How to Perform an RF Calibration

Watch this video for a quick guide to setting and monitoring RF input and output power levels to track change in RF performance.


The Accel-RF AARTS RF system is a multi-channel turnkey test instrument designed to independently set and monitor RF input and output power levels to a population of devices being tested over long durations in order to track change in RF performance. This is accomplished in an accurate and economical manner by using a single RF Power Meter for the absolute measurements and routing all coupled input and output power measurements from each channel through a switch matrix to that reference meter.


Two calibration factors per channel are required to make accurate device-under-test input and output power measurements. This depicts a typical RF block diagram of an AARTS system. The red dashed line represents the CAL1 factor, which relates the power level at the reference sensor, installed in the back of the system, to the real power measured at the DUT input. Since all the elements in this path are configured for use in their linear range, there is minimal level sensitivity. The green dashed line represents the CAL2 factor, which relates the real DUT output power to the level measured at the reference sensor. Each channel in the system will have its own unique cal factors and using these values, the system can measure selected RF signals through the RF switch matrix to determine the actual DUT power levels.


The LifeTest software contains an embedded calibration routine to walk users through this process. RF Calibration should be performed at regular intervals in between tests or whenever changing test frequencies. 


The first step in this process takes place outside of the LifeTest software. The reference power sensors used in the system must first be zeroed and calibrated with the integral power meter in the rack. Exit the LifeTest software to enable local control of the power meter.


Attach the front power sensor to the reference port of the power meter. Use the meter front panel controls to first zero and then calibrate sensor A. Disconnect the sensor when complete.


Detach the back power sensor from the rear of the system. Bring it around to the front of the system and connect it to the power meter reference port and then zero and calibrate sensor B. When finished, install the sensor back in its proper location in the rear of the rack.


Launch the LifeTest software and select RF Calibration from the main menu bar. Note that this routine cannot be performed while any testing is ongoing in the system.


The Cal factors are dependent on frequency, so the first step is populating the Frequency Control box with the appropriate values. When using the internal system VCO source, the LifeTest software will maintain the test frequency within a range of plus or minus 2MHz. With that in mind, we recommend calibrating over five points with a 4MHz spacing centered on the test frequency. Here is an example of the recommended frequency values for testing at 10GHz.


Apply and Exit to register these changes and you can now begin the CAL1 process. Highlight the channels to calibrate in the CAL1 column, right-click and select “Calibrate.” This brings up a new window that will step you through the procedure. 


The basic process for CAL1 involves placing a calibrated power pad on the end of the front power sensor and connecting to the channel being calibrated. This attenuator is sized to prevent damage to the sensor. The system will turn on RF drive to this one channel and measure both the front power sensor and the back power sensor. The difference between these two measurements is the Cal Factor.


Follow the software procedure by clicking through the first message on using an appropriate attenuator. Then, enter the value of the attenuator being used.


The next prompt will ask for the desired DUT drive level. Since the system is configured at the factory for all RF components in the calibration path to be in the linear range, we recommend running the calibration at maximum RF drive so that it runs through the process efficiently. Enter “50” dBm here.


Connect the front power sensor to CH1 when prompted by the software.


Click “OK” when that connection is made, and the system will automatically sweep through the defined frequencies and record the Cal Factor at each point.


Once the software completes all frequencies for the calibration for that channel, you will be prompted to move to the next channel. Repeat this process for all channels and when done, click “Save/Exit” to return to the main Calibration window.

You now have the option to review and plot the CAL1 results. This can be a good check to make sure the measured values are smooth across the frequency range and check to make sure none of the channels are out of family.


To save the calibration results, right-click and select “Store Factors.” You can now move on to CAL2 in order to calibrate the DUT output measurement path.


The CAL2 process uses the results from the CAL1 that you just completed to set the reference power level at the DUT. Rather than use the front power sensor, you simply need to connect up RF thru cables or adapters in order to connect together the DUT input and output cables. You can do this for all channels to be calibrated and the software will automatically iterate through.


In the CAL2 column, right-click and select “Calibrate” to bring up the window that will step you through the process. Click through the first two messages and then enter a drive level of “50” dBm again, similar to CAL1.


A message will then come up prompting you to insert a Thru for channel one, or alternately to install all Thrus and then click “Yes to All”. This will allow the process to cycle through all channels without further input needed from the user.


Once the process automatically completes, click Save/Exit to return to the main Calibration menu. Right-click and select “Store Factors” to complete the RF calibration.


As a final recommended step, you can archive your Cal Factors in order to compare them to previous results for the purposes of traceability. Select File -> Archive Cal Factors and enter the channels that just finished. This will build up your archive of calibration results that can be analyzed and plotted as needed.


From here, you can opt to begin testing or take some additional steps to fine-tune the configuration of the system. For example, you can add offsets to account for the insertion loss of the input and output circuits and de-embed your power measurements down to the DUT. You may also need to perform the VGA Calibration routine at this time if you have changed test frequency. You can refer to our other videos in the Knowledge Base for guidance on these topics. 


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