23
Subject to change without notice
Operating modes of the vertical amplifiers in Yt mode
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 a too high 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
Ω
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 X-
Y mode, a very bright light dot is displayed 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 (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 approximately the same height of both signals
on the screen. The Y deflection coefficients, the time base
coefficients 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,
observe 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
recommended 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
Relatively small phase angles at not too high frequencies can
be measured more accurately in the X-Y mode with Lissajous
figures.
Measurement of an amplitude modulation
The momentary amplitude u at time t of a HF-carrier voltage,
which is amplitude modulated without distortion by a sinusoidal
AF voltage, is in accordance with the equation
where
U
T
= unmodulated carrier amplitude
Ω
= 2
π
F = angular carrier frequency
ω
= 2
π
f = modulation angular frequency
m = modulation factor.
The lower side frequency F-f and the upper side frequency
F+f arise because of the modulation apart from the carrier
frequency F.
The display of an amplitude-modulated HF oscillation can be
evaluated with the oscilloscope provided the frequency
spectrum is inside the oscilloscope bandwidth. The time base
is set so that several cycles of the modulation frequency are
visible. Strictly speaking, triggering should be external with
modulation frequency (from the AF generator or a demo-
dulator). However, internal triggering is frequently possible
with normal triggering using a suitable trigger level setting and
possibly also using the time vernier (variable) adjustment.
