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currents can flow in the shielding of the test cables (voltage
drop between the protective conductor connections,
caused by external equipment connected to the mains/line,
e.g.
signal
generators
with
interference
protection
capacitors).
First Time Operation
Check that the instrument is set to the correct mains/
line voltage. (Refer to page M2).
Before applying power
to the oscilloscope it is recom¬
mended that the following simple procedures are per¬
formed:
- 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 con¬
trols, and HOLD OFF control to their calibrated detent.
- Set all controls with marker lines to their midrange posi¬
tion (marker lines pointing vertically).
- The TV SEP. lever switch and the TRIG, selector lever
switch in the X-field should be set to their uppermost
position.
- 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 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 oscillo¬
scope is now ready for use.
If only a spot appears
{CAUTION!
CRT phosphor can be
damaged.), reduce the intensity immediately and check
that the X-Y 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 measure¬
ment in hand and the ambient light conditions should be
used.
Particular care is required when a single spot is dis¬
played,
as a very high intensity setting may cause damage
to the fluorescent screen of the CRT. Switching the oscillo¬
scope 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 HM 203-7 accepts all signals from DC (direct voltage)
up to a frequency of at least 20MHz (-3dB). For sinewave
voltages the upper frequency limit will be 30-35MHz.
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 25MHz
and the fastest adjustable sweep rate (20ns/div.), one cycle
will be displayed every 2div. The tolerance on indicated val¬
ues amounts to ±3% in both deflection directions. All val¬
ues to be measured can therefore be determined relatively
accurately. However, from approximately 6MHz upwards
the measuring error will increase as a result of loss of gain.
At 12MHz this reduction is about 10%. Thus, approxi¬
mately 11 % should be added to the measured voltage at
this frequency. As the bandwidth of the amplifiers differ
(normally between 20 and 25MHz), the measured values in
the upper limit range cannot be defined exactly. Addition¬
ally, as already mentioned, for frequencies above 20MHz
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
earth's 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 horizon¬
tal center line of the graticule. A few degrees of mis¬
alignment can be corrected by a potentiometer acessi-
ble through an opening on the front panel marked TR.
Probe compensation and use
The attenuator probe must be matched exactly to the input
impedance of the vertical amplifier to ensure an undistorted
display of waveforms. A generator built into the HM 203-7
supplies a square wave signal for this purpose with very
short rise time (<60 ns) at
1 kHz.
The square wave signal
can be taken from the two eyelets beneath the screen. One
output supplies 0.2Vpp ± 1 % for x10 attenuator probes, the
Subject to change without notice
M7 203-7
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