Measurements
R&S
®
FSW
140
User Manual 1173.9411.02 ─ 19
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.
In the "Fast ACLR" mode, the R&S
FSW measures the power of each channel in the
time domain, with the defined channel bandwidth, at the center frequency of the chan-
nel in question. The digital implementation of the resolution bandwidths makes it possi-
ble to select filter characteristics that are precisely tailored to the signal. In case of
CDMA2000, for example, the power in the useful channel is measured with a band-
width of 1.23
MHz and that of the adjacent channels with a bandwidth of 30
kHz.
Therefore the R&S
FSW 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.
Fig. 5-21: Measuring the channel power and adjacent channel power ratio for CDMA2000
signals
with zero span (Fast ACLR)
5.3.3.2
Measurement Repeatability
The repeatability of the results, especially in the narrow adjacent channels, strongly
depends on the measurement time for a given resolution bandwidth. A longer sweep
time may increase the probability that the measured value converges to the true value
of the adjacent channel power, but obviously increases measurement time.
Assuming a measurement with five channels (1 channel plus 2 lower and 2 upper adja-
cent channels) and a sweep time of 100 ms, a measurement time per channel of 20
ms is required. The number of effective samples taken into account for power calcula-
tion in one channel is the product of sweep time in channel times the selected resolu-
tion bandwidth.
Assuming a sweep time of 100 ms, there are (30 kHz / 4.19 MHz) * 100 ms * 10 kHz ≈
7 samples. Whereas in Fast ACLR mode, there are (100 ms / 5) * 30 kHz ≈ 600 sam-
ples. Comparing these numbers explains the increase of repeatability with a 95% confi-
dence level (2δ) from ± 2.8 dB to ± 0.34 dB for a sweep time of 100 ms.
Channel Power and Adjacent-Channel Power (ACLR) Measurement
