25
Subject to change without notice
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
Relatively small phase angles at lower frequencies can be
measured more accurately in the X-Y mode with Lissajous
figures.
Measurement of 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.
Figure 1
Amplitude and frequency spectrum for AM display (m = 50%)
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 demodulator). However, internal
triggering is frequently possible with normal triggering using a
suitable trigger level setting and possibly also using the time
vernier (variable) adjustment.
Figure 2
Amplitude modulated oscillation:
F = 1 MHz; f = 1 kHz; m = 50 %; UT = 28.3 mVrms.
Oscilloscope setting for a signal according to figure 2:
Y: CH. I; 20mV/div; AC.
TIME/DIV.: 0.2ms/div.
Triggering: Normal; with LEVEL-setting; internal
(or external) triggering.
If the two values a and b are read from the screen, the modulation
factor is calculated from
where
a = UT (1+m)
and
b = UT (1-m)
.
The variable controls for amplitude and time can be set arbitrarily
in the modulation factor measurement. Their position does not
influence the result.
Triggering and Time Base
All controls regarding trigger and time base are located on the
right of the VOLTS/DIV. knobs. They are described in the section
“Controls and Readout”
.
Time related amplitude changes on a measuring signal (AC
voltage) are displayable in Yt-mode. In this mode the signal voltage
deflects the beam in vertical direction (Y) while the time base
generator moves the beam from the left to the right of the screen
(time deflection = t).
Normally there are periodically repeating waveforms to be
displayed. Therefore the time base must repeat the time
deflection periodically too. To produce a stationary display, the
time base must only be triggered if the signal height and slope
condition coincide with the former time base start conditions. A
DC voltage signal can not be triggered as it is a constant signal
with no slope.
Triggering can be performed by the measuring signal itself
(internal triggering) or by an external supplied but synchronous
voltage (external triggering).
The trigger voltage should have a certain minimum amplitude.
This value is called the trigger threshold. It is measured with a
sine signal. Except when external trigger is used the trigger
Triggering und Time Base
