The GPIO may be configured as 8 outputs, or 4 outputs and 4 inputs, or 8 inputs. The inputs are
automatically read at the end of each sweep, but may be read between sweeps as well. The
outputs may be written between sweeps, or configured to generate a pattern during each sweep.
Any voltage from 1.8V to 3.3V may be applied to pin 3, and the SM200/SM435 will use this voltage
for the logic levels. Do not ground pin 3. If pin 3 is left unconnected, the default logic level is 1.6V.
The SPI bus writes at about 5 Mbps, and SPI reads are not currently implemented. The clock
idles high, and data transitions on the falling edge of the clock. It can be used to write to most SPI
devices where data is latched on the rising edge of the clock.
2.5.3.2 Applications
A typical application for this GPIO port would be to drive an antenna switch. For example, an
SP8T switch, such as the Peregrine PE42582, has 3 control lines to select one of 8 antennas,
and requires a single 3.3V power supply. A PCB with this switch mounted could be powered from
pin 11. Simply connect pins 3 and 11 to select 3.3V logic. GPIO(2) through GPIO0(0) could control
the switch.
Using the API one could select an antenna, sweep, select a different antenna, and then sweep
the same span again.
A more advanced use of this bus would be to actively control and monitor a device under test.
For example, a user could test a VCO/PLL by sending a SPI command to the PLL, and routing
the SPI select to the trigger in. This would enable the user to make measurements referenced
from the rising edge of the SPI select line, to measure PLL settling time, etc.
2.6 Measurement Descriptions
This section details the core measurement and acquisition types for the SM200/SM435 spectrum
analyzer.
2.6.1 Swept Analysis
This mode of operation is the mode which is commonly associated with spectrum analyzers.
Through the software you will configure the device and request the device perform a single sweep
across your desired span. The SM200/SM435 uses fixed local oscillator (LO) frequencies to
acquire each 40 MHz patch of spectrum. If the start and stop frequency do not map to the same
LO step, multiple 40MHz patches are acquired and concatenated to form the sweep.
The processing performed on each 40MHz patch is determined by the settings provided. A
maximum RBW of 3 MHz and a minimum RBW of 0.1 Hz is available in this mode, but low RBWs
will be further limited by span. For non-buffered sweeps, each time a trace is returned, the device
waits until the next trace request. For buffered sweeps, the next sweep in the queue begins
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