Measurement Overview - HP 84125C User Manual

Measurement Overview

1-2 Product Description
The HP 84125C 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 is a brief description of
how to use the system to test a product to the FCC Part 15 regulation.
A detailed measurement example for measuring an intentional
radiator has been provided in Chapter 6.
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.