Characterizing timing parameters in commercial SERF-OPM multichannel systems for biomagnetic field sensing

Spin-Exchange Relaxation-Free Optically Pumped Magnetometers (SERF-OPMs) are increasingly used in multichannel biomagnetic sensing, yet their timing performance remains poorly characterized. This contribution presents the first cross-platform study of time delay, group delay, intra-channel variability, and settling time across four commercial SERF-OPM systems: Neuro-1 and QZFM Gen.2 (QuSpin Inc.), and HEDscan and FLv2 (FieldLine Inc.). Measurements were performed inside a magnetically shielded room (BMSR-2.1, PTB, Berlin) using an already introduced test bench. Results show frequency-dependent delays of 1-10 ms, intra-channel spreads up to +-1 ms, group delays between 1-15 ms, and settling times of 2-55 ms. Clear differences in manufacturer strategies were observed: QuSpin minimizes intra-channel variability through digital delay equalization, whereas FieldLine employs per-sensor calibration to optimize bandwidth and phase matching. In all systems, the time delay deviation between channels is in the sub-millisecond range in the 20-140 Hz band, which is sufficient for magnetoencephalography source localization. However, longer settling times in some platforms limit performance for rapid stimulation protocols. These findings provide the first vendor-comprehensive benchmarks for timing parameters in commercial SERF-OPM systems. They highlight the trade-offs between bandwidth, delay, and calibration strategies, and underscore the need for rigorous timing characterization to ensure waveform fidelity. The results are directly relevant to applications such as stimulation-evoked responses, brain-computer interfaces, and closed-loop neuromodulation.