Feature sponsored by Calibration
portfolio. It provides two transmit and two receive channels to convert digital IQ bits to RF and RF to digital IQ bits, respectively. It is based on zero-IF architecture, which minimises system power consumption with exceptionally superior transmitter and receiver RF performance. The device can support complete frequency generation using on-chip functions without the need of external components. There are three on- chip frequency synthesisers, and the RF LO synthesiser is one of them. For each synthesiser, there is an integrated VCO and a loop filter. This high level of integration with superior performance provides high flexibility in frequency generation over the whole supported frequency range.
On the digital side, the ADRV9009 employs JESD204B protocol for a serial interface performing digital data transfer. It provides built-in support for multichip synchronisation using the JESD SYSREF signal. Therefore, it is optimum to create large scale phased array and massive MIMO systems. In addition to multichip synchronisation, the ADRV9009 also provides an RF PLL synchronisation feature that enables the internally generated LO signals to be phase coherent and aligned with the applied reference clock. Based on this feature, the following functions could be easily achieved in large scale systems:
Figure 2. Sources of phase imperfections in a multichannel and multi-antenna system.
propagation delay. The variance in the propagation delay causes the variance in phase difference and perturbs the phase coherency in the system.
Calibration techniques are employed to mitigate the practical imperfections. By using a calibration method, the unknown factors are determined and then corrected accordingly. Due to the phase differences, the frequency response of each RF channel in phased array and massive MIMO systems differs from the other channel. Moreover, it is time variant in nature. Static factors in the system that can be measured are compensated through factory calibrations. Deployment dependent factors are mitigated using initial calibrations that could also be
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run upon each system startup. To mitigate dynamic and time variant factors, periodic antenna calibrations are required. In addition to temperature drifts, LO phase drift is one such dynamic factor that varies over multiple channels and over time. When these calibrations are executed during operation, they consume valuable system resources such as time frequency. Hence, an optimisation problem develops that maximises system performance with minimum resources allocated for calibration efforts.
SIMPLIFYING CALIBRATIONS USING THE RF PLL PHASE SYNC FEATURE The ADRV9009 is a dual-channel, highly integrated SDR in ADI’s RadioVerse
If large antenna array systems are built using this device, the RF PLL synchronisation feature could be exploited to simplify the antenna calibrations.
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Phase coherency upon power-up: constant, deterministic, and stable phase value on each power cycle
Phase coherency during operation: tracking of the phase value after startup
Phase coherency among multiple devices: further support to multichip synchronisation
Calibration algorithms require computational and memory resources in the digital hardware. For example, the algorithms are typically implemented in the baseband processing chain and employ FPGA/DSP resources. This feature indirectly reduces power consumption and resources on system calibrations. Hence, enabling this feature optimises the overall system performance and efficiency. Due to complex calibration algorithms, more time is required for initialisation and to reach a steady system state. This time could be minimised by enabling the RF PLL synchronisation feature on initialization. Calibration routines are executed periodically for tracking the drifts in LO phase, especially due to temperature. Otherwise, these drifts affect the beamforming pattern of multi- antenna systems. With the help of the RF PLL synchronisation tracking feature, calibration frequency could be minimised while keeping
March 2023 Instrumentation Monthly
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