32
Subject to change without notice
Triggering and time base
Please note that the Y position settings are also added but are
not affected by the invert function.
Differential measurement techniques allow direct measurement
of the voltage drop across floating components (both ends above
ground). Two identical probes should be used for both Y inputs.
In order to avoid ground loops, use a separate ground connection
and do not use the probe ground leads or cable shields.
X-Y Operation
The important control for this mode is the pushbutton labelled
DUAL
and
MENU (16)
.
In
XY
mode the time base is deactivated. The signal applied
to the input of channel I front panel marking INPUT CHI (X)
causes the X deflection. The input related controls (
AC/DC/
GND
pushbutton and the
VOLTS/DIV
knob) consequently
affect the X deflection. For X position alteration, the
X-POS
.....
control knob must be used, as the
Y-POS./CURS.I
control is
automatically deactivated. The input deflection coefficient
ranges are the same for both channels, because the
X x10
magnifier is inactive in
XY
mode.
The bandwidth of the X amplifier, is lower than the Y amplifier
and the phase angle which increases with higher frequencies,
must be taken into account (please note data sheet).
The Y signal applied at INPUT CHII can be inverted.
Lissajous figures can be displayed in the X-Y mode for certain
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
frequency.
Phase comparison with Lissajous figures
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 formula, and a pocket
calculator with trigonometric functions. Apart from the reading
accuracy, the signal height has no influence on the result.
The following must be noted here:
Because of the periodic nature of the trigonometric
functions, the calculation should be limited to angles
≤
90°
However here is the advantage of the method.
Due to phase shift, do not use too high a test frequency.
It cannot be seen as a matter of course from the screen
display if the test voltage leads or lags the reference
voltage. A CR network before the test voltage input of the
oscilloscope can help here. The 1M Ohm input resistance
can equally serve as R here, so that only a suitable capacitor
C needs to be connected in series. If the aperture width of
the ellipse is increased (compared with C short-circuited),
then the test voltage leads the reference voltage and vice
versa. This applies only in the region up to 90° phase shift.
Therefore C should be sufficiently large and produce only
a relatively small, just observable phase shift.
Should both input voltages be missing or fail in the XY mode, a
very bright light dot is displayed on the screen. This dot can burn
into the phosphor at too high a brightness setting (INTENS. setting)
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 (Yt)
Phase differences between two input signals of the same
frequency and shape can be measured very simply on the
screen in Dual mode. 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. In alternate
triggering condition, phase difference measurement is not
possible.
For greatest accuracy, adjust the time base for slightly over
one period and set approximately the same height of both
signals on the screen. The Y deflection coefficients, the time
base coefficient and the trigger level setting can be used for
this adjustment, without influence on the result. Both base
lines are set onto the horizontal graticule center line using the
Y POS. knobs before the measurement. With sinusoidal
signals, use the zero (crossover point) transitions; the sine
peaks are less accurate. If a sine signal is noticeably distorted
by even harmonics, or if a DC voltage is present, AC coupling
is recom-mended for both channels. If it is a question of pulses
of the same shape, read off at steep edges.
It must be noted that the phase difference cannot be
determined if alternate triggering is selected.
Phase difference measurement in DUAL mode
t
= horizontal spacing of the zero transitions in div
T
= horizontal spacing for one period in div
In the example illustrated, t = 3div and T = 10div The phase
difference in degrees is calculated from
or expressed in radians