform that are displayed on 10
centimeters of the crt screen is dependent on the frequency
of the input waveform. At a sweep rate of 0.1 microsecond
per centimeter, for example, 6 cycles are displayed with
a 6 m c input signal, 5 cycles with a 5 m c signal, and 4 cycles
with a 4 m c input signal. Frequency can usually be meas 
ured faster by the following method than by the one given
in the preceding paragraphs.
If you divide the cycles per unit of length by the time
required for the input waveform to sweep this unit of length,
you get the frequency. For greater accuracy, make your
calculations by using the total number of cycles displayed
in 10 cm. Since the TIME/CM switch gives time for 1 cm,
multiply this setting by 10 to obtain the time required for
10 cm. (See Figure 3-5).
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