External phase autozero loop to extend the voltage and/or current capabilities of a digital phase standard

After reviewing the phase autozero concept used in existing phase standards the authors demonstrate how to extend the technique so as to include an external voltage amplifier and/or an external current amplifier within an external phase autozero loop. A suitably configured phase verification bridge is used as the output phase measuring device in all external cases. The calibration of voltage and current sampling circuits is discussed, and the results for several practical systems are presented. In one case, the available voltage from the phase-controlled system is extended from 100 V RMS to 350 V RMS while the available output current is increased from 15 mA to 7.5 A. This system operates satisfactorily from 50 Hz to at least 5000 Hz. In another case, the system is used for phase measurements of a 50-ohm load at 50 kHz and 90 kHz with an output combination of 40 V and 800 mA (32 W). In all cases, the accuracy of the phase standard is maintained