User manual

27
Subject to change without notice
General informations
High Impedance Probe HZ553
The high impedance probe is used to measure
directly on the com ponent under test or e.g. at
the conductive trace of a PC board. This allows
the precise determination of the emission source.
It features a low input capacitance of only 2pF
(0.3 pF at the HZ545 Low Capacitance Probe) and
supplies only a very small electrical charge to the
device under test. The attenuation of the probe is
in the range of 10:1 to 30:1. Testing is performed
similar to an oscilloscope probe. Due to ist paten-
ted construction a ground termination by cable is
not necessary (see also HZ555).
µ-Magnetic-Field Probe HZ554
The HZ554 is used to check for magnetic distur-
bances at e.g. even smallest SMD devices. It also
detects emissions generated by currents in the
ground plane of PCBs. The probe is sensitive to
changingmagnetic elds (similar to HZ552). The
HZ554’s sensor area is much smaller than that
of the HZ552 probe and is located in the tip of the
probe. Thus the postion of the probe when using
the HZ 554 is vertical (or upright), comapred to a
horizontal position of the HZ552 probe when mea-
suring. Also the measuring distance of both probes
to the emmission source is different. Measurements
with the HZ552 are made at a distance of 1 to 2 cm
from the radiation source, where the HZ554 probe
uses direct mechanical contact of the tip to the
pointunder test. When using the H-eld probes
one observes a rapid increase of the probe’s output
voltage as the interference source is approached.
While investigating a circuit board, the sources are
immediately obvious. It is easily noticed which com-
ponent causes interference and which does not.The
HZ554 is also very good suitable to measure the time
domain results of the emmission by means of a fast
digitizing scope (fBandwidth
> 300 MHz) which gives
important additional information on e.g. decoupling
measures on PCBs.
Low Capacitance Probe HZ555
When measuring at low signal levels and when low
capacitive load is a matter, the low capacitance
probe HZ555 is the right tool. It is a direct-contact
probe to an oscilloscope probe. Due to ist patented
construction a ground termination by cable is not
necessary (see also HZ553). The probe is of very
high impedance (about 250 kOhms) and is loading
the test point with less than 0.3pF (800 Ohms at
1 GHz). Thereby one can measure directly in a
circuitwithoutsignicantlyinuencingtherela-
tionships in the circuit by the probe.
Radiation Probe HZ556
The passive probe HZ556 owns the same cha-
racteristicsas the magneticeld probeHZ552
except,itdoesnotfeatureabuilt-inamplier.Thus
it can be used as an emitting or receiving antenna.
Practical EMI problems
The electronics circuit designer meanwhile
became knowledgeable as regards EMI pre-
vention e.g. on EC boards. The worth of EMI
countermeasures often is seen only when ra-
diation is measured. As the amount of time and
cost for such measurements is high, the effect
of individual circuit changesis seldomly tested.
After several circuit changes were made a test
will not reveal anymore which effect an individual
measure had.
It is hence advantageous to test prior togoing to a
testlabusingthenear-eldprobesresp.sniffer
probes mentioned. The E eldprobe reacts to
electricACelds,theHeldprobeissensitiveto
changesofmagneticux.
Before using these probes one is well advised
torealize which elds play the decisive rolein
modern EC boards. In the case of high voltages
butlowcurrentstheEeldwillbepredominant.
In the case of low voltages and high currents the
Heldwill dominate. The formercasewas the
rule with electron tube circuits.
Modern IC’s operate with low voltages and high
currents. Of course, it is not the amplitude of
a current which counts but in addition its rate
of change (or frequency). If an electromagnetic
wave is generated it is also the rate of change
ofthemagneticeldvs.unitoftimewhichisthe
determining factor.
It is exactly this component which is sensed by
the H eld probe.The amplitude ofthe probe
signalisdirectlyproportionaltotheuxchange
and thus to the change of the current creating the
eld.Hence theseprobes are eminently suited
toarstandroughtestoftheefciencyofEMI
counter-measures.