ETS-Lindgren 7405 E & H Near Field Probe Set - Manual (Page 32)

32 Common Diagnostic Techniques www.ets-lindgren.com
U SING S NIFFER P ROBES
Typically, there are several possible sources for a given signal. To identify the
particular one in question, use the sniffer probes.
1. From a set of loop probes of varying sizes, start with the largest, which
is also the most sensitive. Begin several feet from the unit and look at
the signal of interest. Search for the maximum and approach the unit
along the line of maximum emission.
2. As you near the unit, switch to the next smaller probe; this probe will
be less sensitive but will differentiate the signal source more narrowly.
Often the initial probing locates where the signal is escaping from the
unit, indicating the point of escape from the housing.
3. Once inside the unit and inside any shielding, look for the source of the
signal; use the smallest diameter probe available. You may switch to
the stub probe, which is a small and insensitive E-field probe that can
be used to get close to the signal source.
Finding both the point of escape from the unit and the actual source provides
choice in engineering the solution: you may decide to improve the shielding or to
suppress the source. The more solution alternatives you identify the greater the
chance of identifying one which meets all the requirements of schedule, cost, and
performance.
Another procedure is to use electromagnetic probes in conjunction with regular
scope probes.
1. Connect a regular scope probe and switch back and forth to refine the
offending components as finely as possible. Using this combination
can define a radiating source to a specific signal line.
2. Periodically disable portions of a circuit to make a final determination of
the location of the source. For example, disable a line driver to see if
the radiation is coming from the base unit or from a cable. When
disabling parts of a circuit, use a sensitive probe and take readings
several meters from the unit.
Clear the scope probe out of the unit when making radiated readings;
an attached scope probe can easily radiate and mask the real problem.
When done, you should have a good idea of the exact location of the
offending signal.