Subject to change without notice
32
Component Tester
a fast risetime (max. 5ns). The signal coaxial cable (e.g.
HZ34
)
must be terminated at the vertical input of the oscilloscope
with a resistor equal to the characteristic impedance of the
cable (e.g. with
HZ22
). Checks should be made at 100Hz,
1kHz, 10kHz, 100kHz and 1MHz, the deflection coefficient
should be set at 5mV/div with DC input coupling. In so doing,
the square pulses must have a flat top without ramp-off,
spikes and glitches; no overshoot is permitted, especially at
1MHz and a display height of 4-5div. At the same time, the
leading top corner of the pulse must not be rounded. In
general, no great changes occur after the instrument has left
the factory, and it is left to the operators discretion whether
this test is undertaken or not. A suited generator for this test
is
HZ60
from
HAMEG
.
Of course, the quality of the transmission performance is not
only dependent on the vertical amplifier. The input attenuators,
located in the front of the amplifier, are frequency-compen-
sated in each position. Even small capacitive changes can
reduce the transmission performance. Faults of this kind are
as a rule most easily detected with a square-wave signal with
a low repetition rate (e.g. 1kHz). If a suitable generator with
max. output of 40Vpp is available, it is advisable to check at
regular intervals the deflection coefficients on all positions of
the input attenuators and readjust them as necessary. A
compensated 2:1 series attenuator is also necessary, and this
must be matched to the input impedance of the oscilloscope.
This attenuator can be made up locally. It is important that this
attenuator is shielded.
For local manufacture, the electrical components required are
a 1M
Ω
±1% resistor and, in parallel with it, a trimmer 3-15pF
in parallel with approx. 10pF. One side of this parallel circuit
is connected directly to the input connector of CH I or CH II
and the other side is connected to the generator, if possible
via a low-capacitance coaxial cable. The series attenuator
must be matched to the input impedance of the oscilloscope
in the 5mV/div position (variable control to CAL., DC input
coupling; square tops exactly horizontal; no ramp-off is per-
mitted). This is achieved by adjusting the trimmer located in
the 2:1 attenuator. The shape of the square-wave should then
be the same in each input attenuator position.
Operating Modes: CH.I/II, DUAL, ADD,
CHOP., INVERT and X-Y Operation
In DUAL mode two traces must appear immediately. On
actuation of the Y-POS. controls, the trace positions should
have minimal effect on each other. Nevertheless, this cannot
be entirely avoided, even in fully serviceable instruments.
When one trace is shifted vertically across the entire screen,
the position of the other trace must not vary by more than
0.5mm.
A criterion in chopped operation is trace widening and shad-
owing around and within the two traces in the upper or lower
region of the screen. Set time coefficient to 0.5ms/div, set
input coupling of both channels to GD and advance the
INTENS. control fully clockwise. Adjust FOCUS for a sharp
display. With the Y-POS. controls shift one of the traces to a
+2div, the other to a -2div vertical position from the horizontal
center line of the graticule.
Do not try to synchronize (with the time variable control) the
chop frequency (0.5MHz)! Check for negligible trace widen-
ing and periodic shadowing when switching between 0.5ms/
div and 0.2ms/div.
It is important to note that in the I+II add mode or the I-II
difference mode the vertical position of the trace can be
adjusted by using both the Channel I and Channel II Y-POS.
controls.
In X-Y Operation, the sensitivity in both deflection directions
will be the same. When the signal from the built-in square-
wave generator is applied to the X-input, then, as in Yt (time
base) mode in the vertical direction, there must be a horizontal
deflection of 4div ±1.6div when the deflection coefficient is
set to 50mV/div position. The check of the mono channel
display is unnecessary; it is contained indirectly in the tests
above stated.
Triggering Checks
The internal trigger threshold is important as it determines the
display height from which a signal will be stably displayed. It
should be approx. 0.3-0.5div for the instrument. An increased
trigger sensitivity creates the risk of response to the noise
level in the trigger circuit. This can produce double-triggering
with two out- of-phase traces.
Alteration of the trigger threshold is not possible. Checks can
be made with any sine-wave voltage between 50Hz and
1MHz. The instrument should be in automatic peak (value)
triggering (NM LED dark) and the TRIG. LEVEL knob in
midrange position. It should be ascertained whether the
same trigger sensitivity is also present with Normal Triggering
(NM LED lights). In this trigger mode, TRIG. LEVEL adjust-
ment is absolutely necessary. The checks should show the
same trigger threshold with the same frequency. On chang-
ing the trigger slope, the start of the sweep changes from the
positive-going to the negative-going edge of the trigger signal.
As described in the Operating Instructions, the trigger fre-
quency range is dependent on the trigger coupling selected.
For lower frequencies the LF coupling mode can be selected.
In this mode, triggering up to at least 1.5kHz (sine-wave) is
possible. Internally the instrument should trigger perfectly at
a display height of approx. 0.5div, when the appropriate
trigger coupling mode is set.
For external triggering, the external trigger input connector
requires a signal voltage of at least 0.3Vpp, which is in
synchronism with the Y input signal. The voltage value is
dependent on the frequency and the trigger coupling mode
(AC-DC-HF-LF).
Checking of the TV triggering is possible with a video signal of
any given polarity.
Use the TV-L or TV-F setting for video sync pulse separation.
The correct slope of the sync pulse (front edge) must be
selected and a suitable time coefficient setting must be
chosen. The slope is then valid for both sync frequencies.
Perfect TV triggering is achieved, when in both display modes
the amplitude of the complete TV signal (from white level to
the top of the line sync pulse) is limited between 0.8 and 6div
and sync pulses of more then 0.5 div height. The display
should not shift horizontally during a change of the trigger
coupling from AC to DC when displaying a sine-wave signal
without DC offset.
If both vertical inputs are AC coupled to the same signal and
both traces are brought to coincide exactly on the screen,
when working in the alternate dual channel mode, then no
change in display should be noticeable, when switching from
TRIG I to TRIG II or when the trigger coupling is changed from
AC to DC.
