Calibration
CALIBRATION IMPLEMENTATION TECHNIQUES FOR MULTICHANNEL PHASED ARRAY SUBSYSTEMS
By Siddhartha Das, system applications engineer, and Sam Ringwood, system applications engineering manager, Analog Devices
This article describes the practical implementation and comparative evaluation of two phase-calibration techniques used for characterising a 16 transmit and 16 receive X-band phased array subsystem based on a mixed-signal front end. A passive calibration board enables a fully self-contained transmit-to-receive loopback, allowing deterministic extraction of per-channel phase offsets without external instrumentation. The first technique, a null power approach, determines relative phase alignment through swept phase measurements and provides a noise-tolerant baseline solution. The second technique uses time-interleaved pulsed waveforms and cross-correlation processing to rapidly estimate transmit and receive phase errors with high precision. Experimental results show that the correlation-based method achieves near- theoretical coherent combining performance, producing approximately 23dB transmit combining gain and an additional 12dB improvement in signal-to-noise ratio when both transmit and receive channels are coherently summed. These results demonstrate a scalable and efficient calibration framework suitable for high channel count digital beamforming subsystems.
A
ccurate amplitude and phase calibration is essential to achieving the theoretical performance of phased array antenna systems. Proper calibration applies the correct phase and amplitude weighting to each antenna element. This ensures that desired signals combine constructively (coherently) while noise, being uncorrelated between elements, combines less efficiently (non-coherently). The result is improved signal-to-noise ratio (SNR) and consistent antenna pattern performance across varying frequencies, temperatures, and operating conditions. This article focuses specifically on two phase alignment techniques used for the evaluation and characterisation of the ADXBAND16EBZ reference design. Continuous-wave (CW) waveforms were employed for the analysis, leveraging the hardened digital signal processing (DSP) blocks of the AD9084. The system also utilises prerequisite multichip synchronisation to ensure phase determinism; however, the details of this synchronisation process are outside the scope of this article.
TWO TECHNIQUES EXPLAINED Calibration on the subsystem platform is essential for ensuring that all 16 transmit and receive channels operate coherently. When the channels align in time, phase, and amplitude, the system achieves the expected coherent combining gain of approximately
24dB (20 × log10(16)). Two distinct calibration approaches are considered: the null power method and the cross-correlation method. The first is a traditional approach that excels in robustness, while the second is a modern, quicker alternative leveraged as the primary method for characterisation.
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Figure 1. (a) ADXBAND16EBZ and ADXBAND16EBZ-CAL connection and (b) combined loop back block diagram.
In both cases, a passive calibration board consisting of a power combiner/splitter and switch network is leveraged to enable a self-contained transmit-to- receive loopback (Figure 1). The following sections describe both methods in a detailed, engineering accurate manner and explain how the system extracts the final transmit and receive phase offsets.
NULL POWER CALIBRATION The null power method is based on rotating the phase of one channel relative to a reference
with fixed phase and observing the superposition magnitude of the two combined channels. The peak magnitude is recorded for each phase step and 180° is added to the phase offset corresponding to the two-channel combined minimum (null) as shown in Figure 2. The phase offset is then applied to the numerically controlled oscillator’s (NCO’s) phase shifter of the corresponding channel so that the channel aligns constructively with respect to the selected reference channel. This entire procedure must
September 2026 Instrumentation Monthly
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