Hameg HM 203, Руководство пользователя

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drift are caused by offset current on the gate. The drift is
too high, if the vertical trace position drifts by more than
0.5mm on turning the appropriate attenuator switch
through all 12 steps. Sometimes such effects occur after
long periods of operation.
Calibration of the Vertical Amplifier
A square-wave voltage of 200mVpp ±1 % is present at
the output eyelet marked with a square-wave. If a direct
connection is made between this eyelet and the input of
the vertical amplifier, the displayed signal in the
50mV/cm position (variable control to C) should be 4cm
high (DC i nput coupling). Maximum deviations of 1,2mm
(3%) are permissible. If a X I 0 probe is connected be­
tween the output eyelet and Y input, the same display
height should result in the 5mV/cm position. With higher
tolerances it should first be investigated whether the
cause lies, within the amplifier or in the amplitude of the
square-wave signal. On occasions it is possible that the
probe is faulty or incorrectly compensated. If necessary,
the measuring amplifier can be calibrated with an ac­
curately known DC voltage
(DC input coupling). The
trace position should then vary in accordance with the
deflection coefficient set. With variable control at the at­
tenuator switch fully clockwise, the input sensitivity is in­
creased at least by the factor 2.5 in each position. In the
100mV/cm position, the displayed calibrator signal
height should vary from 2cm to at least 5cm.
Transmission Performance of the Vertical
Amplifier
The transient response and the delay distortion correc­
tion can only be checked with the aid of a square-wave
generator with a fast risetime
[max. 5ns). The signal
coaxial cable (e. g. HZ34) must be terminated at the ver­
tical input of the oscilloscope with a resistor equal to the
characteristic impedance of the cable (e. g. with HZ22).
Checks should be made at 50Hz, 500Hz, 5kHz, 50kHz,
500kHz and 1 MHz, the deflection coefficient should be
set at 5mV/cm with DC input coupling (Y variable con­
trol in C position). In so doing, the square pulses must
have a flat top without ramp-off, spikes and glitches; no
overshoot is permitted, especially at 1 MHz and a display
height of
4-5cm. At the same time, the leading top cor­
ner 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 operator's discretion whether this test
is undertaken or not.
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-compensated in each position. Even small
capacitive changes can reduce the transmission perfor­
mance. Faults of this kind are as a rule most easily
detected with a square-wave signal with a low repetition
rate (e. g. 1 kHz). 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
1 MQ ± 1 % resistor and, in parallel with it, a trimmer
3-1 5pF in parallel with approx. 20pF. One side of this
parallel circuit is connected directly to the input connec­
tor of the vertical amplifier and the other side is con­
nected 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/cm
position (variable control to C, DC input coupling; square
tops exactly horizontal; no ramp-off is permitted). This is
achieved by adjusting the trimmer located in the 2:1 at­
tenuator.
The shape o f the square-wave should then be
the same in each input attenuator position.
Operating Modes:
Mono/Dual, Alt./Chop., I + 11, X-Y Operation
On depressing the DUAL pushbutton, two traces must
appear immediately. On actuation of the
Y-POS. con­
trols, the trace positions should have no affect 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
shadowing around and within the two traces in the upper
or lower region of the screen. Set
TIMEBASE switch to
1/Us/cm, depress the DUAL and ALT/CHOP pushbut­
tons, set input coupling of both channels to GD and ad­
vance the INTENS, control fully clockwise. Adjust
FOCUS for a sharp display. With the Y-POS. controls
shift one of the traces to a + 2cm, the other to a —2cm
vertical position from the horizontal center line of the
graticule. Do not try to synchronize the chop frequency
(1MHz) by means of the TIMEBASE variable control!
Then alternately release and depress the
ALT/CHOP
pushbutton. Check for a negligible trace widening and
periodic shadowing in the chopped mode.
It is important to note that in the I + 11 add mode (only
ALT/CHOP depressed) or the —l + l! difference mode
(INVERT I button depressed in addition) the vertical posi-
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