Rohde & Schwarz R&S FSL3 - Measuring Phase Noise; Measurement Example - Measuring the Phase Noise of a Signal Generator at a Carrier Offset of 10 Khz

R&S FSL Noise Measurements
1300.2519.12 2.23 E-11
The curves in Fig. 2-17 indicate the repeatability obtained with a probability of 95% and 99% depending
o
n 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 confidence level 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 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 501 (= pixels per
trace).
Example
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 probability of 99% is the estimate
that can be derived from Fig. 2-17.
Measuring Phase Noise
The R&S FSL has an easy–to–use marker function for phase noise measurements. This marker
function indicates the phase noise of an RF oscillator at any carrier in dBc in a bandwidth of 1 Hz.
Measurement example – Measuring the phase noise of a signal generator at a
carrier offset of 10 kHz
Test setup:
Signal
generator
R&S FSL
Signal generator settings (e.g. R&S SMU):
Frequency: 100 MHz
Level: 0 dBm
Procedure:
1. Set the R&S FSL to its default state.
 Press the PRESET key.
R&S FSL is in its default state.