ETS-Lindgren 7405 E & H Near Field Probe Set - Manual (Page 8)

8 Introduction www.ets-lindgren.com
Magnetic (H) Field Probes
The Model 7405 includes three
H-field probes of varying size and
sensitivity: models 901, 902,
and 903. These probes are highly
selective of the H-field while being
relatively immune to the E-field.
Each H-field probe contains a
single turn, shorted loop inside a
balanced E-field shield. The loops
are constructed by taking a single
piece of 50 ohm, semi-rigid coax
from the connector and turning it
into a loop. When the end of the
coax meets the shaft of the probe,
both the center conductor and the
shield are 360 degrees soldered to
the shield at the shaft.
Then a notch is cut at the high point of the loop. This notch creates a balanced
E-field shield of the coax shield. The loops reject E-field signals due to the
balanced shield.
Electric (E) Field Probes
The Model 7405 includes two E-field probes: the stub probe (model 904) and the
ball probe (model 905). Due to the small sensing element, the stub probe is
relatively insensitive. This is an advantage when the precise location of a
radiating source must be determined. For example, while moving the stub probe
over the pins of an IC chip, variations can be noted at spaces as close as two or
three pins.
By comparison, the ball probe is much more sensitive. The larger sensing
element does not offer the highly-refined definition of the source location which
the stub probe allows, but it is capable of tracing much weaker signals. The
impedance of the stub probe is essentially the same as that of a non-terminated
length of 50 ohm coaxial cable.