Pre Compliance Emissions Testing: Accounting for Ambient Noise
The first method is to severely narrow the frequency range of the test such that a particular EUT emission stands out from a close by ambient noise signal.
A narrow RBW results in a very long sweep time, which explains why it is normal to first collect data with a wide RBW to collect all emissions quickly. Only afterwards is a particular part of the frequency spectrum ‘homed in upon’.
For maximum pick up of a signal, the receive antenna must be in the same ‘polarization’ as the received radio wave. That is, if an ambient noise transmitting station radiates a vertically polarized signal (E-Field
pointing up and down), ideally the intended receive antenna is perfectly aligned with the transmitted signal, i.e. the antenna rods are also pointing up and down. Conversely, minimum pick up occurs for the same radio wave when the rods are horizontal to the wave.
The test distance can be changed with relative ease, where the nearer the antenna is to the EUT, the larger the measured EUT noise signal is. Meantime, due to the great distance from the source, measured ambient noise signals stay constant.
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