33
Subject to change without notice
Triggering and time base
All controls regarding trigger and time base are located on the
right of the VOL
VOL
VOL
VOL
VOLTS/DIV
TS/DIV
TS/DIV
TS/DIV
TS/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
≤
0.5div. 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 can be drawn from the items
NM - AT - (9) LEVEL (11)
and TRIG. MODE (20) in the section
”Controls and Readout”.
This trigger mode is automatically selected after the
AUTOSET
pushbutton is 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
an input signal or external trigger voltage. A base line will always
be displayed even with no signal. With an applied AC signal,
peak value triggering enables the user to select the trigger
point on the displayed signal, by the adjustment of the trigger
level control. The control range depends on the peak to peak
value of the signal. This trigger mode is therefore called
Automatic Peak (Value) Triggering.
Operation of the scope needs only correct amplitude and time
base settings, for a constantly visible trace. Automatic mode is
recommended for all uncomplicated measuring tasks.
However, automatic triggering is also the appropriate operation
mode for the ”entry” into difficult measuring problems, e.g.
when the test signal is unknown relating to amplitude,
frequency or shape. Presetting of all parameters is now possible
with automatic triggering; the change to normal triggering can
follow thereafter.
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
UT
= unmodulated carrier amplitude
Ω
Ω
Ω
Ω
Ω
= 2?F = angular carrier frequency
ω
ω
ω
ω
ω
= 2
π
f = modulation angular frequency
m
= modulation factor.
As well as the carrier frequency F, a lower side frequency F-f
and upper side frequency F+f arise because of the modulation.
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 1:
Amplitude and fre-quency spectrum for AM display (m = 50%)
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.
Figure 2:
Amplitude modulated oscillation:
F = 1 MHz; f = 1 kHz; m = 50 %; UT = 28.3 mVrms
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
