Tektronix SPECMON3 Manual page 61

General Signal Viewing
Guaranteed Capture of Fast Events
The main reason that swept-tuned and step-tuned spectrum analyzers can't provide 100% Probability of
Intercept, POI, for a signal that isn't continuously present is that they spend only a short period of time
tuned to each segment of their frequency span during each sweep. If something happens in any part of
the span other than where it is tuned at that instant, that event will not be detected or displayed. There
is also a period of time between sweeps, retrace time, during which the analyzer is not paying attention
to the input signal. FFT-based analyzers, including vector signal analyzers, also miss signals during
the time between acquisitions. Their POI depends on a combination of factors including span, number
of FFT points, acquisition time, memory read/write time, and signal processing speed. Vector analyzers
process information sequentially, so when read/write from data and processing is occurring, data is not
being acquired.
RSAs, on the other hand, capture data across all frequencies within their real-time span during every
acquisition. With Tektronix' exclusive Frequency Mask trigger and DPX Density trigger, POI increases to
100%, insuring capture of any spectral event matching the trigger definition. When operating in free run as
a simple signal analyzer, the RSA has a POI similar to other FFT-based analyzers, with gaps between
each acquisition. Processing is done concurrent with the acquisitions.
Guaranteed Capture in DPX Real-Time Spans
The DPX display captures any signal that is at least 10.3 microseconds long (instruments with Option 200)
and within the real-time bandwidth. This performance is possible because the SPECMON computes up to
292,000 spectrum transforms per second. The faster the spectrum updates, the shorter the time between
acquisitions and the greater the probability that any signal will be detected.
Table 1 shows the specified minimum signal duration (MSD) for 100% probability of intercept under
various combinations of Span and RBW in DPX for a representative RSA model. As you can see, MSD is
affected by multiple factors.
Table 2: Minimum signal duration specifications for RSA6100B series signal analyzers
Span
(MHz)
110
110
110
110
110
110
xxx
1 With Options 110 under various combinations of control parameters.
To demonstrate the POI in action, a challenging bi-stable signal is used. A CW sinusoid sits at 2.4453 GHz
most of the time, but every 1.28 seconds, its frequency changes for about 100 μs before returning to
normal. The duty factor of this transient is less than 0.01%.
Figure 16 shows a swept analyzer set up for a 5-second sweep of its MaxHold trace. It shows that there is
something occurring around the signal. This sweep rate was empirically determined to be the optimum rate
SPECMON3 & SPECMON6 Printable Help
RBW Span/RBW
(kHz) Ratio
10000 11
1000 110
300 367
100 1100
30 3667
20 5500
FFT
Length Spectrum/sec
1024
1024 292,969
2048 146,484
4096 73,242
16384 18,311
32768 9,155
DPX Primer
1
MSD for
100% POI (μs)
3.7
5.8
14.8
37.6
134.6
229.2
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