Hameg HM 1004-3 Series, Manual

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Subject to change without notice
Triggering and timebase
Triggering and timebase
All controls regarding trigger and timebase are located on the
right of the VOLTS/DIV. knobs. They are described in the
section
“Controls and Readout” .
Time related amplitude changes on a measuring signal (AC
voltage) are displayable in Yt-mode. In this mode the signal
voltage deflects the beam in vertical direction (Y) while the
timebase generator moves the beam from the left to the right
of the screen (time deflection = t).
Normally there are periodically repeating waveforms to be
displayed. Therefore the timebase must repeat the time
deflection periodically too. To produce a stationary display,
the timebase must only be triggered if the signal height and
slope condition coincide with the former timebase start
conditions. A DC voltage signal can not be triggered as it is a
constant signal with no slope.
Triggering can be performed by the measuring signal itself
(internal triggering) or by an external supplied but synchro-
nous voltage (external triggering).
The trigger voltage should have a certain minimum amplitude.
This value is called the trigger threshold. It is measured with
a sine signal. Except when external trigger is used the trigger
threshold can be stated as vertical display height in div, at
which the timebase generator starts, the display is stable, and
the trigger indicator LED lights or flashes. The internal trigger
threshold of the oscilloscope is given as
≤
0.5div. When the
trigger voltage is externally supplied, it can be measured in
Vpp at that input. Normally, the trigger threshold may be
exceeded up to a maximum factor of 20.
The instrument has two trigger modes, which are character-
ized as Automatic Peak- and Normal-Triggering.
Automatic Peak (Value) -Triggering
Instrument specific information can be drawn from the items
NM - AT - (11) and LEVEL (13) in the section “Controls
and Readout” .
This trigger mode is automatically selected after the AUTOSET
pushbutton is pressed. As the peak value detection makes no
sense in combination with
DC and TV (television) signals, it is
switched off automatically in DC , TVL and TVF trigger coupling
conditions as well as in alternate trigger mode. In this case the
automatic is still present, but a wrong trigger level setting
causes an untriggered display.
In automatic trigger mode the sweep generator can run
without a test signal or external trigger voltage. A base line will
always be displayed even with no signal. With an applied AC
signal the peak value triggering enables the user to select the
voltage point on the trigger signal (trigger point), by the
adjustment of the trigger level control. The control range
depends on the peak to peak value of the signal. This trigger
mode is therefore called Automatic Peak (Value)- Triggering.
Operation of the scope needs only correct amplitude and
timebase settings, for a constantly visible trace.
Automatic mode is recommended for all uncomplicated meas-
uring tasks. However, automatic triggering is also the appropri-
ate operation mode for the “entry” into difficult measuring
problems, e.g. when the test signal is unknown relating to
amplitude, frequency or shape. Presetting of all parameters is
now possible with automatic triggering; the change to normal
triggering can follow thereafter.
The automatic triggering works above 20Hz. The failure of
automatic triggering at frequencies below 20Hz is abrupt.
However, it is not signified by the trigger indicator LED which
is still blinking. Loss of triggering is best recognizable at the left
screen edge (the start of the trace in differing display height).
The automatic peak (value) triggering operates over all varia-
tions or fluctuations of the test signal above 20Hz. However, if
the pulse duty factor of a square-wave signal exceeds a ratio of
100:1, switching over to normal triggering will be necessary.
Automatic triggering is practicable with internal and external
trigger voltage.
Normal Triggering
The description of the most important control elements ( NM
- AT - (11) and LEVEL (13) ) is part of the section
“Controls and Readout” ).
With normal triggering, the sweep can be started by AC signals
within the frequency range defined by the trigger coupling setting.
In the absence of an adequate trigger signal or when the trigger
controls (particularly the trigger LEVEL control) are misadjusted,
no trace is visible.
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 trigger LEVEL
control. If the signal applied at the Y input is used for triggering
(internal trigger source), its adjusting range is directly depend-
ent on the display height, which should be at least 0.5div. If
it is smaller than 1div, the trigger
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 timebase variable control and hold off time control,
mentioned later.
Slope
Please note item (11) in section “Controls and Readout”
for instrument specific information.
The actual slope setting is displayed in the readout. The
setting is not changed by the
AUTO SET function. The slope
setting can be changed in alternate- and B timebase mode for
the B timebase trigger unit if the delay trigger function is
active. The previous slope setting for the A timebase trigger
is stored and still active. For further information
please note
“Controls and Readout” .
The timebase generator can be triggered by a rising or falling
edge of the test signal. Whether the rising or the falling edge
is used for triggering, depends on the slope direction setting.
This is valid with automatic and normal triggering. The positive
slope direction 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.
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.