R&S FSL Measurements on Modulated Signals
1300.2519.12 2.27 E-11
5. Set the optimal reference level and RF attenuation for the applied signal level.
Press the Adjust Ref Level softkey.
The R&S FSL sets the optimal RF attenuation and the reference level based on the
transmission channel power to obtain the maximum dynamic range. Fig. 2-19 shows the result
of the measurement.
Fig. 2-19 Adjacent channel power measurement on a CDMA 2000 MC1 signal
The repeatability of the results, especially in the narrow adjacent channels, strongly depends on
the measurement time since the dwell time within the 30 kHz channels is only a fraction of the
complete sweep time. A longer sweep time may increase the probability that the measured
value converges to the true value of the adjacent channel power, but this increases
measurement time.
To avoid long measurement times, the R&S FSL measures the adjacent channel power with
zero span (fast ACP mode). In the fast ACP mode, the R&S FSL measures the power of each
channel at the defined channel bandwidth, while being tuned to the center frequency of the
channel in question. The digital implementation of the resolution bandwidths makes it possible
to select filter characteristics that is precisely tailored to the signal. In case of CDMA 2000 MC1,
the power in the useful channel is measured with a bandwidth of 1.23 MHz and that of the
adjacent channels with a bandwidth of 30 kHz. Therefore the R&S FSL changes from one
channel to the other and measures the power at a bandwidth of 1.23 MHz or 30 kHz using the
RMS detector. The measurement time per channel is set with the sweep time. It is equal to the
selected measurement time divided by the selected number of channels. The five channels
from the above example and the sweep time of 100 ms give a measurement time per channel
of 20 ms.
Compared to the measurement time per channel given by the span (= 5 MHz) and sweep time
(= 100 ms, equal to 0.600 ms per 30 kHz channel) used in the example, this is a far longer
dwell time on the adjacent channels (factor of 12). In terms of the number of uncorrelated
samples this means 20000/33 Rs = 606 samples per channel measurement compared to
600/33Rs = 12.5 samples per channel measurement.
Repeatability with a confidence level of 95% is increased from ± 1.4 dB to ± 0.38 dB as shown
in Fig. 2-17. For the same repeatability, the sweep time would have to be set to 1.2 s with the
integration method. Fig. 2-20 shows the standard deviation of the results as a function of the
sweep time.
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