Subject to change without notice
19
Thus for fast signals the LED appears to glow continuously,
for low repetition rate signals, the LED flashes at the
repetition rate or at a display of several signal periods not
only at the start of the sweep at the left screen edge, but
also at each signal period.
In automatic triggering mode the sweep generator starts
repeatedly without test signal or external trigger voltage. If
the trigger signal frequency is <20Hz the sweep generator
starts without awaiting the trigger pulse. This causes an
untriggered display and a flashing trigger LED (TR).
Holdoff-time adjustment
If it is found that a trigger point cannot be located on
extremely complex signals even after repeated and careful
adjustment of the
LEVEL
control, a stable display may be
obtained using the
HOLD OFF
control (in the X-field). This
facility varies the holdoff time between two sweep periods
approx. up to the ratio 10:1. Pulses or other signal wave-
forms appearing during this off period cannot trigger the
timebase. Particularly with burst signals or aperiodic pulse
trains of the same amplitude, the start of the sweep can
be delayed until the optimum or required moment.
A very noisy signal or a signal with a higher interfering
frequency is at times displayed double. It is possible
that LEVEL adjustment only controls the mutual
phase shift, but not the double display. The stable
single display of the signal, required for evaluation, is
easily obtainable by expanding the hold off time. To
this end the HOLD OFF knob is slowly turned to the
right, until one signal is displayed.
A double display is possible with certain pulse signals,
where the pulses alternately show a small difference of
the peak amplitudes. Only a very exact
LEVEL
adjustment
makes a single display possible. The use of the
HOLD OFF
knob simplifies the right adjustment.
After specific use the
HOLD OFF
control should be reset
into its calibration detent (fully ccw), otherwise the
brightness of the display is reduced drastically. The function
is shown in the following figures.
Function of var. HOLD OFF control
Fig. 1
shows a case where the
HOLD OFF
knob is in the minimum position
and various different waveforms are overlapped on the screen, making
the signal observation unsuccessful.
Fig. 2
shows a case where only the desired parts of the signal are
stably displayed.
Y Overscanning Indication
This indicator shows any vertical overscan of the usable
(10 x 8) screen area, if any part of the signal or baseline are
outside the graticule. The indication is achieved by 2 light-
emitting diodes, marked
OVERSCAN
, which are located
between the attenuators. Should one LED illuminate
without an input signal, this means that the respective
vertical positioning control has been improperly adjusted.
Because each LED correlates with one of both possible
directions, it can be seen in which direction the trace has
left the screen. With dual channel operation, misadjustment
of both
Y-POS.
controls can occur. If both traces lie in the
same direction, one LED illuminates likewise. If one trace
is positioned above and the other below the graticule, both
LEDs are illuminated. The indication of the Y position after
crossing the graticule area occurs
in each operating
mode
, also when, due to missing time deflection, no
baseline is displayed, or when the oscilloscope is in the X-
Y mode.
As previously written in the paragraph “First Time Opera-
tion”, the
AT/NORM.
pushbutton should be switched in
AT
position, as a baseline is then permanently displayed,
also without any input signal. The trace disappears at
times after applying an input signal. The LED indication
shows, in which direction the trace has left the screen,
above or below the graticule. Illumination of both LEDs at
the same time after applying a signal means that the
vertical deflection has overscanned the graticule edges in
both vertical directions. With DC input coupling and an
applied signal with a relatively high DC offset, smaller
sizes also of displayed signals can overscan the raster
edges, because the DC voltage causes a vertical position
shift of the display height, which seemed correctly adjusted.
In this case, a smaller display height must be accepted, or
AC
input coupling has to be selected.
Component Tester
General
The HM303 has a built-in electronic Component Tester
(
COMP. TESTER
), which is used for instant display of a
test pattern to indicate whether or not components are
faulty. The COMP. TESTER can be used for quick checks
of semiconductors (e.g. diodes and transistors), resistors,
capacitors, and inductors. Certain tests can also be made
to integrated circuits. All these components can be tested
in and out of circuit.
The test principle is fascinatingly simple. A built-in generator
delivers a sine voltage, which is applied across the
component under test and a built-in fixed resistor. The
sine voltage across the test object is used for the horizon-
tal deflection, and the voltage drop across the resistor (i.e.
current through test object) is used for vertical deflection
of the oscilloscope. The test pattern shows a current-
voltage characteristic of the test object.
Since this circuit operates with a frequency of 50Hz
(±10%) and a voltage of 6V max. (open circuit), the
period
heavy parts are displayed
signal
sweep
adjusting
HOLDOFF
time
Fig. 2
Fig. 1
