First Time Operation And Initial Settings; Trace Rotation; Probe Adjustment And Use Of Probes; Khz Adjustment - Hameg HM400 Handbuch

Please note that the display of signals with a low repetition rate at high
sweep speeds will cause the trace to dim, the intensity may be increased
until the trace starts to blur. In such case the time base speed must be
decreased so far that the trace remains visible.
The signals may be connected to the scope either through shielded cables
or by using probes. The use of cables is restricted to low frequencies and
low impedance signal sources because they add typically 100 pF/m load
capacitance. At higher frequencies cables with standard characteristic
impedances like 50 Ω can be used if they are correctly terminated at both
ends. HAMEG HZ22 feedthrough terminations at the scope can be used
together with HAMEG 50 Ω cables such as HZ34. Incorrect or missing
terminations will cause massive pulse distortions. Generators, ampli-
fi ers etc. will only perform to specifi cations if they are feeding properly
terminated cables. The HZ22 is specifi ed for a maximum of 2 W which is
reached if the signal increases to 10 V
With probes no terminations are needed nor allowed, the probes are
directly connected to the scope's BNC connectors. Probes load high
impedance sources only moderately (10:1 probes with 10 MΩ //12 pF,
100:1 with 10 MΩ//5 pF), but this applies only up to appr. 100 KHz, above
the loading increases with increasing frequency, see the probe manuals
for details, also for the necessary derating. Passive probes are unsuited
for measurements on high Q HF circuits!
As mentioned, in most applications probes are used, at least as long
as the loss in sensitivity can be compensated by increasing the scope's
sensitivity. Also, a probe offers protection for the scope's input circuit.
Because probes are manufactured separated from the scopes, they
are only coarsely preadjusted, it is absolutely necessary to adjust each
probe to the input it is used on (see Probe Adjustment).
Probes may decrease the bandwidth of a scope considerably if they
are the wrong type! We recommend to use the HAMEG HZ51 (10:1),
HZ52 (10:1 HF), HZ154 (1:1 and 10:1) probes. Replacement parts may
be ordered from HAMEG and may be exchanged by the user. The
probes mentioned have a HF adjustment in addition to the basic 1 KHz
adjustment. By using the 1 MHz probe adjust signal, the HF adjustment
corrects for group delay aberrations near the –3 dB frequency. With
these probes the HM400 rise time/bandwidth remain nearly constant.
The probe HF adjustment also allows for an optimum pulse response
of the combination probe and scope.
With a 10:1 or 100:1 probe DC coupling has to be used if
the signal voltage exceeds 400 Vp.
As mentioned if AC coupling is used, the 1.6 Hz –3 dB frequency comes
into play which causes distortions with low frequency signals, e.g. STOP
square waves are displayed with tilt. With a 10:1 probe the low frequency
response is improved by a factor of ten (0.16 Hz). If the sensitivity with
this probe is insuffi cient, DC coupling and an external capacitor can
be used e.g. for ripple superimposed on a high dc potential. First the
input must be switched to GND, then the voltage applied, then the input
switched to DC.
The measurement of small voltages requires proper ground connec-
tions as close to the measuring point as possible. Use short ground
cables.
If a probe is to be used to contact a BNC connecto, a probe
to BNC adapter should be used in order to prevent groun-
ding problems.
STOP
If ripple or noise appears on small signals at high sensitivity settings,
multiple grounds resp. ground loops may cause the problem. The mains
safety earth is quite often the cause, because other test equipment will
also be connected to the same safety earth, this can generate currents
through the shields of cables etc. Most instruments have socalled
Y capacitors connected from the mains to safety earth.
or 28.3 V .
rms pp
B a s i c s i g n a l m e a s u r e m e n t

First time operation and initial settings

Prior to any use of the instrument make sure the power plug is inserted
before any other contacts are established.
Turn the instrument on by pressing the red button POWER, several
indicators will light up, the oscilloscope will perform a self-test. If any
errors are detected, there will be 5 acoustic signals; it is recommended
to submit the instrument to a service station. After a successful self-test
the instrument will be ready to operate, it will use the settings stored
from its last use.
If there is no trace after a 20 s wait, press the AUTOSET button. If the
trace is now visible, use the SELECT button, the ADJUST „+" resp. AD-
JUST „–„ buttons to set the desired intensity and optimum focus. For
best focus adjustment it is recommended to display a full screen sine
wave. If no input signal is connected, switch the input to GND in order
to prevent any noise from disturbing the focus adjustment.
In order to extend the life of the crt, set the intensity no higher than
needed for the specifi c measurement under the given ambient light
conditions. Take care not to leave a bright spot as this could burn the crt
phosphor. Do not turn the instrument on and off in short intervals.
It is recommended to always use fi rst the button AUTOSET and select
the buttons AC and CH 1 in the TRIGGER select area.

Trace rotation

In spite of the mumetal crt shield the earth's magnetic fi eld still has
some infl uence on the crt beam. Depending on the orientation of the ins-
trument the trace may not always remain parallel to the graticule lines.
See the chapter Controls for the adjustment of the trace rotation.

Probe adjustment and use of probes.

Probes have to be adjusted to the input they are connected to; this
adjustment has to be performed each time a probe is moved to another
input. A generator in the HM400 delivers a fast rise time (
wave signal of appr. 0.2 Vpp the frequency of which can be selected by
using the knob TIME/DIV (see the chapter Controls). The square wave
signal is accessible at the two contacts below the controls. The 0.2 V
are destined for 10:1 probes, suffi cient for 4 divisons at 5 mV/DIV.

1 kHz adjustment

This basic adjustment compen-
sates for the input impedance
of the scope, the probe's ca-
pacitor is adjusted so that the
capacitive division equals the
resistive division, the division
will thus be identical from DC
to high frequencies. With 1 : 1
probes or probes which can
be switched to 1 : 1, an adjustment is neither necessary nor possible.
Before this adjustment make sure the trace rotation adjustment was
performed (see TRACE rotation).
Connect the 10:1 probe to the input, e.g. CH 1, do not press any button,
set the coupling to DC, the sensitivity with VOLTS/DIV to 5 mV/DIV, the
TIME/DIV switch to 0.2 ms/DIV; make shure that both are calibrated,
i.e. the variables disabled. Connect the probe tip (and ground cable) to
the contact(s) „PROBE ADJUST" (see the photo); a 4 DIV display of two
signal periods should appear. Now adjust the probe capacitor (see the
<
5 ns) square
Subject to change without notice
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33