The Receiver Application
R&S
®
FPC
141
User Manual 1178.4130.02 ─ 07
Selecting the scale of the x-axis
You can display the x-axis in a linear or a logarithmic scale.
In a linear display, the frequencies are distributed linearly across the x-axis. That
means the entire frequency range is divided by the number of measurement points,
and the distance between measurement points is equal. Linear scaling is useful to
determine precise frequencies within a small range.
1 MHz
10 MHz
Figure 15-3: Linear x-axis scaling: the distance between the measurement points is equal, e.g.
200
kHz
However, if high and low frequencies appear in the same display, it is difficult to deter-
mine individual frequencies precisely or to distinguish frequencies that are close
together.
In a logarithmic display, lower frequencies are distributed among a much larger area of
the display, while high frequencies are condensed to a smaller area. Now it is much
easier to distinguish several lower frequencies, as they are spread over a wider area.
Logarithmic scaling is useful for overview measurements when a large frequency
range must be displayed in one diagram.
However, with logarithmic scaling, the frequency resolution between two measurement
points deteriorates with higher frequencies.
1 Hz
1 GHz
1 MHz
Figure 15-4: Logarithmic x-axis scaling: the distance between measurement points is variable
In the spectrum from 10
Hz to 100
Hz, the distance is a few Hz. Between 100
MHz and
1
GHz, the distance is several MHz.
Thus, for logarithmic x-axis scaling, the number of measurement points must be suffi-
ciently high in order to distinguish high frequencies precisely. The resolution bandwidth
should cover at least one measurement point (that means: the distance between two
measurement points should not exceed the RBW). If this condition is not met, signals
or interferers could be missed, especially narrowband interferers.
Insufficient measurement points -
Filter may miss a signal
Resolution filter bandwidth
covers one measurement point
Resolution filter bandwidth covers
several measurement points
1. Press the "Freq" key to open the frequency menu.
2. Select either the "Linear Scale" or "Log Scale" menu items.
The R&S
FPC changes the scale of the x-axis accordingly.
Frequency Configuration
