26
Subject to change without notice
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 (i.a. 1 100%).
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 = 1MHz; f = 1kHz;
m = 50%; U
T
= 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 = U
T
(1+m) and b = U
T
(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
threshold can be stated as vertical display height in div, at
which the time base generator starts, the display is stable, and
the trigger indicator LED lights or flashes.
The internal trigger threshold of the oscilloscope is given as £
5div. When the trigger voltage is externally supplied, it can be
measured in Vpp at that input. Normally, the trigger threshold
may be exceeded up to a maximum factor of 20.
The instrument has two trigger modes, which are characterized
as Automatic Peak and Normal triggering.
Automatic Peak (value) -Triggering
Instrument specific information’s can be drawn from the
items
NM - AT -
(15)
and
LEVEL (17)
in the section
“Controls and readout”.
This trigger mode is automatically selected after the
AUTO-
SET
pushbutton was pressed. As the peak value detection
makes no sense in combination with
DC
and
TV
(television)
signals, it is switched off automatically in
DC
,
TVL
and
TVF
trigger coupling conditions as well as in alternate trigger mode.
In this case the automatic is still present, but a wrong trigger
level setting causes an untriggered display.
In automatic trigger mode the sweep generator can run
without test signal or external trigger voltage. A base line will
always be displayed even with no signal. With an applied AC
signal the peak value triggering enables the user to select the
Triggering and time base
