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X-Y Operation
For
X-Y operation,
the pushbutton in the X field marked X-Y
must be depressed. The X signal is then derived from the
Channel II
(HOR. INP.).
The calibration of the X signal
during X-Y operation is determined by the setting of
the Channel II input attenuator and variable control.
This means that the sensitivity ranges and input imped¬
ances are identical for both the X and Y axes. However, the
Y-POS.II control is disconnected in this mode. Its function
is taken over by the X-POS. control. It is important to note
that the X-MAG. xIO facility, normally used for expanding
the sweep, should not be operated in the X-Y mode. It
should also be noted that the bandwidth of the X amplifier
is >3 MHz (-3dB), and therefore an increase in phase differ¬
ence between both axes is noticeable from 50 kHz
upwards.
The inversion of the X-input signal using the INVERT button
is not possible.
Lissajous figures can
bedisplayed in the X-Ymo</efor cer¬
tain measuring tasks:
- Comparing two signals of different frequency or bringing
one frequency up to the frequency of the other signal.
This also applies for whole number multiples or fractions
of the one signal frequency.
- Phase comparison between two signals of the same fre¬
quency.
Phase comparison with Lissajous figure
The following diagrams show two sine signals of the same
frequency and amplitude with different phase angles.
Calculation of the phase angle or the phase shift between
the X and Y input voltages (after measuring the distances a
and
b
on the screen) is quite simple with the following for¬
mula and a pocket calculator with trigonometric functions
and besides
independent of both deflecting amplitudes
on the screen.
sin cp = |
a
cp = arc sin ^
The following must be noted here:
—
Because of the periodic nature of the trigonometric func¬
tions, the calculation should be limited to angles <90°.
However here is the advantage of the method.
- Do not use a too high test frequency. The phase shift of
the two oscilloscope amplifiers of the HM 203-7 in the X-
Y mode can exceed an angle of 3° above 120 kHz.
- It cannot be seen as a matter of course from the screen
display if the test voltage leads or lags the reference volt¬
age. A CR network before the test voltage input of the
oscilloscope can help here. The 1 MQ input resistance
can equally serve as R here, so that only a suitable
capacitor C needs to be connected in series. If the aper¬
ture width of the ellipse is increased (compared with C
short-circuited), then the test voltage leads the refer¬
ence voltage and vice versa. This applies only in the reg¬
ion up to 90° phase shift. Therefore C should be suffi¬
ciently large and produce only a relatively small just
observable phase shift.
Should both input voltages be missing or fail in the X- Y
mode, a very bright iight dot is dispiayed on the screen.
This dot can burn into the phosphor at a too high
brightness setting (INTENS. knob) which causes either
a lasting loss of brightness, or in the extreme case,
complete destruction of the phosphor at this point.
Phase difference measurement
in DUAL mode
A larger phase difference between two input signals of
the same frequency and shape can be measured very sim¬
ply on the screen in Dual mode (DUAL button depressed).
The time base should be triggered by the reference signal
(phase position 0). The other signal can then have a leading
or lagging phase angle. Alternate mode should be selected
for frequencies >1 kHz; the Chop mode is more suitable
for frequencies <1 kHz (less flickering). For greatest accu¬
racy adjust not much more than one period and approxi¬
mately the same height of both signals on the screen. The
variable controls for amplitude and time base and the
LEVEL knob can also be used for this adjustment - without
influence on the result. Both base lines are set onto the hori¬
zontal graticule center line with the Y-POS. knobs before
the measurement. With sinusoidal signals, observe the
zero (crossover point) transitions; the sine peaks are less
accurate. If a sine signal is noticably distorted by even har¬
monics, or if an offset direct voltage is present, AC coupling
is recommended for
both
channels. If it is a question of
pulses of the same shape, read off at steep edges.
Phase difference measurement in duai mode
t
= horizontal spacing of the zero transitions in cm.
T
= horizontal spacing
for one period In
cm.
In the example illustrated,
t
= 3cm and
T =
10cm. The
phase difference in degrees is calculated from
(p^=—-360°=—-360°= 108°
^
T
10
Subject to change without notice
M9 203-7
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