ETS-Lindgren 7405 E & H Near Field Probe Set - Manual (Page 28)

28 Common Diagnostic Techniques www.ets-lindgren.com
The various specifications are given in the frequency domain, so there are
many dBuV at a particular bandwidth over a given frequency range. However,
most EUT operations are characterized in the time domain: 150 ns memory
access time, 300 V/ms slew rate, and so on. This section presents a technique
that will aid in linking emissions with the signals that create them.
During testing you may receive information indicating, for example, that it failed
by 10 dB at 40 MHz and 3 dB at 120 MHz. The challenge is to find the
EUT function that created the emissions. You may be able to connect the probe
to a spectrum analyzer and locate the source; locating the source of an
emanating signal begins by finding the exit points. Cover seams and air flow
vent holes are primary suspects.
However, many sources can emit at a given frequency. Most of these emissions
are non-propagating, reactive fields. The most helpful first step in locating the
sources of a propagating field is to demodulate the offending signal while it is
being received in the far-field. Demodulation gives a time domain representation
of the signal. This time domain representation will appear in some way similar to
an oscilloscope trace of the radiating signal.