User manual

7
Subject to change without notice
G e n e r a l i n f o r m a t i o n s
High Impedance Probe HZ553
The high impedance probe is used to measure
directly on the component 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 pF
(0. pF at the HZ55 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 0:1. 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 HZ55 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 HZ55). The
HZ55’s sensor area is much smaller than that of
the HZ 55 probe and is located in the tip of the pro
-
be. Thus the postion of the probe when using the HZ
55 is vertical (or upright), comapred to a horizon
-
tal position of the HZ 55 probe when measuring.
Also the measuring distance of both probes to the
emmission source is different. Measurements with
the HZ 55 are made at a distance of 1 to  cm from
the radiation source, where the HZ55 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
component causes interference and which does
not.The HZ55 is also very good suitable to measu
-
re the time domain results of the emmission by
means of a fast digitizing scope (fBandwidth > 00
Mhz) which gives important additional information
on e.g. decoupling measures on PCBs.
Low Capacitance Probe HZ 555
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 HZ55). The probe is of very
high impedance (about 50 kOhms) and is loading
the test point with less than 0.pF (800 Ohms at
1GHz). 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 HZ55
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 beca-
me knowledgeable as regards EMI pre-vention e.g.
on EC boards. The worth of EMI countermeasures
often is seen only when radiation is measured. As
the amount of time and cost for such measure-
ments 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.