Tektronix 585 - Page 27

Applications — Type 585
X
1
10X TIME/CM
Switch Setting
__ Frequency
Fig. 3-5. Measuring the frequency of a repetitive input signal.
To obtain the frequency of a repetitive input signal, per 
form the following steps:
1. Adjust the TIME/CM control to display several cycles of
the input waveform. Insure that the VARIABLE TIME/CM
control is in the CALIBRATED position.
2. Count the number of cycles of the waveform shown on
the 10 centimeters of the graticule.
3. Divide this number by 10 times the TIME/CM switch set 
ting. This gives you the frequency of the input waveform.
For example, assume that when you are using a sweep
rate of 50 milliseconds per centimeter, you count 7.2 cycles
in 10 centimeters. The frequency is 7.2 cycles divided by
50 milliseconds times 10, or 500 milliseconds. 500 milli 
seconds is equal to 500 x .001 second, or 0.5 second. 7.2
cycles per ½½ second gives you 14.4 cycles per second.
Phase Measurements
In many fields of engineering and science, it is frequently
necessary to accurately measure the phase angle between
two electrical signals. One method for making such meas 
urements involves the use of a cathode ray oscilloscope.
In this application phase angle measurements can be made
directly from the crt display.
You will recall that a complete cycle of a sinusoidal
waveform is 360 degrees. Using this fact it is possible to
calibrate the oscilloscope display directly in degrees per
centimeter by means of the TIME/CM controls. For example,
if the TIME/CM controls are adjusted so that one cycle of
the input waveform covers 9 centimeters (see Figure 3-6),
each centimeter then corresponds to 40 degrees. Under
this condition the display is calibrated to 40 degrees per
centimeter.
It is therefore possible to measure phase angles by: (1)
calibrating the display in degrees per centimeter; (2) meas 
uring the displacement between corresponding points on the
two phases; and (3) multiplying the displacement by the
number of degrees per centimeter. This is the method il 
lustrated in Figure 3-7. Note that the relative amplitude of
the two signals does not affect the phase measurement so
long as the signals are both centered about the horizontal
centerline. It is important to note that the two waveforms
shown in the illustration do not appear simultaneously on
the oscilloscope screen. The first waveform is displayed and
positioned to a convenient reference point. The second
waveform is then displayed and compared to this reference
point.
Referring to Fig. 3-8, it can readily be seen that the phase
angle between the two waveforms is 360° x A/B. It is ap 
parent that it is not necessary to calibrate the display in
degrees per centimeter as long as it is possible to measure
A and B accurately. It is particularly important to measure
A accurately when the phase angle is less than 10°. The
delayed sweep feature provides the means for making
these measurements. The phase angle can be found by: (1)
measuring A; (2) measuring B; and (3) multiplying A/B by
360°. Here again the relative amplitude of the two wave 
forms is unimportant as long as the waveforms are centered
about the horizontal centerline of the graticule.
When using the Type 585 Oscilloscope for phase measure 
ments, it is necessary to supply an external triggering signal
to the oscilloscope. This triggering signal serves, in a sense.
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