Hameg HM404-2.02, Руководство пользователя

Страница: 21 / 36

21
Subject to change without notice
First Time Operation
made. Thereafter the instrument will revert to its last used
operating mode.
If after approx. 20 seconds no trace is visible, the AUTO SET
pushbutton should be pressed briefly. This selects the Yt
mode and medium trace and readout intensity (
please note
”AUTO SET” ). Adjust Y-POS.I and X-POS. controls to center
the baseline. Adjust INTENS. (intensity) and FOCUS controls
for medium brightness and optimum sharpness (input(s)
grounded) of the trace. The oscilloscope is now ready for use.
If the AUTO SET function was not used and only a spot
appears (CAUTION! CRT phosphor can be damaged), reduce
the intensity immediately and check that the XY mode is not
selected (XY not displayed in the readout).
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.
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 centered 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 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 the output
socket 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. The
output provides 0.2Vpp ±1% (tr <4ns) for 10:1 probes. When
the Y deflection coefficient is set to 5mV/div, the calibration
voltage corresponds to a vertical display of 4 divisions (10:1
probe).
The output socket has 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 extremely 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. 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 parallel to the horizontal graticule lines is
essential for accurate probe adjustments. (See also ”Trace
rotation TR”).
Connect the probes (Types HZ51 , 52 , 54 , or HZ36 ) to the CH
I input. Set the deflection coefficient to 5mV/div and the input
coupling to DC. The time deflection coefficient should be set
to 0.2ms/div. All deflection coefficients should be calibrated.
Plug the probe tip into the calibrator output socket.
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 capacitor) which permit probe compensation
in the range of the upper frequency limit of the vertical
oscilloscope amplifier. Only this compensative adjustment
ensures optimum utilization 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
oscilloscope 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 at a frequency of
approx. 1MHz. The calibrator output of this instrument 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 deflection coefficient setting.
Insert the probe tip into the output socket. 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)