For resolution bandwidths (RBW) of 30 kHz or greater, spans of 160 MHz or less, and start
frequencies of 650 MHz or greater, the SM200/SM435 can perform real-time spectrum analysis
using overlapping FFTs on its Arria 10 FPGA. The FPGA performs overlapping FFTs at an
overlapping rate of 50%, covering each point of data with 2 FFTs. We take the resulting FFTs and
min/max or average them into a final returned trace, as well as building a persistence image
representing the frequency, amplitude (log scale) points of all FFTs. The number of FFT results
merged depends on Real-Time Accumulation and the RBW. Since most of the number crunching
happens on the FPGA, a dual core i5 processor would typically be sufficient for this mode.
For spans of 40 MHz or less, the SM200/SM435 is capable of streaming 40 MHz of bandwidth
with no time gaps. The PC performs overlapping FFTs at an overlapping rate of 50%, covering
each point of data with 2 FFTs. Since the PC can process larger FFTs than the FPGA, more
RBWs and additional processing options are available in this mode, such as linear scale
persistence plots. Please note that this processing, for spans of 20-40 MHz and low RBWs,
typically requires a fast quad core i7 desktop processor. For slower processors, span may need
to be reduced or RBW increased for the processor to keep up.
The minimum signal duration to guarantee the same amplitude as a CW signal (i.e. 100%
probability of intercept, or POI) in real-time analysis mode is a function of the resolution bandwidth
selected, and is equal to 1.5 times the FFT interval. The FFT interval is approximately 2 / RBW,
so for a 631 kHz RBW, this works out to about 4 microseconds. Lower RBWs will require
proportionally longer signal duration. However, signals of even ¼ this duration will be displayed
only 2-3 dB down.
See the Spike Software manual for further information on Real-time mode.
2.6.2.1 Fast Swept Analysis
When spans wider than 160 MHz must be continuously monitored, the SM200/SM435 can rapidly
sweep the selected span by analyzing 160 MHz patches of spectrum using FFTs on the FPGA.
This mode is capable of 1 THz/s, and can provide 100% POI for a 2 GHz span of about 2 ms.
This mode is used in real-time analysis when span is greater than 160 MHz. In this mode, FFTs
occur on the FPGA of the device. This mode has a maximum RBW of 10 MHz and a minimum
RBW of 30 kHz. VBW must equal RBW. For more information on typical sweep speed
performance see Sweep Speed (Fast).
2.6.3 Zero-Span Analysis and I/Q Streaming
Zero span analysis allows you to view and analyze signals in the time domain using I/Q data
aquisitions from the device. The Spike application can display amplitude, frequency, and phase
vs. time, and display the results through multiple plots. The SM200A can be configured for up to
50 MS/s, the SM200B/SM435B has an additional 250 MS/s option for captures, and the SM200C
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