R&S FSW Series, Руководство пользователя

Страница: 450 / 1450

Common Measurement Settings
R&S
®
FSW
450 User Manual 1173.9411.02 ─ 43
To determine the required offset, consider the external attenuation or gain applied to
the input signal. For attenuation, define a positive offset so the R&S
FSW increases
the displayed power values.
If an external gain is applied, define a negative offset so the R&S FSW decreases the
displayed power values.
Note, however, that the internal reference level (used to adjust the hardware settings to
the expected signal optimally) ignores any "Reference Level Offset". Thus, it is impor-
tant to keep in mind the actual power level the R&S
FSW must handle, and not to rely
on the displayed reference level.
internal reference level = displayed reference level - offset
Example
1. The initial reference level is 2 dBm with no offset.
Both the displayed reference level and the internal reference level are 2 dBm.
2. An offset of 3 dB is defined.
The displayed reference level is adjusted to 5 dBm.
The internal reference level remains at 2 dBm.
(5 dBm (displayed ref level) - 3 dB (offset) = 2 dBm)
3. Now the user decreases the reference level to 1 dBm.
The displayed reference level is adjusted to 1 dBm.
The internal reference level is adjusted to:
1
dBm (displayed ref level) - 3 dB (offset) = -2 dBm.
8.4.1.2 RF Attenuation
The attenuation is meant to protect the input mixer from high RF input levels. The level
at the input mixer is determined by the set RF attenuation according to the formula:
"level
mixer
= level
input
– RF attenuation"
The maximum mixer level allowed is 0 dBm.
Mixer levels above this value may lead to incorrect measurement results, which is indi-
cated by the "RF Overload" status display. Furthermore, higher input levels may dam-
age the instrument. Therefore, the required RF attenuation is determined automatically
according to the reference level by default.
High attenuation levels also avoid intermodulation. On the other hand, attenuation
must be compensated for by re-amplifying the signal levels after the mixer. Thus, high
attenuation values cause the inherent noise (i.e the noise floor) to rise and the sensitiv-
ity of the analyzer decreases.
The sensitivity of a signal analyzer is directly influenced by the selected RF attenua-
tion. The highest sensitivity is obtained at an RF attenuation of 0 dB. Each additional
10 dB step reduces the sensitivity by 10 dB, i.e. the displayed noise is increased by 10
dB. To measure a signal with an improved signal-to-noise ratio, decrease the RF
attenuation.
Amplitude and Vertical Axis Configuration