Calibration
Figure 4. TX_CH<0> (Pulse0) with a seven sequence Barker code and TX_ CH<1> (Pulse1) with a five sequence Barker code.
remaining channels. The relative difference of the complex angle of the peak correlation response for TX_CH<0> across the 16 ADC data frames will determine the ADC NCO phase offsets. The relative difference of the complex angle of the peak correlations of TX_CH<15...1> compared to TX_CH<0> within a single ADC data frame will determine the DAC NCO phase offsets. In both cases, the phase offsets are phase wrapped into the range from –180° to +180° to be compliant with the AD9084 device drivers.
FINAL CALIBRATION RESULTS After calibration is completed and the derived phase offsets are applied, the next step is to validate the effectiveness of the phase alignment procedure. For a fully coherent 16 element array, the signal power is expected to increase by
20log10 (16), while uncorrelated noise power increases by 10log10 (16). Figure 6a shows the magnitude of a single transmit CW waveform
digitised by one ADC channel, along with the FFT of the complex valued data frame. Figure 6b illustrates the result when all 16 transmit CW waveforms are combined and the peak magnitude is measured from the FFT of a single ADC frame; as expected, the coherent combination yields an approximately 23dB increase in peak signal magnitude relative to Figure 6a. Finally, Figure 6c presents the FFT obtained when all 16 transmit waveforms and all 16 receive channels are coherently summed. In this case, the peak magnitude reaches the average expected level due to bit growth effects, while the uncorrelated noise combines non-coherently, resulting in an SNR improvement of approximately 12dB.
CONCLUSION
Figure 5. Coherently summed ADC captures of the combined time interleaved waveform synthesised by all DACs. 2× the samples captured compared to transmitted.
62
This work demonstrates two practical calibration strategies for achieving coherent phase alignment in a 16 by 16 multichannel phased array subsystem. The null power method, while time intensive, offers a highly robust means of determining interchannel phase offsets without reliance on precise timing or correlation metrics, making it well suited for environments with elevated noise or measurement uncertainty. In contrast, the cross-correlation method provides a rapid and highly accurate calibration approach by exploiting time interleaved pulsed waveforms, Barker coded channel identification, and coherent summation of multiple captures to improve SNR and estimation stability.
Figure 6. (a) An FFT plot of a single transmit channel looped back into a single receiver, (b) an FFT plot of 16 combined transmit channels combined and looped back into single receiver, and (c) an FFT plot of 16 combined transmit channels and 16 combined receive channels.
Measured results confirm that the correlation-based calibration achieves near theoretical coherent combining performance, with signal power scaling as expected for a fully aligned 16 element array and noise combining non-coherently across receive channels. The ability to efficiently perform calibration across multiple frequencies enables the generation of high-resolution phase correction tables suitable for temperature and frequency dependent system characterisation. While the current implementation focuses on narrow- band CW excitation, the methodology naturally extends to wideband calibration by leveraging the integrated finite impulse response filtering and digital signal processing capabilities of the AD9084. Future work will explore broadband excitation and frequency dependent calibration to further enhance instantaneous bandwidth performance in large-scale digital beamforming systems.
Analog Devices
www.analog.com September 2026 Instrumentation Monthly
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