AR FI7000, Operating And Service Manual

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PL7004 Kit
16 Rev F
3.3.2.5 Out-of-Band Response and In-Band Response Considerations
Although the specified operating frequency range of a probe may be limited, probes typically respond to signals
both above and below their specified frequencies. Users should be alert to unexplained readings that may be
caused by unintended fields. These fields may be in-band or they may be out-of-band fields. Pay special
attention to fields generated by a UUT or by test equipment located very close to a probe.
Probes can exhibit some response to frequencies as low as the 50/60 Hz line frequency. Locate operating probes
well separated from AC power lines, and power supplies including switching power supplies.
Beyond the upper frequency limit, similar problems can occur. Beyond the specified upper operating frequency,
the dimensions of the probe and any conductive enclosures (housing)
may become appreciable in relation to
ambient field wavelength. This can create responses that are variable due to pattern changes of the sensor or due
to reflections from an electrically conductive housing.
In each case in which active field monitoring is to be used during UUT operation, it is recommended to first
operate the UUT and test equipment together with the probe and
no test field in order to determine the measured
level of any ambient field. The ambient field level should be low enough to provide an error contribution
consistent with the application requirements.
3.3.2.6 Harmonics
Attention should be paid that the error introduced by the level of harmonics, and any other carriers present in the
field when a measurement is made, are consistent with the accuracy requirements. Probes using diode detectors
have a varying sensitivity to harmonics and other carriers. At low field levels the detector typically operates in a
square law range and the nominal effect of a harmonic is to add its
power to the power in the fundamental to
produce a reading. (Reading = square root of sum of squares of the two field levels.) At higher field levels the
detectors may not be in their square law range and tend to respond more to the peak RF level. Readings are more
likely related to the sum of the peak values of the field at each sensor location. This value will vary with the
relative phases of the components. Only the bounds of the response are predictable.
Probe sensors using a single diode detector may produce different readings when the probe is inverted (rotated
180 degrees about the detector in the plane of the sensor). This may be more noticeable at higher field levels
where detectors are not in their square law range. Some probe sensors incorporate diode detectors used in pairs
to reduce this effect.
3.3.2.7 Noise Power
While all RF sources produce broadband noise, some produce more of this noise than others (notably
TWTAs). This noise may produce (non-zero) readings on field probes. Although the noise is at a low noise
power density level, the noise from broadband RF sources accumulates over the broad frequency range of the
probe. This is usually resolved by operating the RF power source near to its rated power. Under this
condition, the noise power level is low enough for most applications. Using space loss or other attenuation, or
using a lower power amplifier (preferred for safety) are among the remedies. This effect is easily checked by
operating the RF source with no carrier and observing the probe response.
3.3.2.8 Measurement of Isotropy
Since typical probes exhibit some axis offset, the offset will likely affect the common isotropy measurement
(about a rotational angle) in which the rotational axis does not coincide with the sensor axis, unless this
measurement is accomplished in a
known uniform field. Since the rotating sensors are moved in the direction
of propagation of the field, this uniform field is valid only for fields that exhibit a very small variation with
distance from the source. This type of field can exist in TEM cells and in radiated fields far from the radiating