Feature sponsored by Calibration
SIMPLIFY ANTENNA CALIBRATIONS USING SDR WITH RF PLL PHASE SYNCHRONISATION FEATURE IN MASSIVE MIMO AND PHASED ARRAY SYSTEMS
In this article from Danish Aziz, staff field applications engineer at Analog Devices, the radio frequency (RF) phase-locked loop (PLL) phase synchronisation feature available in Analog Devices’ software-defined radios (SDRs) is highlighted. This functionality helps in reducing the complexity in antenna calibrations, especially for systems that employ large antenna arrays. Control and configuration of synchronisation is provided in the user guide. This article emphasises its application and benefits...
PHASE COHERENT SIGNALS Coherence is a property of waves that defines the relationships existing in the physical quantities of a single wave or between two or more waves. In electronics, physical systems deal with phase, frequency, and amplitude of continuous wave and clock signals. In general terms, two signals are phase coherent if the difference between their phases stays constant and stable over time. Figure 1a shows the phase of two signals over time. The two signals exhibit a coherent phase relationship, as the phase between them remains constant. Figure 1b compares the starting phase of a reference signal in a system upon different power-up cycles. The coherent phase relationship over every power-up could also be observed here. However, Figure 1c presents an example where the phase is incoherent, as the signal starts with a random phase over every power-up.
PHASE IMPERFECTIONS AND THEIR MITIGATION IN MULTICHANNEL AND MULTI-ANTENNA SYSTEMS Phased array and massive MIMO systems have multiple antennas and multiple RF
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channels. From the digital back end up to the antenna array, phase coherence and timing synchronisation over multiple planes are main requirements in such systems. For example, frame synchronisation is needed on medium access level, coherency is required on digital interface (for example, deterministic latency), synchronisation is necessary in sampling with multiple converters or chips for multiple channels, phase coherency among multiple local oscillators (LO) is essential in generating radio frequencies, and a deterministic phase relationship is needed among the elements of antenna arrays. Thus, maintaining the coherent relationship at different stages is crucial and fundamental. However, it is a challenging task simply due to real-world practical aspects such as part-to- part variation, traces on printed circuit boards, nonlinearities in the components, coupling effects, frequency divider ratios, hardware aging, clock drifts, temperature drifts, and drifts in local oscillators. If multiple RF LOs are used in a system, LO phase drift is an additional factor that varies over multiple channels and over time. Different architecture options are available to
generate coherent RF LO signals. RF LO distribution: The LO signal is generated by a common LO and then distributed in the system. Due to radio frequencies, it is not an easy task. RF losses and RF coupling make it quite difficult. Reference clock distribution: To avoid RF losses, LO signals are generated locally. However, due to variations in PLLs or voltage controlled oscillators (VCOs), extra efforts are needed to synchronise individually generated LO signals. Figure 2 illustrates an example of a multichannel and multi-antenna RF subsystem architecture, which is based on integrated transceiver chips. There is an on- chip frequency synthesiser - a PLL - and a VCO for RF LO generation. The reference clock is generated externally to the transceiver chips and distributed to the device clock inputs respectively to each chip. Further scaling and distribution of the reference clock is done on the chip. In Figure 2, a breakdown of the propagation path is shown from the system reference clock to the antennas. The path could be split into different segments where each segment contributes a
March 2023 Instrumentation Monthly
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