Hold-Off Time Adjustment; Autoset - Hameg HM400 Handbuch

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.
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-
34
OFF/ON .
If no stable display can be obtained even with very careful adjustment of
14
the TRIGGER-LEVEL control in normal trigger mode, an adjustment
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.
period
sweep
adjusting
HOLD OFF time
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
20
and turn the TIME/DIV knob slowly CW until a stable display is
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.
See also the information given under AUTOSET
Controls
As mentioned in the chapter Controls all front panel controls are
electronically read out, hence the instrument can also be completely
electronically controlled. This allows for a fully automatic signal-derived
setting of all controls in the Yt mode. In most cases manual settings will
be superfl uous. When AUTOSET is activated, the instrument will enter
the Yt mode if XY was selected; if it was already in Yt mode, the settings
will remain unaffected unless ADD was selected which will be set to
DUAL. In one-channel mode the sensitivity is automatically chosen so
that the signal will be displayed with appr. 6 divisions; in DUAL channel
mode each channel display will be appr. 4 divisions.
The foregoing and the description of the time base setting apply to
signals which do not differ too much from a duty cycle of 1:1. The auto-
matic selection of a suitable time base speed will show appr. 2 signal
periods. With signals which contain several frequencies the settings
obtained are governed by chance.
Pressing the AUTOSET button will have these results:
– The input coupling (AC, DC) remains unchanged resp. the last
setting before switching to GND is resumed.
– Internal triggering.
heavy parts are displayed
– Automatic triggering.
– Automatic selection of the trigger source
– The trigger level will be set to the center of its range.
– VOLTS/DIV set to calibrated (variables off)
– TIME/DIV set to calibrated (variable off)
signal
– AC or DC trigger coupling unchanged
– Magnifi er off
– X and Y positions automatic
– Trigger slope selection unchanged
Fig. 1
– Visible trace
Selecting AUTOSET will leave the selected AC or DC input coupling
unchanged. In case DC trigger coupling was selected, this will not be
changed to AC. The automatic triggering functions without peak-to-peak
detection. The AUTOSET settings will override any former settings. In
case variables were activated, they will be disabled, such that all settings
will be calibrated. After AUTOSET was activated, manual control can
be executed. Due to the reduced bandwidth in 1 and 2 mV/DIV these
Fig. 2
ranges will not be used in AUTOSET.
STOP
It is recommended to switch to normal trigger mode and to set the
trigger point about 5 mm above or below the screen center. If the TRIG'd
LED lights up, the signal was recognized. In order to render it visible,
the time base speed and the sensitivity must be increased, however, the
trace may dim so much that the pulse may remain invisible.

AUTOSET

If a pulse signal is applied the duty cycle of which reaches
or exceeds 1:400, an automatic display will in most cases
become impossible. In such cases only the freerunning
trace will be visible.
Subject to change without notice
A U T O S E T
10
in the chapter
37