Line Triggering; External Triggering; Triggered State Indicator Led Trig'd; Hold-Off Time Adjustment - Rohde & Schwarz Hameg HM400 Manuel

T r i g g e r i n g a n d t i m e b a s e
otherwise the display will shift vertically depending on the video content.
Use the POSITION control to keep the display within the screen area.
The sync separator circuit is also effective with external triggering. Of
course, the specifi ed voltage range (see the specifi cations) must be
observed. Note that the polarity of external sync signals can be any,
i.e. it can differ from that of the composite signal at the vertical input,
hence the SLOPE must be selected accordingly. In order to check the
external trigger signal, display it by applying it to a vertical input with
internal triggering.

LINE triggering

In the LINE trigger mode a signal from the mains power supply is taken
(50/60 Hz). This mode is recommended for all signals of mains frequency
or synchronous with it. Within limits this also applies for multiples or
submultiples of the mains frequency. LINE triggering will also yield
stable displays if the input signal is very small, i.e. below the trigger
threshold. It is hence especially handy for all kinds of mains frequency
ripple or interference measurements. With LINE triggering the SLOPE
selection will select the positive or negative half wave and not the slope,
hence it may be necessary to pull the mains plug and insert it upside
down if that is possible (not in all countries). In the auto trigger mode
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the TRIGGER-LEVEL will allow to move the trigger point within the
half wave selected. In the normal trigger mode the trigger point can
also be moved outside the selected half wave.
Magnetic interference from the mains can be detected by using a pick-
up coil which allows to determine the direction and the amplitude. The
coil should preferably sport a high number of turns of thin enamel wire
on a small coil former, a shielded cable with a BNC connector should
be used for the connection to the scope. A 100 Ω resistor should be
inserted between the cable and the BNC in order to reduce HF inter-
ference, a ceramic capacitor to ground may be additionally required.
Also, the coil should have a static shield (no short circuit winding). By
turning the coil the minima and maxima of the magnetic interference
are detectable.

External triggering

External triggering is selected by pressing the button EXT
disconnect the internal triggering. The external signal is to be connected
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to the EXT.TRIG/Z-INP BNC connector, see the specifi cations for the
required signal levels. The external trigger signal must be synchronous
to the measuring signal at the Y input(s), but its shape may be entirely
different. Within limits triggering is even possible from multiples or
submultiples of the measuring frequency. A stable phase relationship
is, however, necessary. There may be a phase difference between the
measuring and triggering signals; if the phase difference happens to
be 180 degrees, the other slope has to be selected, else the signal will
be displayed with a starting negative slope although the positive slope
was selected.
The maximum input voltage at the EXT.TRIG./Z-INP
BNC connector is 100 V (DC plus peak AC). The input impe-
dance is 1 MΩ//15 pF.
STOP
The trigger coupling modes are also effective with external triggering.
The only difference to internal triggering is a capacitor in the signal path
(except with DC coupling), the lower bandwidth limit is 20 Hz.

Triggered state indicator LED TRIG'd

The following explanations refer to the TRIG'd – LED – indicator which
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is listed under in the chapter Controls. It will light up if
1. an internal or external trigger signal of suffi cient amplitude is
available at the trigger comparator.
2. and if the reference voltage at the trigger comparator is set to a
level such that the signal slopes will cross this level.
34 Subject to change without notice
If these conditions are fulfi lled, the trigger comparator will deliver
pulses for starting the time base, and a stable display will result.
This indicator is handy for adjusting and controlling the trigger con-
ditions, especially in case of very low frequency signals or very short
pulses. With signals with extremely slow rep rates the LED will light
up pulsed. The indicator will also blink not only when the time base is
started at the left hand side of the screen, but with dual trace operation
with every start of a trace.

Hold-off time adjustment

Further information is available in the chapter Controls under HOLD-
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OFF/ON .
If no stable display can be obtained even with very careful adjustment of
the TRIGGER-LEVEL
of the hold-off time may help.
The hold-off time is required in each analog scope in order to allow
suffi cient time for the retrace of the beam from the right hand to the
left hand side of the screen. During this time trigger pulses from the
trigger comparator are ignored. The HM400 allows for an adjustment
(increase) of 10:1 of the hold-off time. With complex signals, burst
signals or non-periodic signals the time when the time base will accept
the next trigger pulse can then be changed such that a stable display
is achieved.
Sometimes a noisy signal or one which is corrupted by HF will cause
multiple displays. Mostly, the TRIGGER-LEVEL control can only affect
the apparent time difference between the displays. By increasing the
hold-off time a stable display is almost always possible. The following
pictures demonstrate the function of the hold-off.
19 , this will
sweep
31
Fig. 1 shows the screen display with minimum hold-off time (basic
setting). A double display is shown because different portions of the
signal are displayed.
Fig. 2: Here, the hold-off time was adjusted such that always the same
signal portions are displayed, a stable display is obtained.
In order to change the hold-off time, press the HOLD-OFF/ON button
and turn the TIME/DIV
found.
Double displays are also possible with pulse trains when the amplitudes
alternately differ by a small amount. Careful setting of the trigger level
and of the hold-off time may be required for correct displays.
Any time the hold-off time was changed from its basic minimum setting
it should be reset because too long a hold-off time will cause the time
base rep rate to decrease which can dim the display.
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control in normal trigger mode, an adjustment
period
adjusting
HOLD OFF time
15 knob slowly CW until a stable display is
heavy parts are displayed
signal
Fig. 1
Fig. 2