Page 16 of 25
User manual
Antenna model: AT3000
Rev 6.0 – September 2014
Clampco Sistemi is a registered trademark of
Calzavara SpA
– via Corecian, 60 – 33031 – Basiliano (Italy) http://www.clampco-sistemi.com/
It would be possible to establish a SA noise figure and then add the RBW contribution. However, it is better to
consider a given state of the instrument for simplicity. For example, a portable SA with 0 dB input attenuation,
sampling set to “sample”, reference level at –70 dBm, RBW = 1 kHz and VBW = 30 Hz should have a background
noise level
N
dBm
of less than –120 dBm at a frequency of 1 GHz.
Neglecting the contribution from the antenna noise, we can calculate the corresponding equivalent electric field at
the point of measurement due to the SA noise
N
dB V/m
as follows: convert the background noise reading to dBμV,
e.g., if
N
dBm
= -120 dBm,
N
dB V
=
N
dBm
+ 107dB = -13 dBµV, and add both the antenna factor
K
dB (1/m)
and the cable
attenuation
Att
dB
. For example, if
K
dB (1/m)
+
Att
dB
= 46 dB at 1 GHz, then
N
dB V/m
= 33 dBμV/m that corresponds to
about 45 μV/m.
In order to identify a signal, we must obviously distinguish it from the noise. If we want a signal to noise ratio
S/N
dB
of at least 10 dB, the sensitivity level of the measurement system
S
dB V/m
will be
S
dB V/m
=
N
dB V/m
+
S/N
dB
= 43
dBμV/m, corresponding to about 150 μV/m = 0.15 mV/m. The SA set-up could of course be different. However, as
it can be deduced above, the sensitivity level of the entire measurement system depends on the background noise
level displayed by the SA.
Finally, it should be noted that the antenna factor
K
and the background noise of the SA vary with the frequency:
in practice, if the other parameters being equal, the lower the antenna factor
K
, the greater the sensitivity.
Therefore, the user must evaluate, from time to time, the background noise level that is present in his measuring
chain under the conditions in which he intends to perform measurements. With the procedure illustrated, he will
be able to determine the minimum sensitivity threshold. It is worth noting that very small sensitivity values can be
achieved by setting a low RBW value.
3.5. Maximum applicable field strength
AT3000 is a passive device except for its internal three-ways RF switch. The 1 dB compression point of this switch is
about 20 dBm.
This means that, no matter what is RF cable attenuation, the corresponding 1dB compression point, in terms of a
generic electric field component
E
a
(where a can be x, y, or z) is
E
a
[1dB] = 20 dBm + 107 dB +
K
dB (1/m)
, where the
result is expressed in dB V/m.
Using the minimum antenna factor (about 42 dB/m), the minimum compression point is about 300 V/m in the
working frequency band. This value corresponds to the maximum applicable field strength that guarantees a
correct measurement.
Содержание AT3000
Страница 2: ......
