Subject to change without notice
16
Line triggering (~)
A voltage originating from mains/line (50 to 60Hz) is used for
triggering purposes if the trigger coupling (
TRIG.
) is set to
~
.
This trigger mode is independent of amplitude and frequency
of the Y signal and is recommended for all mains/line
synchronous signals. This also applies within certain limits,
to whole number multiples or fractions of the line frequency.
Line triggering can also be useful to display signals below
the trigger threshold (less than 0.5div). It is therefore
particularly suitable for measuring small ripple voltages of
mains/line rectifiers or stray magnetic field in a circuit. In this
trigger mode the ± (
SLOPE
) pushbutton selects the positive
or negative portion of the line sinewave. The
TRIG. LEVEL
control can be used for trigger point adjustment.
Magnetic leakage (e.g. from a power transformer) can be
investigated for direction and amplitude using a search or
pick-up coil. The coil should be wound on a small former
with a maximum of turns of a thin lacquered wire and
connected to a BNC connector (for scope input) via a shielded
cable. Between cable and BNC center conductor a resistor
of at least 100
Ω
should be series-connected (RF decoupling).
Often it is advisable to shield statically the surface of the
coil. However, no shorted turns are permissible. Maximum,
minimum, and direction to the magnetic source are detectable
at the measuring point by turning and shifting the coil.
Alternate triggering
With alternate triggering (
TR I
and
TR II LED´s
lit) it is possible
to trigger two signals which are different in frequency
(asynchronous). In this case the oscilloscope must be
operated in
DUAL
alternate mode and internal triggering each
input signal must be of sufficient height to enable trigger. To
select for alternate triggering the
TRIG.
pushbutton must be
held depressed until both TR I and TR II LED‘s are illuminated.
To avoid trigger problems due to different DC voltage
components, AC input coupling for both channels is
recommended.
The internal trigger source is switched in alternate trigger
mode in the same way as the channel switching system in
DUAL
alternate mode, i.e. after each time base sweep. If a
timebase range (
TIME/DIV
) is chosen where the chopper
generator is automatically activated, switching to alternate
trigger will automatically switch off the chopper generator,
and activate
DUAL
alternate mode. This measure is required
as the chopper generator chops randomly without
synchronization to the time base. Phase difference
measurement is not possible in this trigger mode as the
trigger level and slope setting are equal for both signals. Even
with 180° phase difference between both signals, they appear
with the same slope direction.
External triggering
When in internal trigger mode the
EXT LED
in the Y field is
dark. Depressing the pushbutton below the EXT indicator
switches the EXT LED on. Now the internal triggering is
disconnected and the timebase can be triggered externally
via the
TRIG. INP.
socket using a 0.3Vpp to 3Vpp voltage,
which is in synchronism with the test signal.
This trigger voltage may have a completely different form
from the test signal voltage. Triggering is even possible in
certain limits with whole number multiples or fractions of
the test frequency, but only with synchronous signals.
The input impedance of the
TRIG. INP.
socket is approx.
100k
Ω
II 10pF.
The maximum input voltage of the input circuit is 100V
(DC+peak AC).
It must be noted that a different phase angle between the
measuring and the triggering signal may cause a display not
coinciding with the slope pushbutton setting.
The trigger coupling selection can also be used in external
triggering mode.
Trigger indicator
An LED on condition (above the TRIG. symbol) indicates that
the trigger signal has a sufficient amplitude and the
TRIG.
LEVEL
control setting is correct. This is valid with automatic
and with normal triggering. By observing the trigger LED,
sensitive
TRIG. LEVEL
adjustment is possible when normal
triggering is used, particularly at very low signal frequencies.
The indication pulses are of only 100ms duration.
Thus for fast signals the LED appears to glow continuously,
for low repetition rate signals, the LED flashes at the repetition
rate or at a display of several signal periods not only at the
start of the sweep at the left screen edge, but also at each
signal period.
In automatic triggering mode the sweep generator starts
repeatedly without test signal or external trigger voltage. If
the trigger signal frequency is <20Hz the sweep generator
starts without awaiting the trigger pulse. This causes an
untriggered display and a flashing trigger LED.
Holdoff-time adjustment
If it is found that a trigger point cannot be found on extremely
complex signals, even after careful adjustment of the
TRIG.
LEVEL
control, a stable display may often be obtained using
the
HOLD OFF
control (in the X-field). This facility varies the
holdoff time between two sweep periods approx. up to the
ratio 10:1. Pulses or other signal waveforms appearing during
this off period cannot trigger the timebase.
Particularly with burst signals or aperiodic pulse trains of the
same amplitude, the start of the sweep can be delayed until
the optimum or required time. Another way to trigger such
signals, is to operate the instrument in DELAY mode. The
function of this control is again to delay the sweep start but
the delay time is then visible on the screen as the delay
position
(DEL. POS.)
. See DELAY/After DELAY Triggering.
A very noisy signal or a signal with a higher interfering
frequency is at times displayed double. It is possible
that LEVEL adjustment only controls the mutual phase
shift, but not the double display. The stable single
display of the signal, required for evaluation, is easily
obtainable by expanding the hold off time. To this end
the HOLD OFF knob is slowly turned to the right, until
one signal is displayed.
A double display is possible with certain pulse signals, where
the pulses alternately show a small difference of the peak
amplitudes. Only a very exact
TRIG. LEVEL
adjustment
makes a single display possible. The use of the
HOLD OFF
knob simplifies the right adjustment.
After specific use the HOLD OFF control should be reset
into its calibration detent (fully ccw), otherwise the brightness
of the display is reduced drastically. The function is shown in
the following figures.
