Measurement Basics
R&S
®
FSW-K40
31
User Manual 1173.9286.02 ─ 15
Because the reference level and attenuation are interdependent, changing the attenua-
tion manually may also adjust the reference level.
RF attenuation
RF attenuation is always available. It is a combination of mechanical and IF attenua-
tion.
The mechanical attenuator is located directly after the RF input of the R&S
FSW. Its
step size is 5
dB. IF attenuation is applied after the signal has been down-converted.
Its step size is 1
dB.
Thus, the step size for RF attenuation as a whole is 1
dB. Mechanical attenuation is
used whenever possible (attenuation levels that are divisible by 5). IF attenuation han-
dles the 1
dB steps only.
Example:
If you set an attenuation level of 18
dB, 15
dB are mechanical attenuation and 3
dB are
IF attenuation.
If you set an attenuation level 0f 6
dB, 5
dB are mechanical attenuation and 1
dB is IF
attenuation.
Electronic attenuation
Electronic attenuation is available with R&S
FSW-B25. You can use it in addition to
mechanical attenuation. The step size of electronic attenuation is 1
dB with attenuation
levels not divisible by 5 again handled by the IF attenuator. Compared to RF attenua-
tion, you can define the amount of mechanical and electronic attenuation freely.
4.9 Using Limit Lines
Limit lines provide an easy way to verify if measurement results are within the limits
you need them to be. As soon as you turn a limit line on, the application will indicate if
the phase noise a trace displays is in line with the limits or if it violates the limits.
The application provides two kinds of limit lines. 'Normal' limit lines as you know them
from the Spectrum application and special thermal limit lines for easy verification of
thermal noise results.
Phase noise limit lines
Phase noise limit lines have been designed specifically for phase noise measure-
ments. Their shape is based on the thermal noise floor of the DUT and the typical run
of the phase noise curve.
The typical slope of the phase noise curve depends on the offset from the DUT fre-
quency. In the white noise range (the noise floor), far away from the carrier, the slope
is more or less 0
dB per frequency decade. In the colored noise segment, the slope is
greater than 0
dB. The slope, however, is not constant in that segment, but again is
typical for various carrier offset segments (or ranges).
Using Limit Lines
