12
Subject to change without notice
First Time Operation
Before applying power
to the oscilloscope it is recom-
mended that the following simple procedures are
performed:
• Check that all pushbuttons are in the
out
position, i.e.
released.
• Rotate the variable controls with arrows, i.e.
TIME/DIV.
variable control,
CH.I
and
CH.II
attenuator variable
controls, and
HOLD OFF
control to their calibrated
detent.
• Set all controls with marker lines to their midrange
position (marker lines pointing vertically).
• The
TRIG.
selector lever switch in the X-field should be
set to the position uppermost.
• Both
GD
input coupling pushbutton switches for
CH.I
and
CH.II
in the Y-field should be set to the
GD
position.
Switch on the oscilloscope by depressing the red
POWER
pushbutton. An LED will illuminate to indicate working
order. The trace, displaying one baseline, should be visible
after a short warm-up period of approx. 10 seconds.
Adjust
Y-POS.I
and
X-POS.
controls to center the baseline.
Adjust
INTENS.
(intensity) and
FOCUS
controls for
medium brightness and optimum sharpness of the trace.
The oscilloscope is now ready for use.
If only a spot appears (
CAUTION!
CRT phosphor can be
damaged), reduce the intensity immediately and check
that the
XY
pushbutton is in the released (out) position. If
the trace is not visible, check the correct positions of all
knobs and switches (particularly
AT/NORM.
button in out
position).
To obtain the maximum life from the cathode-ray tube, the
minimum intensity setting necessary for the measurement
in hand and the ambient light conditions should be used.
Particular care is required when a single spot is
displayed,
as a very high intensity setting may cause
damage to the fluorescent screen of the CRT. Switching
the oscilloscope off and on at short intervals stresses the
cathode of the CRT and should therefore be avoided.
The instrument is so designed that even incorrect operation
will not cause serious damage. The pushbuttons control
only minor functions, and it is recommended that before
commencement of operation all pushbuttons are in the
“out” position. After this the pushbuttons can be operated
depending upon the mode of operation required.
The HM303 accepts all signals from DC (direct voltage) up
to a frequency of at least 30MHz (
−
3dB). For sinewave
voltages the upper frequency limit will be 50MHz (
−
6dB).
However, in this higher frequency range the vertical
display height on the screen is limited to approx. 4-5div.
The time resolution poses no problem. For example, with
50MHz and the fastest adjustable sweep rate (10ns/div.),
one cycle will be displayed every 2div. The tolerance on
indicated values amounts to ±3% in both deflection
directions. All values to be measured can therefore be
determined relatively accurately.
However, from approximately 10MHz upwards the
measuring error will increase as a result of loss of gain. At
18MHz this reduction is about 10%. Thus, approximately
11% should be added to the measured voltage at this
frequency. As the bandwidth of the amplifiers may differ
slightly (normally between 30 and 35MHz), the measured
values in the upper frequency range cannot be defined
exactly. Additionally, as already mentioned, for frequencies
above 30MHz the dynamic range of the display height
steadily decreases. The vertical amplifier is designed so
that the transmission performance is not affected by its
own overshoot.
Trace Rotation TR
In spite of Mumetal-shielding of the CRT, effects of
the earths magnetic field on the horizontal trace
position cannot be completely avoided. This is
dependent upon the orientation of the oscilloscope
on the place of work. A centred trace may not align
exactly with the horizontal center line of the graticule.
A few degrees of misalignment can be corrected by a
potentiometer accessible through an opening on the
front panel marked TR.
Probe compensation and use
To display an undistorted waveform on an oscilloscope,
the probe must be matched to the individual input
impedance of the vertical amplifier.
For this purpose a square wave signal with a very fast rise
time and minimum overshoot should be used, as the
sinusoidal contents cover a wide frequency range. The
frequency accuracy and the pulse duty factor are not of
such importance.
The built-in calibration generator provides a square wave
signal with a very fast risetime (<4ns), and switch-
selectable frequencies of approx. 1kHz and 1MHz from
two output sockets below the CRT screen.
This signal should not be used for frequency cali-
bration!
