October 2014
Page
113
4.5
Example 5: Examine the Phase shift between two related
signals
X-Y mode is a very useful when examining the Phase shift of two related
signals. This example takes you step by step to check out the phase change
of the signal after it passes a specified circuit. Input signal to the circuit and
output signal from circuit are used as sources signals.
Set up attenuation coefficient to
10X
through Probe menu for both
CH1 and CH2. Switch Probes to
10X
. For source signals, CH1 takes in
input signal to the circuit and CH2 takes in output signal from circuit.
Press
AUTOSET
, adjust
VOLTS/DIV
for both CH1 and CH2 to the
same amplitude level to get an ellipse.
Press
DISPLAY
to activate
DISP SET
menu, then F3 to select XY
mode. Now the waveform is displayed as
Lissajou's curve
, adjust
VOLTS/DIV
and
VERTICAL POSITION
to get the best possible
display, work out the phase shift as in Fig.108.
Figure 108.
Lissajous Graph.
Based on the expression
sin (q) =A/B or C/D
, there into,
q
is the phase
difference angle, and the definitions of A, B, C, and D are shown as the graph
above. As a result, the phase difference angle can be obtained, namely,
q =±
arcsin (A/B)
or
± arcsin (C/D)
. If the principal axis of the ellipse is in the I
and III quadrants, the determined phase difference angel should be in the I and
IV quadrants, that is, in the range of (0
~
π
/2) or (3
π
/ 2
~
2
π
). If the principal
axis of the ellipse is in the II and IV quadrants, the determined phase difference
angle is in the II and III quadrants, that is, within the range of (
π
/ 2
~
π
) or (
π
~
3
π
/2).
The signal must be
centered and kept in the
horizontal direction.
