Making Measurements - HP 84125A User Manual

Making Measurements

Note
The HP 84125A/B microwave EMI measurement test system may be
used for a variety of measurements. To assure the best, most accurate
measurements of your product, you should read and review the
required product regulations. The following example describes one
method for measuring the intentional radiation of a cellular telephone
in accordance with the current FCC, Part 15 regulation. However,
there are other methods that you can use that may work better for
your testing environment.
Current FCC regulations specify that harmonic emissions measured
above 1 GHz on wireless transmitters should be below 54 dB  V/m
using average detection at a distance of 3 meters. At higher
frequencies, cable loss and antenna factors bring the inherent system
noise level close to the regulatory limit. To improve the signal-to-noise
ratio, the FCC allows positioning the measuring antenna at a distance
closer than 3 meters. The correction to be applied is described in FCC,
Part 15.31 (f)(1) which states:
\At frequencies equal to or above 30 MHz, measurements may be
performed at a distance other than what is speci ed provided:
measurements are not made in the near eld; and, it can be
demonstrated that the signal levels needed to be measured at the
distance employed can be detected by the measuring equipment.
Measurements shall not be performed at a distance greater than
30 meters unless it can be further demonstrated that measurements
at a distance of 30 meters or less are impractical. When performing
measurements at a distance other than what is speci ed, the results
shall be extrapolated to the speci ed distance using an inverse
linear distance extrapolation factor (20 dB/decade)."
This would translate to measuring to a limit of approximately
64 dB  V/m at a distance of 1 meter. Therefore, we recommend that
you rst make a measurement at a distance of 1 meter to the 64
dB  V/m limit line with peak detection. Using a 1 meter measurement
will improve the signal-to-noise ratio making it easier to detect
emissions. Signals with a peak value above the 64 dB  V/m limit line
will need to be remeasured using average detection.
This procedure describes the FCC requirement. Measurements to
other regulatory standards would be similar but should use the
limits, measuring distances, and procedures described in those
regulations.
Even though the measurement is done speci cally for harmonic
emissions, any spurious emission from the test device that exceeds
the limit also fail the speci cation.
6
Making Measurements 6-1