Hameg HM504-2, Руководство пользователя

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Subject to change without notice
positive going edges must be chosen. In the case of sync pulses
below the field/line, the leading edge is negative and consequently
the slope selection must be set for falling edges. Since the invert
function may cause a misleading display, it must not be activated.
On the 2 ms/div setting and field TV triggering selected, 1 field is
visible if a 50 fields/s signal is applied. If the holdoff control is in
fully ccw position, it triggers without line interlacing affects
caused by the consecutive field.
The display can be expanded by switching on the X-MAG. x10
function so that individual lines are recognizable. Commencing
with a frame synchronizing pulse, the display can also be expanded
with the knob
TIME/DIV. But note that this can result in an
apparently unsynchronized display as each frame (half picture)
triggers. This is due to the off set of half a line between frames.
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.
On the 10µs/div setting and line TV triggering selected, approx.
1½ 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 (0.3 V
pp
to 3 V pp ) 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 colour 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.
Line/Mains triggering (~)
The instrument specific information regarding this mode is part of
the section ”Controls and Readout” paragraph
TRIG. MODE
[20] .
This trigger mode is present if the READOUT indicates the “~”
symbol instead of the “trigger source”, “slope” and “coupling”
information. The trigger point symbol is inactive in line/mains
trigger mode as there is no direct amplitude relationship between
the trigger voltage and the signal voltage.
A voltage originating from mains/line (50 to 60 Hz) is used for
triggering purposes if the trigger coupling 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 0.5 div).
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 direction pushbutton selects the
positive or negative portion of the line/mains sine wave. The
trigger 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 centre conductor a resistor of at least 100 Ohm should
be series connected (RF decoupling). Often it is advisable to
statically shield 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
This trigger mode can be selected in DUAL mode by the aid of the
TRIG. SOURCE [17] pushbutton ( please note ”Controls and
Readout”)
. In the case of chopped DUAL mode, selecting
alternate trigger mode automatically sets the instrument to
alternate
DUAL mode.
Under TVL , TVF and line/mains triggering conditions alternate
triggering can not be chosen. Thus only the following trigger
coupling modes are available in alternate trigger mode:
AC , DC ,
HF and LF . The trigger point symbol is not displayed in alternate
trigger mode.
With alternate triggering it is possible to trigger two signals from
different frequency sources (asynchronous). In this case the
oscilloscope must be operated in
DUAL alternate mode with
internal triggering and each input signal must be of sufficient
height to enable trigger. To avoid trigger problems due to diffe-
rent
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. 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.
If signals are applied with a high frequency ratio (difference), the
trace intensity then becomes reduced if the time base is set to
smaller time coefficients (faster sweep). This happens as the
number of sweeps does not increase because it depends on the
lower frequency signal, but with a faster sweep the phosphor
becomes less activated.
External triggering
The external trigger input is activated with the aid of the TRIG.
SOURCE [17] pushbutton (see ”Controls and Readout”), if the
trigger coupling is not set to line/mains trigger coupling. Then the
internal trigger source is deactivated. As the external trigger
signal applied at the
TRIG. EXT socket normally has no relation
to the signal height of the displayed signal, the trigger point
symbol is switched off. The external trigger voltage must have a
minimum amplitude of 0.3 V
pp
and should not increase above
3 V
pp
. The input impedance of the TRIG. EXT. socket is approx.
1 MOhm II 20 pF.
The maximum input voltage of the input circuit is 100V (DC+peak
AC). The external trigger voltage may have a completely different
Triggering and time base