Using Markers and Delta Markers – MKR Key R&S FSL
1300.2519.12 4.56 E-11
AF demodulation
The R&S FSL provides demodulators for AM and FM signals. With these demodulators, a displayed
signal can be identified acoustically by using headphones.
CAUTION
Risk of hearing damage
Check the volume setting carefully before putting on the headphones in order to
protect your hearing.
For span > 0, the demodulation is not continuous. The frequency at which the demodulation takes place
is set by the active marker. If the level of the selected frequency is above the threshold line, the sweep
stops for the selected time (stop time) and the RF signal is demodulated. For span = 0, the
demodulation is continuously active irrespective of the stop time set.
Frequency measurement with the frequency counter
In order to accurately determine the frequency of a signal, the R&S FSL is equipped with a frequency
counter which measures the frequency of the RF signal at the intermediate frequency. Using the
measured IF, the R&S FSL calculates the frequency of the RF input signal by applying the known
frequency conversion factors.
The frequency measurement uncertainty depends only upon the accuracy of the frequency reference
used (external or internal reference). Although the R&S FSL always operates synchronously
irrespective of the set span, the frequency counter delivers a more exact result than a measurement
performed with a marker. This is due to the following:
• The marker measures only the position of the point on the trace and infers from this value the signal
frequency. The trace, however, contains only a limited number of points. Depending upon the
selected span, each point may contain many measurement values, which therefore limits the
frequency resolution.
• The resolution, with which the frequency can be measured with a marker, is dependant on the
selected resolution bandwidth which in return affects the necessary measurement time. For this
reason, the bandwidth is normally made as wide as possible and the sweep time as short as
possible. This results in a loss of frequency resolution.For the measurement with the frequency
counter, the sweep is stopped at the reference marker, the frequency is counted with the desired
resolution and then the sweep is allowed to continue.
Measurement of noise density
During noise measurement, the noise power density is measured at the position of the marker. For
span = 0, all points of the trace are used to determine the noise power density. For span > 0, two points
to the right and left of the marker are used for the measurement to obtain a stable result.
The noise power density is indicated in the marker field. With logarithmic amplitude units (dBm, dBmV,
dBmMV, dBMA), the noise power density is output in dBm/Hz, i.e. as level in 1 Hz bandwidth with
reference to 1 mW. With linear amplitude units (V, A, W), the noise voltage density is evaluated in
MV/Hz, the noise current density in MA/Hz or the noise power density in MW/Hz.
In the default setting, the R&S FSL uses the sample detector for the noise function.
With the sample detector, the trace can additionally be set to Average to stabilize the measured values.
With RMS detector used, trace averaging must not be used since in this case it produces too low noise
levels which cannot be corrected. Instead, the sweep time can be increased to obtain stable
measurement results.
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