Biomedical Engineering Reference
In-Depth Information
magnetic heads from a discarded drum. Even a very worn-out head will work well in this
application. Very soiled heads should be cleaned with a swab and pure alcohol. Degaussing
will also help improve the sensitivity of an old head. All other aspects of constructing and
using this probe are the same as for the ferrite-bead probe.
For E-
eld probe is a coax cable in which a short segment of
the center conductor extends beyond the braid at the unterminated end of the coax. Similar
to the loop probe, a longer wire will pick up a stronger signal at the expense of speci
fi
elds, the simplest near-
fi
city
and bandwidth. In general, the length of the wire should be selected so that measurements
can be performed with a sensitivity of approximately 3 mV/m. At this level, potentially
problematic emissions can be identi
fi
fi
ed without causing undue concern about low-level
emissions.
Constructing the ideal H- or E-
fi
eld probe for a speci
fi
c job may take some trial and
error, since the e
ort of electromagnetic modeling required for proper design is most prob-
ably an overkill for most applications. One test that you may nevertheless want to perform
on a probe is to determine the existence of resonances within the desired spectral range.
To conduct the test, a wideband probe should be connected to an RF generator set up to
track the tuning frequency of a wideband spectrum analyzer. The probe under test should
be located in close proximity to the emitting probe and connected to the input of the spec-
trum analyzer. The limit of the useful bandwidth of a probe is the point at which the
ff
fi
rst
abrupt resonance appears.
Before even plugging the spectrum analyzer to the power line, however, the
fi
first step in
conducting a near-
eld EMI study should be to draw a component placement diagram of
the assembly to be probed. The diagram should indicate circuit points identi
fi
ed in the
mathematical circuit harmonic analysis as potential sources for EMI radiation. Only after
this preliminary work has been done should bench testing begin. A coarse near-
fi
eld sweep
should be conducted at relatively high gain to identify EMI hot spots in the assembly. A
technique that works well is to log the frequencies at which strong components appear
when scanning the unit under test. Detailed scanning using a more discriminating probe
can then concentrate on the hot spots to identify the culprit circuit generating o
fi
ff
ending
emissions.
A very valuable source of clues for future troubleshooting can be built along the way by
printing the spectral estimate at each point in which measurements strongly agree or dis-
agree with the circuit's harmonic analysis. In any case, keep detailed and organized notes
of the near-
fi
eld scans, since these will certainly prove to be invaluable when attempting
quick
fi
fixes while the clock is running at the far-
fi
eld compliance-testing facility.
BARE-BONES SPECTRUM ANALYZER
While an ac voltmeter can provide an indication of the
field strength to which a probe is
exposed, it does not provide any indication of the spectral contents of an emission. A spec-
trum analyzer is a tool that certainly cannot be beaten in the search for o
fi
ending signals.
Unfortunately, spectrum analyzers are often beyond the reach of a designer on a tight budget.
For near-
ff
fi
eld sni
ng, however, even the crudest spectrum analyzer will do a magni
fi
cent
job.
Figure 4.9 shows a simple home-brewed adapter to convert any triggered oscilloscope
into a spectrum analyzer capable of providing qualitative spectral estimates with a band-
width of 100 kHz to 400 MHz. As shown in Figure 4.10, a voltage-controlled TV tuner IC1
forms the basis of the spectrum analyzer. Most any voltage-controlled tuner will work, and
you may be able to get one free from a discarded TV or VCR printed circuit board. The con-
nection points and distribution vary from device to device, but the pinout is usually
identi
fi
ed by stampings on the metallic can of the device.
Search WWH ::
Custom Search