Common Measurement Settings
R&S
®
FSW
394
User Manual 1173.9411.02 ─ 19
The optimum reference level for the current measurement settings can be set automat-
ically by the R&S
The reference level determines the amplitude represented by the topmost grid line in
the display. When you change the reference level, the measurement is not restarted;
the results are merely shifted in the display. Only if the reference level changes due to
a coupled RF attenuation (see
ment is restarted.
In general, the R&S
FSW measures the signal voltage at the RF input. The level dis-
play is calibrated in RMS values of an unmodulated sine wave signal. In the default
state, the level is displayed at a power of 1 mW (= dBm). Via the known input impe-
dance (50 Ω or 75 Ω, see
on page 319), conversion to other units is pos-
sible.
6.4.1.2
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 indicated by the "RF OVLD" status display.
Furthermore, higher input levels may damage 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.
For ideal sinusoidal signals, the displayed signal level is independent of the RF attenu-
ation.
Depending on the type of measurement evaluation that is required, a compromise
must be found between a low noise floor and high intermodulation levels, and protect-
ing the instrument from high input levels. This is best done by letting the R&S
FSW
determine the optimum level automatically (see
Amplitude and Vertical Axis Configuration
