The Fast Sweep processing block takes a short burst of 250 MSPS I/Q data, does an FFT,
converts to dB, and stores the result with 0.01 dB resolution into a 16-bit register. This, combined
with a fast-switching LO, enables THz/sec sweep speeds with a 30 kHz RBW.
The Real-Time processing block takes a continuous stream of 250 MSPS I/Q data and does 50%
overlapping FFTs. For the real-time frame buffer, the results of these FFTs are converted to dB,
and plotted on a two-dimensional image showing how many times that frequency was at that
amplitude during the real-time frame interval. The offset and scaling, from dB to pixels, is
controlled by your reference level and dB/div.
For the real-time trace buffer, either min/max or average is selected. In the case of average, the
results of the FFT is converted to power and summed. When min/max is selected, the FFT is
converted to 0.01 dB resolution, and processed through a min hold and max hold trace buffer.
The SM200B and SM435B include a 2 GB DDR capture buffer at the 250 MSPS I/Q rate.
The SM200A/B and SM435B I/Q Streaming processing block first tunes the 250 MSPS I/Q data
to a new center frequency, and then decimates by 5, to provide 50 MSPS I/Q data with 40 MHz
useable bandwidth. There are additional decimate-by-2 stages to further decimate the data to 25,
12.5 or 6.25 MSPS if desired. This can significantly reduce the PC’s processing requirements for
smaller bandwidth signals. For LTE applications, the hardware can also resample to 61.44, 30.72,
15.36, and 7.68 MHz.
The SM200C and SM435C I/Q streaming processing block first tunes the 250 MSPS I/Q data to
a new center frequency, and then resamples by 4/5, to provide 200 MSPS I/Q data with 160 MHz
useable bandwidth. There are additional decimate-by-2 stages to further decimate the data to
100, 50 or 25 MSPS if desired. This can significantly reduce the PC’s processing requirements
for smaller bandwidth signals. For LTE applications, the hardware can also resample to 122.88,
61.44, 30.72, and 15.36 MHz.
I/Q sample rates of 50 MS/s or lower are fully corrected. Sample rates above 50 MS/s, especially
when using frequencies below 700 MHz, may use interpolated or extended correction data, and
are not fully calibrated. For these high sample rates, specified accuracy is typically maintained,
but is not guaranteed.
3.4 Residual and Spurious Signals
3.4.1 Residual Signals
A residual signal appears even when there is no signal input. The SM200 has some low level
residual signals, especially above 10 GHz. The SM435 does as well, but typically below the noise
floor in most cases.
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