R&S FSQ Signal Analyzer - Rohde & Schwarz

R&S FSQ Getting Started
Repeatability can be estimated from the following diagram:
max. error/dB
0
0.5
1
1.5
2
2.5
3
Noise Measurements
10 100 1000
10000 100000
Number of samples
Fig. 2.16 Repeatability of channel power measurements as a function of the number of samples
used for power calculation
The curves in Fig. 2.16 indicates the repeatability obtained with a probability of 95%
and 99% depending on the number of samples used.
The repeatability with 600 samples is ± 0.5 dB. This means that – if the sample
detector and a channel bandwidth over the whole diagram (channel bandwidth =
span) is used - the measured value lies within ± 0.5 dB of the true value with a probability
of 99%.
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 (Ndecorr) is calculated as follows:
Ndecorr = SWT × RBW
The number of uncorrelated samples per trace pixel is obtained by dividing Ndecorr
by 625 (= pixels per trace).
Example
95 % Confidence
level
99 % Confidence
level
At a resolution bandwidth of 30 kHz and a sweep time of 100 ms, 3000 uncorrelated
samples are obtained. If the channel bandwidth is equal to the frequency display
range, i.e. all trace pixels are used for the channel power measurement, a repeatability
of 0.2 dB with a confidence level of 99% is the estimate that can be derived
from Fig. 2.16.
2.24 Operating Manual 1313.9681.12 - 01