Hameg HM304, Руководство пользователя

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15 Subject to change without notice
coupling mode. In this case the automatic is still present, but
a wrong
TRIG. LEVEL setting causes an untriggered display.
Automatic triggering is practicable with internal and external
trigger voltage.
In alternate triggering mode (TR I and TR II lit) the peak value
detection is switched off.
Normal Triggering
With normal triggering (both NORM pushbuttons depressed
until the NM LED is lit) and TRIG. LEVEL adjustment, the
sweep can be started by AC signals within the frequency
range defined by the TRIG. coupling setting. In the absence
of an adequate trigger signal or when the trigger controls
(particularly the TRIG. LEVEL control) are misadjusted, no
trace is visible, i.e. the screen blanked completely.
When using the internal normal triggering mode, it is possible
to trigger at any amplitude point of a signal edge, even with
very complex signal shapes, by adjusting the TRIG. LEVEL
control. Its adjusting range is directly dependent on the display
height, which should be at least 0.5div. If it is smaller than
1div., the TRIG. LEVEL adjustment needs to be operated with
a sensitive touch. In the external normal triggering mode,
the same applies to approx. 0.3Vpp external trigger voltage
amplitude.
Other measures for triggering of very complex signals are
the use of the time base variable control and HOLDOFF time
control, hereinafter mentioned.
Slope
The time base generator can be triggered by a rising or falling
edge of the test signal. The
± pushbutton marking selects
the slope polarity. If the LED above the pushbutton is lit, the
(
- ) falling edge is used for triggering. This is valid with
automatic and normal triggering. The positive ( + ) slope
direction (LED dark) means an edge going from a negative
potential and rising to a positive potential. This has nothing
to do with zero or ground potential and absolute voltage
values. The positive slope may also lie in a negative part of a
signal. A falling (
- ) edge will trigger the timebase when the
minus symbol is lit.
However the trigger point may be varied within certain limits
on the chosen edge using the LEVEL control. The slope
direction is always related to the input signal and the non
inverted display.
Trigger coupling
The coupling mode and accordingly the frequency range of
the trigger signal can be changed using the upper or lower
NORM pushbutton. The selected coupling mode is indicated
on the LED scale above.
AC: Trigger range <20Hz to 100MHz .
This is the most frequently used trigger mode. The trigger
threshold is increasing below 20Hz and above 100MHz.
The AUTO SET function always selects AC trigger
coupling.
DC: Trigger range DC to 100MHz .
DC triggering is recommended, if the signal is to be
triggered with quite slow processes or if pulse signals
with constantly changing pulse duty factors have to be
displayed.
With DC- or LF-trigger coupling, always work with nor-
mal triggering (NM) and TRIG.LEVEL adjustment. If
automatic (peak-value) triggering was in use, the peak
value detection is then switched off automatically.
LF: Trigger range DC to 1.5kHz (low-pass filter).
The LF coupling is often more suited for low-frequency
signals than the DC coupling, because the (white) noise
in the trigger voltage is strongly suppressed. So jitter or
double-triggering of complex signals is avoidable or at
least reduced, in particular with very low input voltages.
The trigger threshold increases above 1.5kHz.
TV-L / TV-F: The built-in active TV-Sync-Separator provides
the separation of sync pulses from the video signal.Even
distorted video signals are triggered and displayed in a
stable manner.
Video signals are triggered in the automatic mode (NM LED
dark). The internal triggering is virtually independent of the
display height, but the sync pulse must exceed 0.5div. height.
TV-L is for line sync pulse separation and triggering, while
TV-F is for field sync pulse separation and triggering.
The slope of the leading edge of the synchronization pulse is
critical for the
SLOPE selection. If the displayed sync pulses
are above the picture (field) contents (leading edge positive
going), then the positive going
SLOPE (+) must be chosen.
In the case of sync pulses below the field/line, the leading
edge is negative and - (minus) symbol above the ± pushbutton
must lit. Since the INV (invert) function may cause a
misleading display, it must not be activated (INV LED dark).
On the 2ms/div setting and field TV triggering (TV-F) selected
1 field is visible if a 50 fields/s signal is applied. If the hold off
control is in fully ccw position, it triggers without line
interlacing affects caused by the consecutive field. More
details in the video signal become visible if in delayed trigger
mode the timebase speed is increased (see
DELAY / AFTER
DELAY TRIGGERING ). The X-MAG. (x10) expansion may also
be used (x10 LED lit). The influence of the integrating network
which forms a trigger pulse from the vertical sync pulses
may become visible under certain conditions. Due to the
integrating network time constant not all vertical sync pulses
starting the trace are visible.
Disconnecting the trigger circuit (e.g. by double depressing
and releasing the
EXT. button next to the TRIG. INP.
BNC socket in the Y field) can usually result in triggering the
consecutive (odd or even) field.
On the 10µs/div setting and line TV triggering (
TV-L ) selected,
approx. 1 ½ lines are visible. Those lines originate from the
odd and even fields at random.
The sync-separator-circuit also operates with external
triggering. It is important that the voltage range (0.3Vpp to
3Vpp) for external triggering should be noted. Again the
correct slope setting is critical, because the external trigger
signal may not have the same polarity or pulse edge as the
test signal displayed on the CRT. This can be checked, if the
external trigger voltage itself is displayed first (with internal
triggering).
In most cases, the composite video signal has a high DC
content. With constant video information (e.g. test pattern
or color bar generator), the DC content can be suppressed
easily by AC input coupling of the oscilloscope amplifier.With
a changing picture content (e.g. normal program), DC input
coupling is recommended, because the display varies its
vertical position on screen with AC input coupling at each
change of the picture content. The DC content can be
compensated using the
Y-POS. control so that the signal
display lies in the graticule area. Then the composite video
signal should not exceed a vertical height of 6div.