The bridge is rendered self-evaluating
by choice of orthogonal adjustments for resistance and reactance
balance, and by including provision for the associated adjustment
parameters to be measured. The theoretical basis on which frequency-dependent
deviations of the parameters from their calibration settings
can be converted into performance data is given.
Both Faraday-shielded and unshielded
versions of the current transformer were tried. In both cases,
with the bridge maintained at constant temperature; the maximum
maximum peak-to-peak phase runout was about ±0.03°;
and the maximum magnitude runout was about ±0.04%. The
ultimate magnitude accuracy is limited by the uncertainty in
the value of the reference load resistance used during calibration.
Using a 4½-digit Fluke 8060A multimeter for the resistance
measurement gave a final magnitude accuracy of about ±0.13%.
The results obtained constitute a two orders-of-magnitude improvement
over typical Douma (CVS) transmission-bridge designs.
The effect of temperature variation
on phase accuracy is investigated. This issue is related to the
temperature coefficient of the permeability of the transformer
core. The effect is only significant at low frequencies. A compensation
scheme using a Linear PTC thermistor is proposed but not tested. |