Tutorial on performing custom calibration on a real-time spectrum analyzer-EEWORLD

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ThinkRF R5500 Real-time Spectrum Analyzer allows end users to load custom calibration files on the device to optimize the device, reduce differences between devices or enhance the parameters most important to the end user's application. Calibration files are used to map reference levels for various radio settings and physical parameters such as frequency, temperature and insertion loss of attenuators.

This application note provides a step-by-step calibration procedure for end-users to perform calibration.

1 Introduction

ThinkRF R5500 provides a built-in feature that allows end users to load custom calibration files on the RTSA device. Calibration files are used to map reference levels for various radio parameter settings and physical parameters such as frequency, temperature, and insertion loss of attenuators. Custom calibration files allow end users to perform specific optimization of the device for the target user's application. It can be used to reduce the impact of differences between devices and enhance the parameters that are most important to the end user's application.

This application note is intended to assist the end user in performing the calibration process over a frequency range. A step-by-step calibration procedure is provided below. This procedure assumes that the user is familiar with the operation of the R5500, including how to communicate and control the device using SCPI commands (refer to the R5500 User Guide and Programmer Guide as needed).

2. Setting requirements

The following items are required to perform the calibration:

An R5500, its power adapter, and a CAT-5 Ethernet cable with RJ-45 connector.

An RF signal generator that synthesizes signals within a target frequency range.

An RF coaxial cable with a coaxial SMA connector on one end to connect to the R5500 input and a suitable connector on the other end to connect to the RF signal generator. Make sure the cable is appropriate for the frequency range being calibrated.

An application software for determining the input signal power level, such as the R5500 API, PyRF library or its GUI, all provided by ThinkRF.

The following options are available:

A cable similar to the one above is used to connect the 10 MHz reference clock source to the R5500.

An RF power meter to accurately measure the signal power supplied to the R5500.

3 Setup steps

3.1 Setting up the RF signal generator

Please connect the RF cable to the RF signal generator. Do not connect the signal source to the R5500 until the R5500 is turned on and the attenuation status is determined in Section 3.2.

Set up the signal generator. Before connecting the signal source directly to the R5500's RF IN connector, use a power meter to verify the signal source from the signal generator.

Warning: The maximum input signal allowed by ThinkRF receiver at the RF IN port is +10 dBm. Connecting a signal level exceeding the specified level may cause permanent damage to the receiver.

3.2 Setting up the R5500

Connect the R5500 to its power adapter and turn on the device. Make sure the status LEDs on the device indicate that the device is operating normally. Refer to the R5500 User Guide for more information.

Use the steps described in the R5500 User Guide to determine the IP address unit of the device.

Use the following SCPI command to turn off the input attenuator of the R5500:

:INPUT:ATTENTUATOR 0 # For -408 model

only :INPUT:ATTENTUATOR:VARIABLE 0 # for -418 and -427 models

Please connect the SMA plug to the RF IN port of R5500.

3.3 Optional

If using a 10 MHz external reference source:

Please make sure that the external 10 MHz reference level is between -10 dBm and 0 dBm.

WARNING: Levels exceeding 0 dBm can cause permanent damage to the internal clock circuits. In addition, the 10 MHz reference level must be turned off before powering off the R5500.

Please connect a suitable cable from your 10MHz reference source to the 10 MHz IN port on the R5500.

Please switch the R5500 to use an external reference clock:

:SOURCE:REFERENCE:PLL EXT

4. Calibration steps

Please ensure that the sampling factor is set to 1 (no sampling) and the frequency shift is set to zero by issuing the following SCPI command or *RST command:

:SENSE:DEC 1

:SENSE:FREQ:SHIFT 0

Please set the R5500 to the target RFE (Receiver Front End) mode:

:INPut:MODE

Refer to the R5500 Programmer's Guide for available modes.

Please adjust the R5500 to the target frequency, such as 2450MHz:

:SENSE:FREQ:CENTER 2450 MHz

Please set the RF signal generator to generate a frequency visible within the bandwidth of the target RFE mode. Please note that in ZIF mode, please avoid connecting a signal frequency that is the same as the R5500 center frequency to avoid DC offset.

Please note the reference levels reported in the VRT environment packet (R_old). The environment packet is returned along with the VRT packet. See the Reference Levels section of the Programmer's Guide for more details.

Use the R5500 S240 application or a user-specified measurement application to record the resulting power level at the frequency of the applied input signal.

Use the following formula to calculate the new reference level:

R_new=R_old-(P_observed-P_input)

Where:

P_new = new reference level in dBm

R_old = Current reference level in the VRT environment data packet, in dBm

P_observed = Power level observed on the R5500 GUI (or your application) in dBm

P_input = RF power level supplied to the R5500 input connector in dBm

Repeat steps 2 through 7 for all frequencies of interest. See the Interpolation Calculation section for information on how reference levels are interpolated to create new reference levels at frequencies between two calibrated points.

Repeat steps 1 to 8 for any other target RFE patterns.

Once all the data has been collected, create a custom calibration file by following the instructions in Appendix A: Creating a Custom Calibration File below. An example of a custom calibration file is provided in Appendix B: XML Calibration File Example.

Refer to the "Customizing the Calibration of the R5500" section in the User Guide for instructions on how to upload a custom calibration file to the R5500 using the Web Management Console.

4.1 Interpolation calculation

When the R5500 is set to a frequency that has not been directly calibrated, the required calibration value can be found by interpolating between two adjacent calibration points. This interpolation is calculated using the following formula:

R_new=(|R_above-R_below |*(F_current-F_below ))/(F_above-F_below)+R_below

Where:

R_new = interpolated reference level in dBm

R_above = calibration reference level at the calibration frequency just above the current center frequency, in dBm

R_below = calibration reference level at the calibration frequency just below the current center frequency, in dBm

F_current = the current center frequency of the R5500, in Hz

F_above = the calibration frequency just above the current center frequency, in Hz

F_below = calibration frequency just below the current center frequency, in Hz

NOTE: If many calibration points are loaded:

After a custom calibration file has been uploaded, it will take longer to start the R5500 for the first time.

Since the embedded software requires longer lookup times, the scan rate may be affected. The worst-case lookup time is determined by the big-O algorithm O(log(N)), where N is the number of points to lookup.

5. Additional Information

During normal operation of the R5500, when the attenuator is set to a non-zero value, that attenuation value is automatically added to the reference level reported by the VRT environment data packet.

Each R5500 unit should be calibrated individually because unit-to-unit variations in gain in the signal path due to component tolerances can be significant.

When you factory reset the R5500, the calibration values are reset to the system defaults, effectively deleting the custom calibration file.

To better understand how the R5500 overrides the factory/system reference values with custom calibration values, and how to select a calibration source when multiple sources are available, refer to the "Calibration File Source Selection" section of the R5500 User Guide.

Appendix A: Creating a Custom Calibration File

The calibration settings for the R5500 are stored in XML files. The XML file name must have a .xml extension. You can create your own calibration file according to the structure and format described below (below is a description of the XML tags). Replace the italic text with the appropriate values.

YYYY/MM/DD HH:MM:SS

YY-XXXX

XYZ

ref_level_1

ref_level_2

... more frequency steps and more...

... more frequency ranges and more…

ref_level_1

ref_level_2

... more temperatures and more...

... more RFE modes and more...

ref_level_1