Setting Of The Bandwidth (Bandw); Fig. 5.2: The Selections Offered In The Rbw Menu; Setting Of The Sweep - Hameg HMS 3000 Handbuch

The reference level represents the amplitude level which is
displayed at the upper trace screen boundary. The actual set-
ting is shown in the third line left in the readout. Adjusting the
reference level automatically switches attenuator, gain and the
optional preamplifier. Lowering the reference level increases
sensitivity. Normally the reference level is chosen to display
the whole dynamic range on screen. For strong input signals
the reference level must be set high in order to prevent over-
driving of the signal amplifier chain and in order to keep the
signal within the visible display window. For spectra with many
signals, the reference level should be so high that all signals
remain within the display area.
The receiver input will be overloaded by a disadju-
sted reference level.
The basic unit (UNIT) of the reference level is the unit dBm.
Alternatively, the unit dBµV can be selected by pushing the
soft menu key and using the knob. The range (RANGE) defines
the resolution of the amplitude axis of the display. The basic
scaling is in dB. The standard scaling is 10 dB/DIV. In order to
obtain a higher visual resolution, the spectrum analyzer also
offers the scalings 5 dB/DIV, 2 dB/DIV, and 1 dB/DIV. A higher
resolution does not increase the accuracy, it only improves
the readability. Using an appropriate combination of reference
level and vertical scale can be used to get a more detailed
display of the trace.
The reference offset is used to vertically shift the trace if trace
math is switched on.
The setting of the reference level will also directly affect the
amount of RF attenuation at the input of the spectrum analyzer.
The attenuation setup menu is used to influence the thresholds
used for automatic attenuator selection when the reference
level is adjusted.
The instrument offers two different modes of coupling which
are selected via the softkey ATT-SETUP:
– LOW NOISE: When adjusting reference level switching
thresholds for attenuator and gain are optimised to get the
best signal/noise ratio.
– LOW DISTORTION: When adjusting reference level switching
thresholds for attenuator and gain are optimised for lowest
possible distortion.
If the unit contains the option „Preamplifier" this soft key is
used to activate or deactivate the preamplifier. The preamplifier
increases the signal/noise ratio by 10 dB (refer to chapter 12.1
for activate the optional preamplifier).
5.5 Setting of the bandwidth
(BANDW)
Spectrum analyzers resolve the spectral content of a signal
and display a frequency spectrum. The quality of the resolution
is determined by the resolution bandwidth. Additionally, the
spectrum analyzers offer 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 RBW
(resolution bandwidth) selected (provided the user did not set
the span to manual operation).
The video bandwidth affects the smoothing (reduction of noise)
of the displayed curve. 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 bandwidth
I n s t r u m e n t f u n c t i o n s
RBW
100 Hzk
200 Hzk
1 kHz
3 kHz
10 kHz
30 kHz
100 kHz
200 kHz
300 kHz
1MHz
Table 5.1: Available RBW and VBW settings
has no influence on the resolution properties of the spectrum
analyzer.
If the span was set manually too wide or the sweep
time to too high, the amplitudes will be displayed
with incorrect level; in such cases a red UNCAL
message will warn. The span must then be reduced
until the UNCAL message disappears.
By pushing the key BANDW you will enter the menu for setting
the bandwidths. Both the resolution bandwidth (RBW) and the
video bandwidth (VBW) may be set within the specified limits.
The table 5.1 shows the step sizes which are available.
Additionally automatic selection for both bandwidths (AUTO
RBW/AUTO VBW) may be chosen with the respective soft menu
key. The knob is used for the setting of the parameters.

Fig. 5.2: The selections offered in the RBW menu

5.6

Setting of the SWEEP

At a frequency spectrum of f 1 0 Hz the sweep time is the time
the spectrum analyzer requires for sweeping the selected
frequency range to measure the spectrum. Certain limits have
to be observed (e.g. the resolution bandwidth set) in order to
obtain a correct display.
Pushing the key SWEEP will call the selection menu. The
SWEEP TIME can be varied within the specified limits. The
VBW
10 Hzk
30 Hzk
100 Hzk
300 Hzk
1 kHz
3 kHz
10 kHz
30 kHz
100 kHz
300 kHz
1 MHz
3 MHz
43
Subject to change without notice