Response-separation-guided line impedance stabilization network-probe apparatus for fixed-plane in-circuit impedance measurement.

Sensitive in-circuit impedance measurement is often limited by the response separation between practical short and open standards after receiving-network, cable, and fixture residuals are included. We present a response-separation-guided line impedance stabilization network (LISN)-probe apparatus for fixed-plane impedance extraction. The calibrated asymmetric LISN-probe combination uses a LISN for isolation and coupling, while the current-probe interface lowers equivalent receiving-port impedance. An equivalent-port model, short-open-load (SOL) calibration at a fixed device-under-test (DUT) plane, and a response-noise-aware local impedance-resolution proxy separate global end point-response contrast from one-standard-deviation noise-equivalent local impedance fluctuation. At 1 MHz, measured short-open response-separation gains are 24.6 dB over the dual-LISN path and 33.6 dB over the dual-probe-plus-LISN path, corresponding to ∼17-fold and 48-fold increases in global short-open response ratio. Local fluctuation is evaluated independently through the SOL-inversion Jacobian and measured repeated-DUT-response covariance. Apparatus-level validation is restricted to 0.5-10 MHz. Passive-standard tests show that increased response separation is not a universal accuracy improvement: the LISN-probe method gives 95th-percentile errors of 1.5% and 8.1% for the 255 Ω and 1.5 kΩ standards, while the dual-LISN path is preferable at 5 Ω. Probe and LISN fits give normalized root-mean-square errors of 2.72% and 2.62%, respectively. The apparatus also resolves current-dependent magnetic-component impedance changes, with a series-equivalent decomposition quantifying resistance and inductance reductions. Results establish the sensitivity benefit, measurement-performance boundary, and validated frequency range of this fixed-plane apparatus, without claiming a general modal electromagnetic-interference compliance measurement.