



A study of the causes of inaccuracy in broadband RF transmission
bridges, and a demonstration of the methods that can be used
to correct them. Main document (version 1.01, 10th Feb. 2014) Work files (spreadsheets can be opened using Apache OpenOffice) testbrg6112_1.ods  Prototype test bridge data analysis. capcal_p500.ods  500 pF reference capacitor calibration curve. capcal_848.ods  40 pF reference capacitor calibration curve. testbrg6112_2.ods  Optimised test bridge with 500 pF reference capacitor. testbrg6112_3.ods  Optimised test bridge with 40 pF reference capacitor. testbrg6112_4.ods  Generator side shield earth, 500 pF ref.cap. testbrg6112_5.ods  Generator side shield earth, 40 pF ref.cap. mismatch_sim.ods  Effect of throughline mismatch on inphase response. testbrg6112_6.ods  Apparent sec. capacitance with 75.5 Ω ref. load testbrg6112_7.ods  Apparent sec. capacitance with 50 Ω ref. load testbrg6112_8.ods  No Faraday shield. Uncompensated. 50 Ω ref. load. testbrg6112_9.ods  No Faraday shield. Uncompensated. 75.5 Ω ref. load. testbrg611210.ods  No Faraday shield. Load port capacitance neutralisation. testbrg611211.ods  No Faraday shield. Herzog neutralisation. testbrg611212.ods  Load port capacitance neutralisation. testbrg611213.ods  Quadrature current neut. via Faraday shield. testbrg611214.ods  Herzog neutralisation. testbrg611215.ods  Loadside voltage sampling testbrg6113_1.ods  No Faraday shield. Phase shift neutralisation. testbrg6113_2.ods  Quad. current neut. to unloaded aux. winding. testbrg6113_3.ods  Quad. current neut. to loaded aux. winding. Fixed load. testbrg6113_4.ods  Quad. current neut. to loaded aux. winding. Var. load. testbrg6113_5.ods  Phase shift neutralisation. testbrg6114_1.ods  No Faraday shield. Quad voltage neut. 40 pF ref. cap. testbrg6114_2.ods  Quad voltage neut. 40 pF ref. cap. testbrg6114_3.ods  Quad voltage neut. 3 pF  30 pF trimmer. 
Online references A selfevaluating precision reference bridge. D W Knight  Followup article US Pat. No. 2808566  Douma's bridge . AC electrical theory, by DWK (source of the phasor theorems used here). Scientific Data Analysis, by DWK (linear regression procedures and statistical methods). Amplitude response of conventional and maxflat current transformers, DWK. + spreadsheets Maxflat_test1.ods , & Maxflat_test2.ods . Current transformer efficiency factor (DWK) + spreadshet Itr_effy.ods . Hatfield LE300A/1 TRAB (Laboratory bridge used for the L and C measurements). US Pat. No. 4739515  Herzog's compensation method. US Pat. No. 2134589  Stanek's Faraday shielded current transformer. 
Abstract An impedance monitoring bridge can be characterised by choosing two independent (or nearly independent) circuit parameters related to the magnitude and phase of the load impedance at balance. By adjusting the selected parameters to balance the bridge exactly with a reference load attached, the deviations of the parameters from their target values can be used to compute the bridge error at a given frequency. In a bridge that uses a capacitive potential divider for voltage sampling (Douma's bridge), suitable parameters are the lower voltagesampling network capacitance and the LFcompensation resistance. The balance point can be located with great precision by using a communications receiver as the detector. Shielding and the use of commonmode chokes in the earthloop between the signal generator and receiver prevents errors due to spurious signal injection. The optimised system can make relative phase measurements with an RMS uncertainty of about ±0.0075 degrees. The effect of the series inductance of the lower voltage sampling capacitor is clearly determined by the data. Compensation for this parasitic reactance can be obtained by inserting a small adjustable inductance in series with the upper voltagesampling arm. Magnitude flatness of around ±0.03% over 5 octaves is possible by this method. The parallelequivalent secondaryinductance of the current transformer is a strongly conserved model parameter. The measurement of parallel secondary capacitance is however skewed by throughline mismatch and other parasitic reactances, to the extent that it may appear to be positive, negative, or accidentally zero. A perturbation series is derived to account for the various contributions, and includes a hitherto undocumented effect of Faraday shield displacement current. Control of parasitics is needed if bridges built by different individuals are to give comparable results. The data show a linear relationship between phase error and frequency except for a small deviation attributable to a dispersion region in the premeability of the ferrite transformer core. This supports the view that the phase error can be considered as a time delay ocurring primarily in the transformer. Various phase compensation schemes are proposed and evaluated. These lead to bridge designs with 2point frequency tracking that can easily achieve a maximum phase error of better than ±0.2° and a maximum magnitude error of better than ±0.3% over the 1.6 MHz to 30 MHz range. A 3point tracking scheme that gives a maximum phase error of ±0.04° is also demonstrated. The need for the transformer Faraday shield is investigated. Theory indicates that the effect of the parasitic capacitance from line to detector port is correctable depending on the coupling factor. An unshielded bridge with 2point frequency tracking gave a maximum phase error of ±0.05°over the 1.6 MHz to 30 MHz range, close to the ±0.03° limit imposed by dispersion effects in the ferrite used. 



