Hameg HM304, Руководство пользователя

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11 Subject to change without notice
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.
As the squarewave signals are used for probe compensation
adjustments, neither the frequency accuracy nor the pulse
duty factor are of importance and therefore not specified.
One output provides 0.2Vpp ±1% (tr <4ns) for 10:1 probes,
and the other 2Vpp for 100:1 probes. When the Y deflection
coefficients are set to 5mV/div., these calibration voltages
correspond to a screen amplitude of 4div.
The output sockets have an internal diameter of 4.9mm to
accommodate the internationally accepted shielding tube
diameter of modern Probes and F-series slimline probes. Only
this type of construction ensures the extremly short ground
connections which are essential for an undistorted waveform
reproduction of non-sinusoidal high frequency signals.
Adjustment at 1kHz
The C-trimmer adjustment (low frequency) compensates the
capacitive loading on the oscilloscope input (approx. 20pF
for the HM304). By this adjustment, the capacitive division
assumes the same ratio as the ohmic voltage divider to ensure
the same division ratio for high and low frequencies, as for
DC. (For 1:1 probes or switchable probes set to 1:1, this
adjustment is neither required nor possible). A baseline par-
allel to the horizontal graticule lines is essential for accurate
probe adjustments. (See also
„Trace rotation TR“ ).
Connect the probes (Types HZ51, 52, 53, 54, or HZ36) to the
CH.I input. One deflection coefficient in the
VOLTS/DIV
section of channel I must lit. If this is not the case depress
the
CHI pushbutton once and switch off channel II by
depressing the CHII pushbutton once. Set input coupling CH
I to DC (LED illuminates) and check that GD is switched off.
The CHI deflection coefficient must be 5mV/div., and TIME/
DIV. should be set to 0.2ms/div., and all variable controls to
CAL. position. Plug the the probe tip into the appropriate
calibrator output socket, i.e. 10:1 probes into the 0.2V socket,
100:1 probes into the 2V socket.
incorrect correct incorrect
Approximately 2 complete waveform periods are displayed
on the CRT screen. The compensation trimmer should be
adjusted. The location of the low frequency compensation
trimmer can be found in the probe information sheet. Adjust
the trimmer with the insulated screw driver provided, until
the tops of the square wave signal are exactly parallel to the
horizontal graticule lines (see 1kHz diagram). The signal height
should then be 4div. ± 0.16div. (= 4 % (oscilloscope 3% and
probe 1%). During this adjustment, the signal edges will
remain invisible.
Adjustment at 1MHz
Probes HZ51, 52 and 54 can also be HF-compensated. They
incorporate resonance de-emphasing networks (R-trimmer
in conjunction with inductances and capacitors) which per-
mit probe compensation in the range of the upper frequency
limit of the vertical oscilloscope amplifier. Only this
compensative adjustment ensures optimum utilisation of the
full bandwidth, together with constant group delay at the
high frequency end, thereby reducing characteristic transient
distortion near the leading edge (e.g. overshoot, rounding,
ringing, holes or bumps) to an absolute minimum.
Using the probes HZ51, 52 and 54, the full bandwidth of the
HM304 can be utilized without risk of unwanted waveform
distortion.
Prerequisite for this HF compensation is a square wave
generator with fast risetime (typically 4ns), and low output
impedance (approx. 50
Ω
), providing 0.2V and 2V at a
frequency of approx. 1MHz. The calibrator output of the
HM304 meets these requirements when the
CAL.
pushbutton is depressed.
Connect the probe to CH.I input. Depress the CAL.
pushbutton for 1MHz. Operate the oscilloscope as described
under 1kHz but select for 0.2µs/div
TIME/DIV. setting.
Insert the probe tip into the output socket marked 0.2V. A
waveform will be displayed on the CRT screen, with leading
and trailing edges clearly visible. For the HF-adjustment now
to be performed, it will be necessary to observe the rising
edge as well as the upper left corner of the pulse top. The
location of the high frequency compensation trimmer(s) can
also be found in the probe information sheet. These R-
trimmer(s) have to be adjusted such that the beginning of
the pulse is as straight as possible. Overshoot or excessive
rounding are unacceptable. The adjustment is relatively easy
if only one adjusting point is present. In case of several
adjusting points the adjustment is slightly more difficult, but
causes a better result. The rising edge should be as steep as
possible, with a pulse top remaining as straight and horizon-
tal as possible.
incorrect correct incorrect
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 HM304. Furthermore, the HF-adjustment feature
is nearly always missing so that waveform distortion can not
be entirely excluded.