Hameg HM304, Руководство пользователя

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Subject to change without notice 10
sources are only slightly loaded (approx. 10M
Ω
II 16pF or
100M
Ω
II 9pF with HZ53). Therefore, if the voltage loss due
to the attenuation of the probe can be compensated by a higher
amplitude setting, the probe should always be used. The series
impedance of the probe provides a certain amount of protection
for the input of the vertical amplifier. Because of their separa-
te manufacture, all attenuator probes are only partially
compensated, therefore accurate compensation must be
performed on the oscilloscope (see Probe compensation ).
Standard attenuator probes on the oscilloscope normally
reduce its bandwidth and increase the rise time. In all cases
where the oscilloscope bandwidth must be fully utilized (e.g.
for pulses with steep edges) we strongly advise using the
probes HZ51 (x10) HZ52 (x10 HF) and HZ54 (x1 and x10).
This can save the purchase of an oscilloscope with larger
bandwidth.
The probes mentioned have a HF-calibration in addition to
low frequency calibration adjustment. Thus a group delay
correction to the upper limit frequency of the oscilloscope is
possible with the aid of an 1MHz calibrator, e.g. HZ60.
In fact the bandwidth and rise time of the oscilloscope are
not noticably changed with these probe types and the
waveform reproduction fidelity can even be improved because
the probe can be matched to the oscilloscopes individual pulse
response.
If a x10 or x100 attenuator probe is used, DC input
coupling must always be used at voltages above 400V.
With AC coupling of low frequency signals, the
attenuation is no longer independent of frequency,
pulses can show pulse tilts. Direct voltages are
suppressed but load the oscilloscope input coupling
capacitor concerned. Its voltage rating is max. 400 V
(DC + peak AC). DC input coupling is therefore of quite
special importance with a x100 attenuation probe
which usually has a voltage rating of max. 1200 V (DC
+ peak AC). A capacitor of corresponding capacitance
and voltage rating may be connected in series with
the attenuator probe input for blocking DC voltage (e.g.
for hum voltage measurement).
With all attenuator probes, the maximum AC input voltage
must be derated with frequency usually above 20kHz.
Therefore the derating curve of the attenuator probe type
concerned must be taken into account.
The selection of the ground point on the test object is
important when displaying small signal voltages. It should
always be as close as possible to the measuring point. If this
is not done, serious signal distortion may result from spurious
currents through the ground leads or chassis parts. The
ground leads on attenuator probes are also particularly critical.
They should be as short and thick as possible. When the
attenuator probe is connected to a BNC-socket, a BNC-
adapter, should be used. In this way ground and matching
problems are eliminated. Hum or interference appearing in
the measuring circuit (especially when a small deflection
coefficient is used) is possibly caused by multiple grounding
because equalizing 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
Switch on the oscilloscope by depressing the red POWER
pushbutton. The instrument will revert to its last used
operating mode. Except in the case of COMP. TESTER mode,
where a trace appears on the screen if the INTENS. knob is
in center position, all LED‘s should remain unlit. The trace,
displaying one baseline or the shorter COMP TESTER
baseline, should be visible after a short warm-up period of
approx. 10 seconds. If the COMP TESTER mode is active,
depress the COMP TESTER pushbutton once to switch to
XY or Yt mode. In XY mode the XY LED in the TIME/DIV
section is lit, in this case depress the XY pushbutton once to
switch over to Yt mode. 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.
• 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).
• • • • • Depress the upper NORM. pushbutton until the AC
symbol on the trigger coupling scale is lit.
• Both GD input coupling pushbutton switches for CH.I and
CH.II in the Y-field should be set to the GD position (GD lit).
If only a spot appears ( CAUTION! CRT phosphor can be
damaged ), reduce the intensity immediately and check that
the XY mode is not selected ( XY LED dark). If the trace is
not visible, check the correct positions of all knobs and modes
(particularly
NM LED - normal triggering - LED on).
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 HM304 accepts all signals from DC (direct voltage) up to
a frequency of at least 35MHz (-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 35 and 38MHz), the measured values in the upper
frequency range cannot be defined exactly. Additionally, as
already mentioned, for frequencies above 35MHz 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