R&S ZNB Series User Manual page 229

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R&S ZNB/ZNBT
EMC Test Site Validation
EMC test sites for radiated emission measurements rely on free-space conditions to
minimize the influence of reflections on the received signal. Practically, near free-space
conditions are achieved by shielded enclosures fully lined with RF absorbing material.
Site validation determines deviations from free-space conditions that must meet the
acceptance criterion for making EMC compliance measurements in a FAR (= fully
anechoic room). It is performed by measuring the site voltage standing-wave ratio
(S
VSWR
caused by interference between direct (intended) and reflected signals.
blue arrow
red and green arrows = reflected signal
TD S
The ANSI time-domain method relies on a complex transmission measurement (S
using a vector network analyzer (VNA).
A time-domain transformation of the frequency domain data shows the impulse
response between the two antennas. Since the direct antenna-to-antenna response is
related to the shortest distance, the earliest impulse is the direct antenna response.
Reflections from the test site are from farther distance away, thus come at later time. It
is therefore possible to separate the direct antenna response S
S
21, reflected
TD S
User Manual 1173.9163.02 ─ 62
), which is the ratio of maximum received signal to minimum received signal,
= direct signal
Calculation
VSWR
by using time gating and to calculate
= (1+r) / (1-r) ,
VSWR
Concepts and features
Optional extensions and accessories
from reflections
21, direct
)
21
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