R&S FSQ Signal Analyzer - Rohde & Schwarz

R&S FSQ Getting Started
Noise Measurements
Fig. 2.15 Approximating the channel filter by sweeping with a small resolution bandwidth
The following steps are performed:
• The linear power of all the trace pixels within the channel is calculated.
Pi = 10(Li/10)
where
Pi = power of the trace pixel i
Li = displayed level of trace point i
• The powers of all trace pixels within the channel are summed up and the sum is
divided by the number of trace pixels in the channel.
• The result is multiplied by the quotient of the selected channel bandwidth and the
noise bandwidth of the resolution filter (RBW).
Since the power calculation is performed by integrating the trace within the channel
bandwidth, this method is also called the IBW method (Integration Bandwidth
method).
Bandwidth selection (RBW)
For channel power measurements, the resolution bandwidth (RBW) must be small
compared to the channel bandwidth, so that the channel bandwidth can be defined
precisely. If the resolution bandwidth which has been selected is too wide, this may
have a negative effect on the selectivity of the simulated channel filter and result in
the power in the adjacent channel being added to the power in the transmit channel.
A resolution bandwidth equal to 1% to 3% of the channel bandwidth should, therefore,
be selected. If the resolution bandwidth is too small, the required sweep time
becomes too long and the measurement time increases considerably.
Detector selection
Resolution filter
Sweep
Channel bandwith
Since the power of the trace is measured within the channel bandwidth, only the
sample detector and RMS detector can be used. These detectors provide measured
values that make it possible to calculate the real power. The peak detectors (Pos
Peak, Neg Peak and Auto Peak) are not suitable for noise power measurements as
no correlation can be established between the peak value of the video voltage and
power.
With the sample detector, a value (sample) of the IF envelope voltage is displayed
at each trace pixel. Since the frequency spans are very large compared with the resolution
bandwidth (span/RBW >500), sinewave signals present in the noise might be
2.22 Operating Manual 1313.9681.12 - 01
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