A Digital Setup for Determining Magnetic AC Properties

In order to determine the ac properties of laminated magnetic-core materials, a digital assembly has been developed to measure the mean half-period value nearly independent of frequency. Conventional integrating digital voltmeters are unsuitable for such measurements if the frequency fs of the unknown voltage does not fit the equation f
s
?? T
o
= ``integer,'' where T
o
is the integrating time of the DVM. This error can be reduced considerably by the designed double-integration method. A FET switch supplies the analog signal to a low drift integrator generating a dc output corresponding to the voltage-time integral over n periods (n = integer). The signal switch is triggered internally by the unknown voltage or externally by one or two reference signals of the same frequency. Thus it is possible to sample any interval of the unknown voltage by appropriate phase shifting. The integrator dc-output voltage is converted to a proportional frequency, which is fed to a counter with variable gate time T
m
. If the signal frequency f
8
= 1 / T
8
= (n + k)/To, (-1/ 2 ??? k ??? + 1/2), T
m
will be reduced or increased by kT
s
. The systematic error is a function of (k/f)
²
. The overall accuracy in the frequency range 25 Hz-1 kHz and the voltage range 15 mV-7.5 V is better than 5-10
^???4
. Additionally, the equipment measures the arithmetic mean and, with an internal dc reference, the angle between the zero crossings of two voltages.