Performance variance of low-noise resonant capacitance bridges with ungapped MnZn ferrite core replacements

Precision AC resonant capacitance bridges, with a planar printed circuit board transformer using an ungapped MnZn ferrite core, have shown excellent noise performance in high-precision measurements. Several applications use an ensemble of bridges; therefore, consistency of performance is critical to the functionality of the systems. When part of the same manufacturing batch and scaled to the same frequency, the cores exhibit a noise performance at 293 K with a weighted mean variance of <0.3%, and all cores fall within a ±1% band. At 140 K, the weighted mean variance is <0.6%, and the 90% inclusion band is ±1.5%. Ten cores of SIFERRIT Material N41, supplied by TDK, were tested at room temperature (293 K) and in a liquid nitrogen dewar (140 K). Fitted to a parabolic function extending to +3 dB on both frequency sides, the weighted mean of the noise minima at 293 K was 0.3027 aF/√Hz, with a variance of 0.0012 aF/√Hz, spanning a frequency range of 85.0 ± 2.7 kHz. When scaled to 100 kHz, the weighted mean and variance at 293 K were 0.2788 and 0.0008 aF/√Hz, respectively. At 140 K, the corresponding noise values were 0.1815 aF/√Hz with a variance of 0.0016 aF/√Hz for a frequency range of 152.7 ± 6.3 kHz, and 0.1835 aF/√Hz with a variance of 0.0011 aF/√Hz when scaled to 150 kHz. After scaling for resonant frequency and temperature, these results are consistent with a previous measurement of another ungapped core (from the same supplier and type but a different manufacturing batch), with discrepancies of 9% and 6% at 293 and 140 K, respectively.