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ger signal rnay not have the same polarity or pulse edge as
the test signal. This can be checked, if the external trigger
voltage itself is displayed at first (with internal triggering).
Generally, the composite video signal has a high DC con¬
tent. With a 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 height 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 6cm.
External triggering
The internal triggering is disconnected by depressing the
EXT. button. The timebase can be triggered
externally
the TRIG. INP. socket using a O.SVpp to 5Vppvoltage, which
is in syncronism with the test signal. This trigger voltage
may have completely different form from the test signal
voltage. Triggering is even possible - in certain limits -
with whole number multiples or fractions of the test fre¬
quency, but only in locked phase relation.
The input impedance of TRIG. INP. socket is approx.
1 MQII35pF. The maximum input voltage of the input cir¬
cuit is 100V (DC-f peak AC). Only 5Vpp maximum are
required for a good external triggering.
Trigger indicator
An LED on condition (to the left of the TRIG, switch) indi¬
cates that the sweep generator is triggered. This is valid
with automatic and with normal triggering. The indication of
trigger action facilitates a sensitive LEVEL adjustment, par¬
ticularly at very low signal frequencies. The indication
pulses are of only 100 ms duration.
Thus for fast signals the LED appears to glow continuously,
for low repetition rate signals, the LED flashes at the repet¬
ition 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.
Holdoff-time adjustment
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 double displayed. It is possible
that
LEVEL
adjustment only controls the mutual phase
shift, but not the double display. The stable single dis¬
play of the signal, required for the evaluation, is easily
obtainable by expanding the holdoff time. To this end
the
HOLDOFF
knob is slowly turned to the right, until
one signal is dispiayed.
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 re-set
into its calibration detent, 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 dis¬
played.
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 in the
normal triggering
mode, a stable display may be obtained using the HOLD¬
OFF control (in the X-field). This facility varies the hold-off
time between two sweep periods approx, up to the ratio
10:1. Pulses or other signal waveforms appearing during
Component Tester
General
The HM 203-7 has a built-in electronic Component Tester (ab¬
breviated
CTi.
which is used for instant display of a test pat¬
tern to indicate whether or not components are faulty. The
Subject to change without notice
Ml3 203-7
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