Measurements
R&S
®
FSW
142
User Manual 1173.9411.02 ─ 19
All instrument settings for the selected channel setup (channel bandwidth, channel
spacing) can be optimized automatically using the "Adjust Settings" function (see
mized Settings (Adjust Settings)"
The easiest way to configure a measurement is using the configuration "Overview",
see
chapter 6.1, "Configuration Overview"
Sweep Time
The sweep time is selected depending on the desired reproducibility of results. Repro-
ducibility increases with sweep time since power measurement is then performed over
a longer time period. As a general approach, it can be assumed that approx. 500 non-
correlated measured values are required for a reproducibility of 0.5 dB (99 % of the
measurements are within 0.5 dB of the true measured value). This holds true for white
noise. The measured values are considered as non-correlated if their time interval cor-
responds to the reciprocal of the measured bandwidth.
With IS 136 the measurement bandwidth is approx. 25 kHz, i.e. measured values at an
interval of 40 µs are considered as non-correlated. A measurement time of 40 ms is
thus required per channel for 1000 measured values. This is the default sweep time
which the R&S
FSW sets in coupled mode. Approx. 5000 measured values are
required for a reproducibility of 0.1 dB (99 %), i.e. the measurement time is to be
increased to 200 ms.
The number of A/D converter values, N, used to calculate the power, is defined by the
sweep time. The time per trace pixel for power measurements is directly proportional to
the selected sweep time.
If the sample detector is used, it is best to select the smallest sweep time possible for a
given span and resolution bandwidth. The minimum time is obtained if the setting is
coupled. This means that the time per measurement is minimal. Extending the mea-
surement time does not have any advantages as the number of samples for calculating
the power is defined by the number of trace points in the channel.
If the RMS detector is used, the repeatability of the measurement results can be influ-
enced by the selection of sweep times. Repeatability is increased at longer sweep
times.
If the RMS detector is used, the number of samples can be estimated as follows:
Since only uncorrelated samples contribute to the RMS value, the number of samples
can be calculated from the sweep time and the resolution bandwidth.
Samples can be assumed to be uncorrelated if sampling is performed at intervals of 1/
RBW. The number of uncorrelated samples is calculated as follows:
N
decorr
= SWT * RBW
(N
decorr
means uncorrelated samples)
The number of uncorrelated samples per trace pixel is obtained by dividing N
decorr
by
1001 (= pixels per trace).
The "Sweep Time" can be defined using the softkey in the "Ch Power" menu or in the
"Sweep" configuration dialog box (see
Channel Power and Adjacent-Channel Power (ACLR) Measurement
