Experiences with a two terminal-pair digital impedance bridge
  This paper describes the realization of a two terminal-pair digital impedance bridge and the test measurements performed with it. The bridge, with a very simple architecture, is based on a commercial two-channel digital signal synthesizer and a synchronous detector. The bridge can perform comparisons between impedances having arbitrary phase and magnitude ratio: its balance is achieved automatically in less than a minute. $R$-$C$ comparisons with calibrated standards, at kHz frequency and 100 kohm magnitude level, give ratio errors of the order of $10^{-6}$, with potential for further improvements. 
Luca Callegaro, Vincenzo D'Elia, Marian Kampik, Dan Bee Kim, Massimo   Ortolano, and Faranak Pourdanesh
physics.ins-det