14
Subject to change without notice
After completion of the HF-adjustment, the signal amplitude
displayed on the CRT screen should have the same value
as during the 1kHz adjustment.
Probes other than those mentioned above, normally
have a larger tip diameter and may not fit into the
calibrator outputs. Whilst it is not difficult for an
experienced operator to build a suitable adapter, it
should be pointed out that most of these probes have
a slower risetime with the effect that the total bandwidth
of scope together with probe may fall far below that of
the HM303. Furthermore, the HF-adjustment feature is
nearly always missing so that waveform distortion can
not be entirely excluded.
The adjustment sequence must be followed in the order
described, i.e. first at 1kHz, then at 1MHz. The calibrator
frequencies should not be used for timebase calibration.
The pulse duty cycle deviates from 1:1 ratio.
Prerequisites for precise and easy probe adjustments, as
well as checks of deflection coefficients, are straight
horizontal pulse tops, calibrated pulse amplitude, and
zero-potential at the pulse base. Frequency and duty cycle
are relatively uncritical. For interpretation of transient
response, fast pulse risetimes and low-impedance
generator outputs are of particular importance.
Providing these essential features, as well as switch-
selectable output-frequencies, the calibrator of the HM303
can, under certain conditions, replace expensive
squarewave generators when testing or compensating
wideband-attenuators or -amplifiers. In such a case, the
input of an appropriate circuit will be connected to one of
the CAL.-outputs via a suitable probe.
The voltage provided at a high-impedance input (1M
Ω
II15-
50pF) will correspond to the division ratio of the probe
used (10:1 = 20mV
pp
, 100:1 = also 20mV
pp
from 2V
output). Suitable probes are HZ51, 52, 53, and 54.
Operating modes of the vertical amplifiers
The vertical amplifier is set to the desired operating mode
by using the 3 pushbuttons (CH I/II, DUAL and ADD) in the
Y field of the front panel. For
Mono
mode all 3 buttons
must be in their released positions; only channel I can then
be operated. The button CH I/II-TRIG.I/II must be depressed
in mono mode for Channel II. The internal triggering is
simultaneously switched over to Channel II with this
button.
If the
DUAL
button is depressed, both channels are
working. Two signals can be displayed together in this
button position (alternate mode) if the time-base setting
and the repetition frequency of the signal are suited. This
mode is not suitable for displaying very slow-running
processes. The display then flickers too much or it appears
to jump. If the
ADD
button is depressed
in addition
to
DUAL
, both channels are switched over constantly at a
high frequency within a sweep period (
CHOP
mode). Low
frequency signals
below 1kHz, or with periods longer
than 1ms
are then also displayed without flicker. CHOP
mode is not recommended for signals with higher repetition
frequencies.
If only the
ADD
button is depressed, the signals of both
channels are algebraically added (±I ±II). Whether the
resulting display shows the
sum
or
difference
is dependent
on the phase relationship or the polarity of the signals
and
on the positions of the
INVERT
buttons.
In-phase input voltages:
Both
INVERT CH.I
and INVERT CH.II
buttons
released or depressed = sum.
Only one INVERT button depressed = difference.
Antiphase input voltages:
Both
INVERT
buttons released or depressed
= difference.
INVERT CH.I
or INVERT CH.II
button depressed = sum.
In the
ADD
mode the vertical display position is dependent
upon the
Y-POS.
setting of
both
channels. The same
attenuator switch position is normally used for both
channels with algebraic addition.
Please note that the Y-POS. settings are added too but are
not affected by the INVERT pushbuttons.
Differential measurement
techniques allow direct
measurement of the voltage drop across floating
components (both ends above ground). Two identical
probes should be used for both vertical inputs. In order to
avoid ground loops, use a separate ground connection and
do not use the probe ground leads or cable shields.
incorrect
incorrect
correct
Adjustment
1MHz
