Calibration


TX_CH<0> has a unique sequence of phase shifts. This helps distinguish the reference channel compared to the other channels by a higher SNR signature in the correlation response. Channels 1 to 15 all have a five sequence Barker code with its own unique phase shift sequence. A comparison of TX_CH<0> and TX_CH<1> can be observed in Figure 4.


Figure 2. Magnitude response for null power method comparing phase contributions of two combined channels.


be repeated for each channel and for each frequency of interest.


Although this approach requires significant time due to its swept nature, it is extremely robust in noisy or unpredictable environments. It leverages a sinusoidal waveform and does not rely on timing alignment or correlation-based measurements. The main drawbacks are the execution time and the limited information it provides since it yields only phase offset information. Additional optimisation can be implemented for quicker sweeps using this method. Instead of a fixed phase step, one could start with a coarse step size to find a general range for the null location and use a finer phase step size to dial in the exact offset for the corresponding null.


CROSS-CORRELATION CALIBRATION The cross-correlation method is the standard calibration procedure leveraged because it aligns phase rapidly and with high precision. The method begins by configuring the calibration board into either combined loopback mode. All transmit and receive NCO phases, including both main and per channel adjustments, are set to zero so that any measured phase error originates solely from the hardware signal chain.


A pulsed baseband waveform is then generated with one pulse assigned to each digital-to-analogue converter (DAC) channel in a time division fashion akin to time-division multiple access (TDMA). These pulses are placed in discrete time slots in a time interleaved fashion to produce a frame containing 16 non-overlapping pulses once all channels are combined. This


Instrumentation Monthly September 2026


waveform is transmitted at the operating RF frequency and serves as the reference for the correlation-based alignment.


The pulse corresponding to TX_CH<0> has a seven sequence Barker code, where each sinusoidal period within the pulse envelope of


The calibration process starts by each DAC transmitting its respective time interleaved pulsed waveform and all analogue-to-digital converters (ADCs) capturing the 10 data frames of the combined waveform. Each data capture for all 16 ADCs is taken at an arbitrary time resulting in each data frame shifted by an unknown sample count relative to the others. Sample misalignment correction is done by selecting an arbitrary data frame as a reference and executing an autocorrelation against all remaining data frames. Each capture is then sample shifted by the appropriate number of samples to achieve alignment based on the autocorrelation sample delay results. Once aligned, the data frames are coherently summed, thus improving the SNR for accurate post-processing as shown in Figure 5. The next step cross correlates the coherently summed data (Figure 4) and the original transmitted time interleaved pulse train waveform (Figure 3). The resulting cross correlation matrix will aid in determining the phase offsets relative to a defined reference for all ADCs and DACs. The reference channels TX_ CH<0> help identify its location in the time interleaved pulses and can be used to determine the relative phase offsets of the


Figure 3. All DAC waveforms combined to create time interleaved pulsed waveform. Each pulse train corresponds to an independent transmit channel.


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