24.11.2015
The Swedish national measurement institute SP and PTB compared their novel impedance bridges, which are based on Josephson junction arrays.
SP and PTB are the only metrology institutes in the world that develop Josephson impedance bridges. Previously, these bridges have been validated only by comparison with conventional bridges. An extended comparison between both impedance bridges was performed in June 2015 for capacitance ratios of 1:1 and 1:10, as part of the European project "Aim-QuTe".
Two temperature controlled capacitors from Andeen Hagerling with a nominal value of 100 pF as well as two capacitors from General Radio with nominal values of 1 nF and 10 nF where chosen as transfer standards. The temperature of latter capacitors was monitored manually and corrections were applied where necessary.
The main difference between the impedance bridges from SP and PTB is the approximation to a perfect sinusoidal waveform. PTB implemented a square wave, which has only two voltage transients per period, allowing for a frequency range up to 20 kHz. SP’s bridge simulates a sine wave using up to 256 transients. This enables the calibration of higher capacitance values, but it reduces the accuracy at audio frequencies.
An agreement of 1×10-8 at 100 pF and 1 kHz could be achieved.