Tektronix 585 - Page 28

Applications — Type 585
Fig. 3-6. One method for calibrating the oscilloscope display in
degrees per centimeter.
as a reference signal. The two input signals are compared
indirectly to this reference and directly to each other. Con 
sequently the triggering stability must be maintained to
permit accurate phase angle measurements. The external
triggering signal must have sufficient amplitude to insure
stable triggering. The triggering signal must also be related
in frequency to the waveforms on which phase measure 
ments are to be made, however, the actual phase of the
triggering signal is not critical. If you so desire, you may
use one of the signals to be measured as the external trig 
gering signal. It is essential that once the triggering condi 
tions have been established, they are not changed during
any phase measurement.
In phase measurements it is very important that the width
of the trace is not included in the measurements. Measure 
ments must consistently be made from either the top or
Fig. 3-7. Measurement of the phase angle between two electrical
waveforms. It is important to note that the two displayed wave 
forms do not appear simultaneously on the oscilloscope screen.
bottom of the trace. The height of the display should be
made as large as possible to improve accuracy. Accuracy
of the measurements also depends to a large extent on
how well the waveforms are centered about the horizontal
centerline of the graticule. Care should be taken to insure
that the waveforms are properly centered.
Two primary methods of phase measurements have been
described thus far. Method 1 is somewhat easier than
method 2. However, method 2 provides more accurate
results, particularly with small phase angles. Method 1
should be used when moderate accuracy is sufficient and
when large numbers of phase measurements are to be made.
Method 2 should be used when more precise measurements
are required. The two methods are summarized as follows:
Method 1:
1. Connect the external triggering signal to the Time Base
A TRIGGER INPUT connector. Place the HORIZONTAL
DISPLAY switch in the A position and adjust the Time Base
A triggering controls for stable triggering.
Fig. 3-8. An alternate method for measuring the phase angle
between two electrical signals.
2. Connect the first (leading) waveform to the oscilloscope
input. Adjust the Time Base A TIME/CM controls so that
one cycle of the input waveform covers exactly 9 centi 
meters. This corresponds to 40 degrees per centimeter.
3. Carefully center the display waveform about the hori 
zontal centerline. Switch on the magnifier and adjust the
horizontal position of the trace so that the displayed curve
crosses the horizontal centerline at the extreme left mark 
ing of the graticule (see Fig. 3-9). Make this adjustment
with the black HORIZONTAL POSITION control. The
VERNIER HORIZONTAL POSITION control should be placed
in the fully clockwise position.
4. Disconnect the first waveform and connect the second
(lagging) waveform to the oscilloscope. Switch off the mag 
nifier and using only the VERNIER HORIZONTAL POSI 
TION control, position the curve onto the screen as far
Fig. 3-9. Establishing the reference point with the leading wave 
form.
3-6