59
Instrument Functions
The preamplifier increases the signal/noise ratio by 10 dB.
6.4.1 Reference Offset
The reference offset is used to vertically shift the trace if
trace math is switched on. The reference offset adds a se-
lectable value to the reference level. This is useful when
prior to the RF input, an attenuator or an amplifier is used.
The spectrum analyzer automatically takes the loss or gain
into account when the level is displayed and no manual
calculations are necessary. A loss introduced at the RF in-
put must be entered as a positive number and a gain as a
negative number. The value input of the reference offset
(REF. OFFSET) is always given in dB, even if the reference
level is set to a different unit.
6.5 Setting of the bandwidth (BANDW)
Spectrum analyzers resolve the spectral content of a signal
and display a frequency spectrum. The quality of the reso-
lution is determined by the resolution bandwidth (RBW).
Additionally, the spectrum analyzer offers a selectable
video bandwidth. The instrument will automatically (or, if
desired, manually) choose a slower sweep time if the span
was set too wide for the selected RBW (provided that the
user did not set the span to manual).
The video bandwidth (VBW) affects that the video voltage
is smoothed by video filtering to, say, reduce noise on the
trace. It is determined by the bandwidth of the low pass
filter inserted between the video signal and the display.
In contrast to the resolution bandwidth the video band-
width has no influence on the resolution properties of the
spec-trum analyzer. By pushing the BANDW key the set-
ting menu of the bandwidths will open. Both the resolution
bandwidth (RBW) and the video bandwidth (VBW) may
be set within the specified limits. The table 6.2 shows the
The reference offset function is only selectable if the trace math
is activated (TRACE). If the trace math is activated the reference
offset function is not grayed out anymore.
If a wide span was set manually or the sweep time was too high,
the amplitudes will be displayed with incorrect level; in such ca-
ses a red UNCAL message will warn. The span must then be re-
duced until the UNCAL message disappears.
step sizes which are available: Additionally automatic se-
lection for both bandwidths (AUTO RBW/AUTO VBW) may
be chosen with the respective softkey. The knob is used
for the setting of the parameters.
6.6 Setting of the SWEEP
With a frequency spectrum of f > 0 Hz the sweep time is
the time the spectrum analyzer requires for sweeping the
selected frequency range to measure the spectrum. Cer-
tain limits have to be observed (e.g. the resolution band-
Fig. 6.2: The selections offered in the RBW menu
Table 6.1: Relation between reference level and automatic setting of RF attenuation
Preamplifier OFF
Preamplifier ON
Reference Level
ATT-Setup
Low Noise
ATT-Setup
Low Distortion
ATT-Setup
Low Noise
ATT-Setup
Low Distortion
Preamplifier
20 dBm
30 dB
30 dB
30 dB
30 dB
OFF
15 dBm
30 dB
30 dB
30 dB
30 dB
OFF
10 dBm
20 dB
30 dB
20 dB
30 dB
OFF
5 dBm
20 dB
30 dB
20 dB
30 dB
OFF
0 dBm
10 dB
20 dB
10 dB
20 dB
OFF
–5 dBm
10 dB
20 dB
10 dB
20 dB
OFF
–10 dBm
0 dB
10 dB
0 dB
10 dB
OFF
–15 dBm
0 dB
10 dB
10 dB
10 dB
ON
–20 dBm
0 dB
0 dB
10 dB
10 dB
ON
≤ –25 dBm
0 dB
0 dB
0 dB
0 dB
ON
RBW
VBW
100 Hz *)
10 Hz *)
300 Hz *)
30 Hz *)
1 kHz *)
100 Hz
*)
3 kHz *)
300 Hz
*)
10 kHz
1 kHz
30 kHz
3 kHz
100 kHz
10 kHz
200 kHz
30 kHz
300 kHz
100 kHz
1 MHz
200 kHz
300 kHz
1 MHz
Table 6.2: Available RBW and VBW settings
*) only available in combination with R&S®HMS-EMC resp. R&S®HV213
Содержание HMS-X
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