User manual
7
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
difcult. 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 electromag-
netic energy inside from the unit under test, for
instance - reects 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
±0dB 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 77Ohms.
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 HZ50 probe
set is indispensable for measurements due to its
ultra-low input capacitance of pF 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 ms 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 0 to 0 MHz for 0 dB uV/m,
and 0 to 1000 MHz for 7 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
10-0 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 -
meter test distance, or by use of a pre-selector
or preamplier (which is built-in for example in
the E-Field probe of the HZ50) with the spec
-
trum analyzer. Each of these methods effectively
increases 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.
Co m m o nly a s ked Q u est i o n s
C o m m o n l y a s k e d Q u e s t i o n s