First Time Operation And Initial Settings; Trace Rotation; Probe Adjustment And Use Of Probes; Khz Adjustment - Rohde & Schwarz Hameg HM400 Manuel

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.
If ripple or noise appears on small signals at high sensitivity settings, STOP
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.

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 + / – 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
F i r s t t i m e o p e r a t i o n a n d i n i t i a l s e t t i n g s
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
probe manual for its location) until the square wave is perfectly fl at, i.e
there are neither under- nor overshoots. The transitions are invisible at
this sweep speed (see the pictures). The amplitude of the square wave
should be within 4 ±0.12 DIV.
undershoot

1 MHz adjustment

The probes supplied with the scope have additional adjustment ele-
ments which allow to correct for aberrations at high frequencies.
After this adjustment maximum bandwidth and best pulse response of
the combination scope and probe are obtained by achieving maximally
fl at group delay; overshoots, undershoots, ripple are minimized.
This adjustment requires a fast square wave generator (
output impedance (50 ohms) which delivers 0.2 Vpp at 1 MHz; the PROBE
ADJUST output of the scope fulfi lls these requirements.
Connect the 10:1 probe to the input to be used. Select PROBE ADJUST
signal 1 MHz with the knob „TIME/DIV (see the chapter Controls) switch
the coupling to DC, the VOLTS/DIV selector to 5 mV/DIV and the TIME/DIV
selector to 100 ns/DIV. Connect the probe tip and ground to the two
PROBE ADJUST contacts. The square wave will now be visible and also
its rising and falling slopes. See the probe manual for the location of
the adjustment elements.
Adjustment criteria:
Only the rising slope and the top of the square wave are of concern,
disregard the other portions of the signal.
– Short rise time
– Clean transition from the rising slope to the top of the square
wave with no over- or undershoot, fl at top.
< 5 ns) square
pp
undershoot
The amplitude of the square wave should be identical to that with the
1 kHz signal. It is important to always fi rst perform the 1 kHz adjustment,
in general a readjustment of the 1 kHz will not be necessary. Please
note that the probe adjust frequencies are not precise and hence must
not be used for any checks of the accuracy of the time base, also their
duty cycle is not controlled.
The probe adjust signal must conform to the requirements of zero
potential at the bottom of the square wave, precise amplitude and fl at
tops; its frequencies and duty cycles need not be precise.
correct overshoot
<
5 ns) and low
correct overshoot
Subject to change without notice
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