Master the challenges of OTA calibration

Integrating antennas to the radio frequency chipset without connectors is increasingly common with both low cost IoT devices and especially in highly integrated 5G mobile communication equipment operating at mmWave frequencies. This makes testing a device under test (DUT) over the air (OTA) mandatory. To tackle the challenges of OTA testing it is key to understand the demands of antenna measurement and chamber setup.

Calibration is an important first step in the preparation for OTA testing. The measurement results of a DUT should not be setup-depended, requiring a calibration for each measurement setup. The OTA link adds a significant amount of attenuation to the measurement setup due to the free space path loss (FSPL), in addition to the losses from cables and other connected components. Unlike with cables, a stable measurement distance and perfect mechanical alignment is crucial for OTA measurement accuracy in order to avoid differences in attenuation caused by positioning errors.

Port calibration, for example by using a network analyzer, cannot be conducted to an air interface. Therefore, another type of calibration is required. Commonly, antenna gain measurements are conducted by comparing an unknown antenna to a reference antenna. Following this principle, an antenna with known gain is required to identify the calibration information of the OTA test setup and store these calibration data either on the measurement instrument or some external software controlling the measurement setup.

The high FSPL, longer cables due to the required measurement distance as well as the use of high frequencies require excellent dynamic range from the test and measurement equipment.

To address these different challenges, to calibrate OTA test setups and to carry out OTA measurements, Rohde & Schwarz has a diverse portfolio of vector network analyzers like the R&S®ZNA, vector signal generators, such as the R&S®SMW200A and signal and spectrum analyzers such as the R&S®FSW. Rohde & Schwarz also offers complete OTA test solutions, such as the R&S®ATS1000 antenna test chamber. Combined with Rohde & Schwarz test instruments, it is the ideal environment for 5G antenna characterization throughout the entire process from R&D to production for both active and passive devices.

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