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Connection of Test Signal
Caution:
When connecting unknown signals to the oscillo¬
scope input, always use automatic triggering and set the
DC-AC input coupling switch to AC. The attenuator switch
should initially be set to 5 V/div..
Sometimes the trace will disappear after an input signal has
been applied. The attenuator switch must then be turned
back to the left, until the vertical signal height is only 3-8div.
With a signal amplitude greater than lOOVpp, an attenuator
probe must be inserted before the oscilloscope's vertical
input. If, after applying the signal, the trace is nearly
blanked, the period of the signal is probably substantially
longer than the set value on the TIME/DIV. switch. It
should be turned to the left to an adequately larger time
coefficient.
The signal to be displayed can be connected directly to the
Y-input of the oscilloscope with a shielded test cable such
as HZ 32 and HZ 34 or attenuated through a xIO or xlOO
attenuator probe. The use of test cables with high imped¬
ance circuits is only recommended for relatively low fre¬
quencies (up to approx. 50 kHz). For higher frequencies, the
signal source must be of low impedance, i.e. matched to
the characteristic resistance of the cable (as a rule 50 Ohm).
Especially when transmitting square and pulse signals, a
resistor equal to the characteristic impedance of the cable
must also be connected across the cable directly at the Y-
input of the oscilloscope. When using a 50 Ohm cable such
as the HZ 34, a 50 Ohm through termination type HZ22 is
available from HAMEG. When transmitting square signals
with short rise times, transient phenomena on the edges
and top of the signal may become visible if the correct ter¬
mination is not used. A terminating resistance is some¬
times recommended with sine signals as well. Certain
amplifiers, generators or their attenuators maintain the
nominal output voltage independent of frequency only if
their connection cable is terminated with the prescribed
resistance. Here it must be noted that the terminating resis¬
tor HZ 22 will only dissipate a maximum of 2 Watts. This
power is reached with 10 V^ms or - at 28.3 Vpp with sine sig¬
nal.
If a x10 or x100 attenuator probe is used, no termination is
necessary. In this case, the connecting cable is matched
directly to the high impedance input of the oscilloscope.
When using attenuators probes, even high internal imped¬
ance sources are only slightly loaded (approx. 10 MQ II 16
pF or 100 MQ II 7 pF with HZ 53). Therefore, if the voltage
loss due to the attenuation of the probe can be compen¬
sated by a higher amplitude setting, the probe should
always be used. The series impedance of the probe pro¬
vides a certain amount of protection for the input of the ver¬
tical amplifier. Because of their separate manufacture, all
attenuator probes are only partially compensated, therefore
accurate compensation must be performed on the oscillo¬
scope (see "Probe compensation" page M7).
Standard attenuator probes on the oscilloscope normally
reduce its bandwidth and increase the rise time. In all cases
where the oscilloscope band width must be fully utilized
(e.g. for pulses with steep edges) we strongly advise using
the
modular probes HZ 51
(x10)
HZ52{x^
0 HF) and
HZ54
(x1 and xIO, see oscilloscope accessories, page Z1). This
can save the purchase of an oscilloscope with larger
bandwidth and has the advantage that defective compo¬
nents can be ordered from HAMEG and replaced by one¬
self. The probes mentioned havea 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 1 MHz calibrator, e.g. HZ60. In
fact the bandwidth and rise time of the oscilloscope are not
noticably changed with these probe types and the wave¬
form reproduction fidelity can even be improved because
the probe can be matched to the oscilloscope's individual
pulse response.
If a xIO orxIOO attenuator probe is used,
DC
input cou¬
pling must always be used at voltages above 400 V.
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 oscil¬
loscope input coupling capacitor concerned. Its voltage rat¬
ing is max. 400 V (DC -F peak AC). DC input coupling is
therefore of quite special importance with a xlOO attenua¬
tion probe which usually has a voltage rating of max. 1 200
V (DC
+
peak AC). A
capacitor
of corresponding capaci¬
tance 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 volt¬
age
must be derated with frequency usually above 20 kHz.
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 impor¬
tant 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 cur¬
rents 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, which is often supplied as probe accessory, 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
M6 203-7
Subject to change without notice
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