Frequency Range Displayed (Span); Setting Of The Amplitude Parameters (Ampl); Fig. 6.1: Hf Signal Modulated By A Sine Wave Signal And The Resultant Video Signal Vs. Time - Rohde & Schwarz HMS-X Benutzerhandbuch

Instrument Functions
6.3

Frequency range displayed (SPAN)

The span is the frequency range centered on the center
frequency which a spectrum analyzer displays on the
screen. In principal there are two methods to define the
displayed frequency range: defining start and stop
frequency or center frequency and span. The frequency
range called span is the range on both sides of the center
frequency which a spectrum analyzer displays on its
screen. The span to be selected depends on the signal to
be analyzed, in general, it should be at least twice as wide
as the bandwidth of the signal. With the span the
bandwidth of the signal to be analyzed is set.
Mathematically the span is the difference between stop
and start frequency. In simple terms, the span is the
magnitude of the spectral range and the center frequency
defines the position in the spectrum.
Fig. 6.1: Hf signal modulated by a sine wave signal and the resultant
video signal vs. time
The R&S®HMS-X resp. the R&S®HMS-X incl. options of-
fers the following frequency ranges (spans):
R&S®HMS-X (basic unit)
R&S®HMS-X +
R&S®HMS-3G / R&S®HV212
In zero span mode (0 Hz - Zero) the spectrum analyzer acts
similar to a receiver tuned to the center frequency. In this
case the trace display does not represent a spectrum, but
the amplitude over time. In other words the spectrum ana-
lyzer acts like a selective oscilloscope. For example the
zero span mode can be used to determine the maximum
of fluctuating signals at a specific frequency more detailed
or to represent parts of an amplitude modulation. Use the
softkey FULL to select the full span from 1MHz (100Hz)
to 1.6 GHz resp. 3 GHz. The softkey LAST will restore the
last span setting. The setting is performed as described in
chapt. 5.
6.4

Setting of the amplitude parameters (AMPL)

The key AMPL is used for all settings of the displayed
amplitude. The reference level (soft menu item REF.
LEVEL) is identical to the top graticule line of the display.
The setting is performed as described in chapter 5. The
reference level represents the amplitude level which is
displayed at the upper trace screen. The RF attenuation
setting at the input of the spectrum analyzer is directly
coupled to the reference level. Lowering the reference
level increases the sensitivity. Normally the reference level
58
1 MHz to 1.6 GHz
1 MHz to 3 GHz
is chosen to display the whole dynamic range on screen. If
the input signals are strong, a high reference level must be
set to prevent the analyzer signal path from being over-
driven and to keep the signal display within the display
range. When displaying the spectrum of a composite
signal, the reference level should be at least high enough
to ensure that all the signals are within the measurement
diagram.
The receiver input will be overloaded by a disadjusted reference
level.
The RF attenuation setting at the input of the spectrum
analyzer is directly coupled to the reference level. If the re-
ference level is high, RF attenuation is switched on in steps
according to the table 6.1, so that the input mixer always
remains in the linear range.
The default unit (UNIT) of the reference level is dBm. Al-
ternatively, the unit dBµV or the linear unit V and W can
be selected using the knob. The scaling of the linear units
V and W is set dynamically. Unit selection is of most rele-
vance to the marker level display as the marker level is dis-
played in the unit of the reference level.
If the linear unit V or W is selected, the reference level is adjus-
ted automatically. The linear scaling is only available in combina-
tion with R&S®HMS-EMC resp. R&S®HV213.
The measurement range (RANGE) determines the reso-
lution along the level axis in the measurement diagram.
The basic scaling is in dB. The standard scaling is 10 dB/
DIV. The spectrum analyzer also provides the level ranges
5 dB/DIV, 2 dB/DIV and 1 dB/DIV which enhance resolution
along the level axis. A higher resolution does not increase
the accuracy, it only improves the readability. An approp-
riate combination of reference level and vertical scale can
be used to get a more detailed display of the trace. If the
Unit is set to dBm or dBµV, the scaling of the reference le-
vel can be set to LIN% (linear percentage display). This me-
ans that a logarithmic unit is represented as a percentage
value of the reference level. This mode is useful if you want
to display, for example, a carrier being amplitude modula-
ted in the time domain (Span = 0 Hz).
The RF attenuation setting at the input of the R&S®HMS-X
is directly coupled to the reference level. 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:
❙ When adjusting reference level switching
LOW NOISE:
thresholds for attenuator and gain are optimised to get the
best signal/noise ratio.
❙ When adjusting reference level
LOW DISTORTION:
switching thresholds for attenuator and gain are optimised
for lowest possible distortion.
If the spectrum analyzer contains the R&S®HMS-EMC op-
tion resp. R&S®HV213 voucher incl. preamplifier the soft-