Hameg HM504-2, Manual

Page: 33 / 42

31
Subject to change without notice
The following explanation assumes that the trace starts on the
left vertical graticule line.
Photo 1
(composite video signal)
MODE: “DEL. MODE” OFF
TIME/DIV: 5ms/div
Trigger coupling: TvF
Trigger slope: falling (-)
Switching over from undelayed to delayed time base automatically
sets the holdoff time to minimum so that the HO LED extinguishes,
the DEL. POS. knob function changes from holdoff time to delay
time control and the READOUT indicates “sea”.
In search (“sea”) mode a part of the previously complete visible
trace becomes blank. The length of the blanked sector depends
on the delay time
(DEL. POS.) setting and can be set between
approx. two and seven divisions after the normal trace start
position. Consequently the trace is displayed with reduced length.
If the maximum delay time is not sufficient, the time coefficient
must be increased (
TIME/DIV -knob) and the DEL. POS.- knob set
to the later starting point.
Note:
Actually the trace start is not really delayed in “sea” (search)
condition, as the blank sector serves only as an adjusting indicator
making visible the delay time which will be active after selecting
“del” (delay time base) or “dTr” (delay time base in triggered
condition).
Photo 2
MODE: “sea” (SEARCH)
TIME/DIV: 5ms/div
Trigger coupling: TvF
Trigger slope: falling (-)
Delay time:
4div x 5ms = 20ms
Figure 2 shows that the delay time can be measured. It is identical
with the displacement of the start of the trace. One can calculate
this by multiplying the blanked out section (horizontal) by the time
deflection coefficient setting.
The full length trace will be visible when switched from “sea”
(SEARCH) to “del” (DELAY), starting with the section previously
selected, providing the (stored) current time deflection coefficient
is not too small.
If the trace is invisible or hardly visible because of too much
expansion (too small deflection coefficient), the time deflection
coefficient must be increased with
TIME / DIV knob. A larger
deflection coefficient than in the “sea” (SEARCH) mode cannot
be set.
Example: The SEARCH setting selected in figure 2 is 5 ms/cm.
The display in “del” (DELAY) mode, also with 5ms/div is delayed
but unexpanded (1:1). A further increase in the deflection
coefficient, e.g. 10ms/div would be meaningless and therefore
automatically blocked.
Please note that the previous time coefficient chosen in “del”
and “dTr” mode is stored and automatically set after activating
one of those modes. If the stored time coefficient in “del” /
”dTr” mode was higher than the actual value in “sea” (search)
mode, the time coefficient in “del” / ”dTr” mode is automatically
set to the value used during “sea” (search) operation.
Photo 3
MODE: “del” (DELAY)
TIME/DIV: 5ms/div
Trigger coupling: TvF
Trigger slope: falling (-)
Delay time:
4div x 5ms/div = 20ms
Reducing the time coefficient (increasing the time base speed)
now expands the signal. If the signal start position is not set to the
optimum, it can still be shifted in the X direction by changing the
delay time.
Photo 4 shows a 50 fold X magnification caused by setting the
time coefficient to 0.1 ms/div (5 ms/div : 0.1 ms/div = 50). The
reading accuracy also increases with higher X magnification.
Photo 4
MODE: “del” (DELAY)
TIME/DIV: 0.1ms/div
Trigger coupling: TvF
Trigger slope: falling (-)
Delay time: 20ms
The delayed and expanded signal display can be triggered again
if a signal slope suitable for triggering appears after the delay
time. For this, one must switch to
“dTr” (2nd triggering after the
expiry of the delay time - after Delay Triggering). The settings
selected before switching, automatic Peak value triggering /
Normal triggering, trigger coupling, the trigger
LEVEL setting and
slope setting, remain valid and trigger the start of the delay time.
The ”After Delay” Triggering automatically switches to normal
triggering (indicated by the NM LED ) and DC trigger coupling.
These default conditions cannot be changed. But the trigger level
(LEVEL) and the trigger slope direction can be altered in order to
enable the triggering at the desired signal section. The trace does
not start and the screen remains blank if the signal amplitude is
not sufficient for triggering or if the setting of the trigger
LEVEL
is unsuitable.
The expanded display can also be displaced in the X direction by
changing the delay time (
DEL.POS. ) under suitable settings.
However, the displacement is not continuous as in the untriggered
“del” (DELAY) operation but jumps from one trigger slope to
another - with most signals this is not evident. This means, in the
case of TV Triggering, that it is possible to trigger not only with
line synchronizing pulses but also on suitable slopes occurring
within the line.
Of course, the magnification is not restricted to a factor 50 as
mentioned in the example. The limit is given by the increasing
loss of trace intensity as the magnification is increased.
Triggering and time base