Instrument Interfacing

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This guide explains how the Automated Radio Evaluation Suite (ARES) communicates with lab instruments using standard APIs and SCPI commands. Whether you’re measuring power amplifier (PA) performance or characterizing antennas, this section helps you interface with your instruments through MATLAB using VISA libraries.

VISA Library Overview

Virtual Instrument Standard Architecture (VISA) is a test and measurement industry-standard communication API (Application Programming Interface) for use with test and measurement devices regardless of their communication protocol (e.g., LAN, USB, GPIB, etc.). The VISA libraries are the communication drivers that allow ARES to control the instruments using Standard Commands for Programmable Instruments (SCPI).

ARES uses the VISAdev interface, introduced in MATLAB R2021a. VISAdev replaces the older VISA interface and integrates better with modern instrument drivers. Read more about the necessary software in MATLAB: Get Started with VISA.

SCPI Command Basics

Standard Commands for Programmable Instruments (SCPI) are text-based commands used to configure and query instruments. These commands are device-independent and can be used over any supported communication interface.

Keysight Command Expert allows searching the necessary SCPI strings interactively and provides seamless integrations with different VISA libraries.

There are two types of SCPI strings:

• Command String: :KEY <parameter>

• Query String: :KEY?

Command strings are reserved for writing to the instrument, while query strings are reserved for reading back from the instrument. The KEY represents the name of the instrument feature to adjust, while ‘’ is the value to be sent to the instrument. Complex SCPI strings can be made by combining several KEYs and parameters into one string. You can chain multiple levels of control using colons, i.e., :KEY1:KEY2:KEY3 <parameter 1> <parameter 2>.

Transitioning from VISA to VISAdev

ARES uses the visadev interface instead of the older visa interface. Here’s a summary of equivalent MATLAB commands:

VISA Interface	VISAdev Interface	Purpose
visa()	visadev()	Connect to an instrument.
fprintf()	writeline()	Send a command.
fscanf()	readline()	Read a response.
query()	writeread()	Send a query and read the response.
binblockread()	readbinblock()	Read binary data blocks.
clrdevice()	flush()	Flush I/O data buffers.
fclose()	clear()	Disconnect the instrument object.

Common SCPI Strings Across Instruments

Many instruments share a basic set of SCPI commands, regardless of vendor or type. These are especially helpful for setup, synchronization, and debugging.

Type	SCPI Command/Query	Purpose	When to Use
Identification Instrument	*IDN?	Returns instrument model and info.	Verify the instrument after connecting.
Reset to Default State	*RST	Resets instrument to factory defaults.	Use at the start of the session unless your instrument uses a preconfigured environment.
Clear Status Register	*CLS	Clears error and communication flags.	Prevent corrupted data in sweeps.
Operation Complete	*OPC?	Queries if previous commands finished	For command debugging.
Wait for Completion	*WAI	Waits until prior operations are complete.	Improve speed/accuracy.

SCPI Data Format Commands

The following commands control how numerical data is formatted and transmitted between the instrument and controller, impacting compatibility, speed, and precision during data transfer.

Type	SCPI Command/Query	Purpose	When to Use
Set Byte Order	:FORM:BORD: SWAP	Configures Little-endian binary format.	Required by MATLAB for binary reads.
Set Byte Order	:FORM:BORD: NORM	Configures Big-endian binary format.	Used in other environments (non-MATLAB).
Query Byte Order	:FORM:BORD?	Queries current byte order.	Verifying before reading binary data.
Set Data Format	:FORM:DATA ASCii,0	Sets ASCII data format.	Useful for debugging, not efficient.
Set Data Format	:FORM:DATA REAL,32	Sets 32-bit real binary format.	Faster transfer, sufficient for most tasks.
Set Data Format	:FORM:DATA REAL,64	Sets 64-bit real binary format.	Highest precision, use for double-precision.
Query Data Format	:FORM:DATA?	Queries current data format.	Verifying before reading trace/sample data.

Sample SCPI Commands for PA Measurement Instruments

The following table lists frequently used SCPI commands for configuring and controlling instruments in the power amplifier (PA) measurement setup. These commands help automate voltage/current sourcing, RF signal generation, and signal analysis tasks.

Instrument	Command / Query	SCPI String	Input Parameter
Power Supply	Set voltage and current	:APPLy <channel>,<voltage>,<current>	Voltage and current within instrument limits.
Power Supply	Measure DC voltage	:MEAS:SCAL:VOLT:DC? <channel>	Use @1, @2, or @1,2 to specify channel(s).
Power Supply	Measure DC current	:MEAS:SCAL:CURR:DC? <channel	Use @1, @2, or @1,2 to specify channel(s).
Power Supply	Enable or disable output	:OUTP:STAT <state>, (<channel>)	ON/OFF or 1/0. Channel in parentheses: (@1), (@2), or (@1,2).
Signal Generator	Set RF power	:SOUR1:POW:LEV:IMM:AMPL <power>	Power in dBm, within allowed range.
Signal Generator	Set frequency	:SOUR1:FREQ:CW <frequency>	Frequency in Hz, within allowed range.
Signal Generator	Enable or disable output	:OUTP1:STAT <state>	ON/OFF or 1/0.
Signal Analyzer	Set number of sweep points	:SENS:SWE:POIN <points>	Integer between 1 and the maximum supported value..
Signal Analyzer	Set frequency span	:SENS:FREQ:SPAN <span>	Span in Hz, within valid range.
Signal Analyzer	Set center frequency	:SENS:FREQ:CENT <frequency>	Center frequency in Hz, within valid range.
Signal Analyzer	Enable continuous measurement	:INIT:CONT <state>	ON/OFF or 1/0.
Signal Analyzer	Restart current sweep	:INIT:IMM	No input. Immediately starts or restarts a sweep.
Signal Analyzer	Query sweep data	:TRAC:DATA? <trace>	Trace name, e.g., TRAC1, TRAC2, …, TRAC6.

For the Power Supply Unit (PSU):

• channel: Use @1 for channel 1, @2 for channel 2, and @1,2 to address both simultaneously.

Sample SCPI Commands for Antenna Measurement Instruments

The following table lists frequently used SCPI commands for configuring and controlling instruments in the antenna measurement setup. These commands allow automation of RF sweeps, mechanical motion, and data acquisition during the antenna measurements.

Instrument	Command / Query	SCPI String	Input Parameter
VNA	Set start frequency	:SENS<cnum>:FREQ:START <num>	Frequency in Hz; within VNA’s allowed range.
VNA	Set stop frequency	:SENS<cnum>:FREQ:STOP <num>	Frequency in Hz; minimum is often 70 Hz, depending on model.
VNA	Set sweep points	:SENS<cnum>:SWE:POIN <num>	Integer between 1 and max supported points.
VNA	Set sweep mode	:SENS<cnum>:SWE:MODE <mode>	Mode can be HOLD, CONT (continuous), or SING (single).
VNA	Enable smoothing	:CALC<cnum>:SMO:STAT <state>	ON/OFF or 1/0.
VNA	Set smoothing aperture	:CALC<cnum>:SMO:APER <num>	Percentage between 1 and 25.
EM Center	Set rotation speed	1<slot-letter>:SPEED <speed>	Speed between 1 and 100.
EM Center	Seek to angle	1<slot-letter>:SK <angle>	Angle in degrees.
EM Center	Stop movement	1<slot-letter>:ST	No parameters. Immediately halts motion.
Linear Slider	Set speed	AXIS1:S <speed	Speed preset from 1 (slowest) to 8 (fastest).
Linear Slider	Seek to position	AXIS1:SK <position	Position in cm (within travel limits).
Linear Slider	Query current position	AXIS1:CP?	Returns current position in cm.
Linear Slider	Enable scan mode	AXIS1:SCAN	No parameters. Starts programmed scan routine.
Linear Slider	Enable homing mode	AXIS1:HOME	No parameters. Returns the axis to mechanical zero.

For the Vector Network Analyzer (VNA):

• cnum is the channel number, which defaults to one if not specified.

• mnum is the measurement number, and mname is the name of the measurement.

For the EM Center, which controls the anechoic chamber:

• Slot 1 contains both tower and table actuators.

• Use A for the table and B for the tower as slot-letter designations (e.g., 1A, 1B).