Keysight Technologies N437 Series Installation and operating instructions

Application Note

Keysight Technologies

N437x Lightwave Component Analyzer

Automation – Getting Started with SCPI

The Keysight N437x Series Lightwave Component Analyzer (LCA) is a solution which

integrates a vector network analyzer (VNA) along with a calibrated lightwave test set

to measure the transfer function of electrical to electrical (EE), electrical to optical

(EO), optical to electrical (OE), and optical to optical (OO) components. Similar to a

vector network analyzer both the magnitude and phase response of a component are

characterized as a function of frequency. Typical measurements performed by the LCA

include: gain, bandwidth, frequency response, flatness, and electrical return loss. A key

value of the LCA is that the calibrated response of the lightwave test set is automatically

de-embedded from the measurement and the reported results are isolated to the optical

component under test. Additional information on the measurement fundamentals for the

LCA can be found in the Keysight application note, High Speed Lightwave Component

Analysis. [1] In this paper, we will review the steps required to automate the N437xA

LCA and provide suggestions to help a new user quickly integrate the LCA into their

automated test environment.

The LCA application running on the network analyzer integrates the VNA with the

lightwave test set. Automating the LCA requires that commands are sent to both

the VNA and the LCA. Here is a summary of the typical steps required to perform

measurements along with the application which is automated for each step.

Introduction

03 | Keysight | N437x Lightwave Component Analyzer Automation – Getting Started with SCPI - Application Note

i) Define VNA measurement settings and perform electrical calibration – VNA

application

The VNA settings such as start and stop frequency, number of measurement points,

and IF bandwidth are set for a specific measurement. Once the instrument state is

defined an electronic or mechanical calibration is performed to define the electrical

reference plane and compensate for losses in the test fixturing and/or cabling. After

the calibration is complete, the instrument state is saved so it can be quickly recalled.

Additional information on performing the VNA calibration is available in the specific

N437x LCA user guide. [6, 7, 8 9]

ii) Start LCA application – LCA application

The LCA application is started and the measurement type (EO, OE, or OO) is selected.

For remote automation, rather than starting the LCA application the user will start the

LCA SCPI Server. More details on this are provided in the section, LCA Automation.

iii) Define LCA measurement settings – LCA application

The LCA measurement settings including: wavelength, optical power, electrical

power, and modulator optimization are defined in the LCA application for a given

measurement. The user can also enable additional optical path or electrical path

de-embedding along with specifying a path for the appropriate s-parameter file.

iv) Initialize LCA – LCA application

The LCA is initialized with the settings defined in step iv). Initializing a measurement

will set up the instrument and load the appropriate lightwave test set calibration data.

For example, initializing an OE measurement will setup the LCA to de-embed the

response of the optical modulator in the lightwave test set so the final measurement

result is isolated to the optical to electrical component under test. Once the LCA is

initialized for a given measurement, multiple sweeps can be performed to characterize

multiple devices.

v) Perform LCA Measurement – LCA application

The LCA application sweeps the VNA over the defined frequency range to measure

the magnitude and phase response of the component under test. This step can be

repeated multiple times once the LCA is initialized in step iv). If the measurement type

or any of the LCA parameters are changed then step iv) needs to be repeated.

vi) Query VNA Trace Data / Save *.csv File – VNA application

The final step once a measurement sweep is completed is to query the VNA trace data

so the gain, bandwidth, frequency response, flatness, and electrical return loss can be

determined. It is also possible to position markers directly on the VNA to gather this

information.

To completely automate an LCA measurement, the control program needs to interface

with both the VNA application in steps i), vi) and the LCA application in steps ii), iii), iv),

and v). The remainder of this paper will outline the steps required to interface with both

the VNA and the LCA application.

04 | Keysight | N437x Lightwave Component Analyzer Automation – Getting Started with SCPI - Application Note

PNA/ENA Automation

There are two series of Keysight VNAs which are integrated into the N437x Series LCA.

The low frequency N4374B LCA incorporates an E5071C ENA, while the higher frequency

N437x LCA use the N522x PNA. There are a number of standard IO programming

interfaces available for each instrument, as summarized in Table 1.

Table 1: ENA/PNA Programming Interfaces

Instrument LAN

(remote GPIB)

LAN

(VXI-11)

LAN

(Socket)

LAN

(HiSlip)

USB GPIB

ENA – E5071C NO YES YES NO YES YES

PNA – N522xA YES YES YES YES YES YES

Once an IO interface is selected, there are a few options for sending instructions to the

VNA. Both the ENA and PNA can be automated using an IVI driver or by sending ASCII

text commands to the instrument using direct IO. The ASCII text commands follow a

convention known as ‘Standardized Commands for Programmable Instruments’ which

is abbreviated as SCPI. There are trade-offs between using an IVI driver vs. SCPI

commands. [2] This paper will focus on using SCPI commands to automate the VNA;

SCPI is a simple and intuitive approach for basic instrument operations. Each front-panel

keystroke performed by the user has an equivalent SCPI command which is sent to the

instrument to perform an operation. Typical VNA operations which require automation,

along with the corresponding SCPI command, are detailed in Table 2. These SCPI

commands are typically sent to the instrument using the Keysight VISA IO library. [2]

The remote command structure for the ENA and PNA need to manage the general case

where there could potentially be multiple channels, windows, and traces used as part of

the measurement. Managing the data generated by the LCA application is simplified as

only a single VNA window and a single VNA channel are generated. For a single ended

E/O or O/E measurement there are two traces generated, electrical return loss and

insertion loss. While for a differential E/O or O/E measurement there are a total of three

traces generated, an electrical return loss and two insertion loss traces.

The SCPI commands in Table 2 are formatted for a single channel, as set up by the LCA

application. Many of the ENA and PNA commands are identical; the main exception are

commands for querying trace data and saving/recalling files. Complete documentation

for the ENA and PNA SCPI commands are available in the on-line help files. [3][4]

05 | Keysight | N437x Lightwave Component Analyzer Automation – Getting Started with SCPI - Application Note

Table 2: SCPI Commands for ENA/PNA Automation

Operation PNA ENA

Instrument Reset :SYST:PRES :SYST:PRES

Save Instrument State :MMEMory:STORe 'filename.csa' :MMEM:STOR 'filename.sta'

Recall Instrument State :MMEM:LOAD 'filename.csa' :MMEM:LOAD 'filename.sta'

Start Frequency :SENS1:FREQ:STAR <double> :SENS1:FREQ:STAR <double>

Stop Frequency :SENS1:FREQ:STOP <double> :SENS1:FREQ:STOP <double>

IF Bandwidth :SENS1:BWID <double> :SENS1:BAND <double>

Number of Points :SENS1:SWE:POIN <int> :SENS1:SWE:POIN <int>

Autoscale Trace :DISP:WIND:TRAC:Y:AUTO :DISP:WIND:TRAC:Y:AUTO

Query Available Traces DISP:WIND1:CAT? CALC1:PAR:COUNT?

Query Trace Data CALC1:PAR:MNUM <int>

CALC1:DATA? FDATA

SDATA - Complex Measurement Data

FDATA - Formatted Measurement Data

where <int> represents the trace number

:CALC1:TRAC<int>:DATA:FDAT?

SDATA - Complex Measurement Data

FDATA - Formatted Measurement Data

where <int> represents the trace number

Operation Complete *OPC? *OPC?

Save *.CSV File MMEM:STORe:DATA <string>,’CSV Formatted

Data’,’Trace’,’Displayed’, <int>

where <string> represents the file name and <int>

represents the trace number

:CALC1:PAR<int>:SEL

:MMEMory:STORe:FDATa <string>

where <string> represents the file name and <int>

represents the trace number

It is important to make sure that the automation program is synchronized with the

operations being performed by the test equipment. Often this is accomplished using the

operation complete query, ‘*OPC?’, which returns a ‘1’ when an operation has finished.

The instruments in this example will wait until the operation is complete and then return

a ‘1’. In other words, nothing is returned until the operation is complete. It is important

that the interface timeout is set for a value which is longer the operation. For example,

if the interface timeout is set to 1 second but the measurement requires 3 seconds to

complete then a timeout error will result in response to the ‘*OPC?’.

Here is a brief example which summarizes typical operations and SCPI commands

required to setup the VNA. The syntax of the example in this section has been

abbreviated to highlight the commands required for each step. Comments are included

on lines starting with the ‘#’ character. A complete Python example which sends

SCPI commands using the PyVisa libraries is provided in Appendix B. The individual

SCPI commands sent to the instrument using the ‘myVNA.write()’ statements are the

important commands which can be transported to a different language. The example

code below will query one of the VNA traces as a comma separated string which can be

parsed and formatted into an array. The trace is returned as a real and imaginary pair

if ‘SDATA’ is specified or in the currently displayed format (i.e. log magnitude, phase,

group delay, etc.) if ‘FDATA’ is used in the command string. The example program also

highlights the command to save *.csv file to the VNA hard disk drive.

06 | Keysight | N437x Lightwave Component Analyzer Automation – Getting Started with SCPI - Application Note

1. #Define VNA as PNA or ENA

2. inst_VNA = 'PNA'

4. #Preset instrument and wait until complete

5. myVNA.write(':SYST:PRES')

6. myVNA.write('*OPC?')

7. print ('VNA Reset Complete: ' + myVNA.read())

9. #Recall instrument state which includes electrical calibration

10. if inst_VNA == 'PNA':

11. myVNA.write(':MMEM:LOAD "C:\\Users\\Public\\PNA_state.csa"')

12. myVNA.write('*OPC?')

13. print ('Recall PNA State Complete: ' + myVNA.read())

14. elif inst_VNA == 'ENA':

15. myVNA.write(':MMEM:LOAD "C:\\Users\\Public\\ENA_state.sta"')

16. myVNA.write('*OPC?')

17. print ('Recall ENA State Complete: ' + myVNA.read())

18.

19.

20. #Insert Code here for LCA - See next section on LCA Automation

21.

22. #Query VNA trace data

23. if inst_VNA == 'PNA':

24. #Define trace data transfer as ASCII

25. myVNA.write(':FORMAT ASC')

26. myVNA.write(':FORMAT?')

27. print('Data Xfer Format: ' + myVNA.read())

28. #Select measurement data using trace number

29. myVNA.write('CALC1:PAR:MNUM 1')

30. #Query data as formatted - FDATA

31. #Query data in complex format - SDATA

32. myVNA.write('CALC1:DATA? FDATA')

33. myydata = myVNA.read()

34. elif inst_VNA == 'ENA':

35. #Define return trace format

36. myVNA.write(':FORM:DATA ASC')

37. myVNA.write(':FORM:DATA?')

38. print('Data Xfer Format: ' + myVNA.read())

39. #Query data as formatted - FDATA

40. #Query data in complex format - SDATA

41. #Note on ENA FDATA:

42. #Array returned for ENA formatted data will have 2x the number of data

43. #points in the returned array. The data points of interest are

44. #at the odd elements (i.e. n*2-2 where n = 1 to number of points)

45. myVNA.write(':CALC1:TRAC1:DATA:FDAT?')

46. myydata = myVNA.read()

47.

48. #Save VNA *.csv file

49. filename = '"C:\\Users\\Public\\trace1.csv"'

50. if inst_VNA == 'PNA':

51. #Save trace 1

52. #Append filename to the command string

53. command = ':MMEM:STORe:DATA ' + filename

54. command = command + ',"CSV Formatted Data","Trace","Displayed",1'

55. myVNA.write(command)

56. myVNA.write('*OPC?')

57. complete = myVNA.read()

58. elif inst_VNA == 'ENA':

59. #Select and save trace 1

60. command = ':MMEMory:STORe:FDATa ' + filename

61. myVNA.write('CALC1:PAR1:SEL')

62. myVNA.write(command)

63. myVNA.write('*OPC?')

64. complete = myVNA.read()

65.

66. #Query VNA Errors

67. err_num = 1

68. while (err_num != 0):

69. myVNA.write(':SYST:ERR?')

70. err_VNA = myVNA.read()

71. print('VNA_err: ' + err_VNA)

72. err_VNA = err_VNA.split(',')

73. err_num = int(err_VNA[0])

74.

07 | Keysight | N437x Lightwave Component Analyzer Automation – Getting Started with SCPI - Application Note

LCA Automation

Traditionally, the LCA application is automated using Microsoft .NET remoting over a LAN

interface. This requires that a remote client is installed on the control PC. To automate

the LCA, the control program calls the methods and properties of the remote client

which in turn communicates with the server running on the LCA via LAN. Additional

information on programming the LCA using .NET remoting is available in the N437x

LCA Programming Manual. [5] An important point is that the .NET remoting exposes

the complete functionality of the LCA and is the most flexible interface for controlling

the instrument. However, for programming environments other than Microsoft Visual

Studio .NET, interfacing with the required drivers can require extra steps. Using the .NET

remoting also limits the control PC to a Windows OS and requires communication via

LAN. Finally, for test and measurement engineers not familiar with automation using

drivers, there is a learning curve to use the .NET remoting. To overcome these challenges

and simplify the steps required to automate the LCA, a SCPI server has recently been

added. This allows users to directly send SCPI commands to the LCA SCPI server over a

LAN Socket or USB interface. Similar to using SCPI commands to control the VNA, using

SCPI commands to automate the LCA is a simple and intuitive approach.

One point to consider at present is that commands to automate the external optical input

and the 75 Ohm selection in the LCA application are not currently available with the

SCPI server. These are on the roadmap for a future release of the LCA application. The

next section of this paper will summarize the required automation steps for the LCA SCPI

Server.

LCA Automation – SCPI

LCA automation using SCPI commands is a straight forward and intuitive approach; each

front panel operation performed by the user has an equivalent SCPI command. The LCA

user interface tab for performing OE measurements is provided in Figure 1. For a typical

measurement, the user will select LCA parameters including: wavelength, optical power,

and RF power. Additional selections are available for optical and RF path de-embedding.

08 | Keysight | N437x Lightwave Component Analyzer Automation – Getting Started with SCPI - Application Note

Figure 1. Typical LCA Macro GUI tab for performing OE Measurements

After the measurement parameters are defined, the instrument is initialized by

pressing the ‘Start’ button which sends the parameters to the LCA to set up the

measurement. This is important to remember; when the user selects the wavelength

for an OE measurement (either manually in the LCA GUI or by sending the

‘:PARameter:WAVelength’ command) the actual wavelength of the source is not changed

until the ‘Start’ button is selected or the ‘:MEASurement:INITialize:OE’ command is

sent. After initialization is complete the user is able to perform a single measurement

or continuously repeat the measurement scan. If any of the LCA settings need to be

changed, the appropriate command is sent to the LCA along with the command to

initialize the measurement. A summary of the specific SCPI commands corresponding to

each of the LCA Macro entries is summarized in Table 3. Complete documentation for the

LCA SCPI commands are available in the N437x programming guide. [5]

09 | Keysight | N437x Lightwave Component Analyzer Automation – Getting Started with SCPI - Application Note

Table 3: SCPI Commands for each LCA parameter

GUI Control SCPI Command Measurement Mode

1Start :MEAS:INIT:EE

:MEAS:INIT:EO

:MEAS:INIT:OE

:MEAS:INIT:OO

2Abort :MEAS:ABOR EE / EO / OE / OO

3Advanced :PAR:ADVA:MODE 0/1 EO / OE / OO

4 Measure Mode :PAR:MEAS:MODE SING/DIFF EO / OE

5Wavelength :PAR:WAV 850/1310/1550 EO / OE / OO

6Optical Power :PAR:OPT:OUT:POW <double> OE / OO

External Input Not Currently Available OE / OO

7Optical Input :PAR:OPT:INP:POW:HIGH 0/1 EO / OO

8 Modulator Optimization :PAR:MODU:BIAS:MODE CONT/EVER/ONCE OE / OO

9 Forward RF Power :RF:POW:FWD <double> EO / OE / OO

10 Optical Path De-embedding :PAR:OPT:PATH:DEEM 0/1 EO / OE / OO

11 De-embedding Data :PAR:OPTical:S2PFile:USE 0/1 EO / OE / OO

12 Source Length :PAR:SOUR:PATH:LENG <double> EO / OE / OO

13 Source Refractive Index :PAR:SOUR:REFR:IND <double> EO / OE / OO

14 Source Attenuation :PAR:SOUR:REFR:IND <double> EO / OE / OO

15 Receiver Length :PAR:REC:PATH:LENG <double> EO / OE / OO

16 Receiver Refractive Index :PAR:REC:REFR:IND <double> EO / OE / OO

17 Receiver Attenuation :PAR:REC:ATT <double> EO / OE / OO

18 S2P File (Source) :PAR:OPT:SOUR:S2PF <string> EO / OE / OO

19 S2P File (Receiver) :PAR:OPT:SOUR:S2PF <string> EO / OE / OO

20 RF Path De-embedding :PAR:ELEC:PATH:DEEM 0/1 EO / OE / OO

21 S2P File (Src 1) :PAR:ELEC:SOUR:S2PF1 <string> EO / OE / OO

22 S2P File (Src 2) :PAR:ELEC:SOUR:S2PF2 <string> EO / OE / OO

23 S2P File (Rcv 1) :PAR:ELEC:REC:S2PF1 <string> EO / OE / OO

24 S2P File (Rcv 2) :PAR:ELEC:REC:S2PF2 <string> EO / OE / OO

75 Ohm Not Currently Available EO / OE / OO

10 | Keysight | N437x Lightwave Component Analyzer Automation – Getting Started with SCPI - Application Note

LCA Automation – Starting the SCPI Server

Prior to sending any SCPI commands to the LCA, the user needs to close the LCA

application and start the LCA SCPI Server as shown in the images below. This is an

important point; either the LCA Macro or the LCA SCPI Server can be active, but not

both at the same time. The SCPI Server provides connections to the LCA using either a

LAN Socket connection on port 5026 or by directly connecting to the VNA with a USB

cable. The user can select to start the SCPI Server with either a LAN Socket or USB

interface active. The steps to close the LCA macro and start the LCA SCPI server are

detailed in Figure 2 and 3. One point is that the VNA only has a single USB device port.

This can be used to control either the VNA application or the LCA application via SCPI. If

the VNA is being controlled using the USB port then the LAN interface needs to be used

for LCA automation.

Figure 2. Close LCA Application prior to starting LCA SCPI Server

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