User manual

37
Subject to change without notice
How will the level of ambient signals
affect my radiated emissions measure-
ments and will using fully compliant
measuring equipment help?
The level of ambient signals that exist in your
laboratory or at a particular “open area test site”
directly affects your ability to make radiated
emissions measurements. In some industrial
environments, the existing ambient may make
obtaining radiated emissions measurements very
difcult.Thereisnoadvantageinusingcompli-
ance instruments in such locations.
Will a screened room overcome the
problem of ambient signals?
Any Faraday cage structure provides screening
from externally generated ambient signals. Howe-
ver, in an unlined screened room, any electroma-
gnetic energy inside from the unit under test, for
instance-reectsfromallthemetallicsurfaces
of the room. The result is a very uneven and posi-
tionsensitiveeld,andyouwillmeasuredifferent
levels depending on the positions of both the unit
and the antenna. You may encounter errors up to
±40dB irrespective of the instruments you use.
Can I make meaningful radiated emis-
sions measurements with a near eld
probe instead of an antenna?
The two types of measurements are different. In
fact, they are complementary and not alternatives.
Aneareldprobeonlyoperatesinthe“neareld”,
whereasanantennaoperatesinthe“fareld”.In
theneareldyoucannoteasilyquantifytheim-
pedanceofthesource,whereasinthefareldthe
impedance of free space is a constant 377Ohms.
Forexample,anear eldprobe allows youto
detect RF currents on printed circuit boards, and
consequently enables you to pin-point problem
signalsthatyourstobservedusinganantenna
on an open area test site. If you are going down to
detect the source of the radiation (e.g. a transistor,
IC, or any other component) on the contact-level,
the high-impedance probe of the HZ530 probe
set is indispensable for measurements due to its
ultra-low input capacitance of 2pF only.
Do spectrum analyzers have any advan-
tages over receivers?
Yes.Spectrumanalyzersaregenerallymoreexi-
ble, particularly for design and diagnostic testing.
The main advantage is that you can observe large
frequency spans, whereas a receiver only displays
one amplitude and one frequency at a time. An
additional advantage of the HM5000 Spectrum
Analyzer series is its fast sweep rate. The display
is “updated” every 23ms which allows very fast
tracking of the test signal.
Will a spectrum analyzer allow me to
make sensible emissions measure-
ments?
Themainproblemwithnoiseooroccurswhen
you make radiated emissions measurements.
Radiated emissions limits for residential, com-
mercial, or light industrial environments with a
10-meter separation between a unit under test
and an antenna are 30 to 230 MHz for 30 dB uV/m,
and 230 to 1000 MHz for 37 dB uV/m. A typical ma-
ximumreceivernoiseooris1-5dBuV,whereas
foraspectrumanalyzerthenoiseoorisaround
10-20 dB uV. Clearly, sensible measurements at
some frequencies are not easily obtained with
aspectrum analyzerbecause the noise ooris
comparable with the limit.
You can overcome this limitation by using a 3-me-
ter test distance, or by use of a pre-selector or
preamplier(whichisbuilt-inforexampleinthe
E-Field probe of the HZ530) with the spectrum
analyzer. Each of these methods effectively in-
creasesthelimitorreducesthenoiseoorofthe
measurement.
On the other hand we observed in many cases
clearly noticeable radiation from the test recei-
ver or measuring equipment itself. Concerning
this phenomena the HM5000 series spectrum
analyzer has a big advantage due to its strictly
analog design. There is no self generated radia-
tion measurable from the analyzer within its own
sensitivity range.
Commonly asked Questions Commonly asked Questions