Ac-based gate detection method and device for the sizing of colloidal particles,cells and bacteria in liquids

An AC-based detection method and device, whereby an LF signal generator applies LF power in parallel to an active cell with a gate and a device matched in impedance to the active cell. The LF carrier signal applied to the active cell is modulated with a measurement signal when a particle to be measured passes through the gate. The outputs from the cell and the matched impedance device are fed to an AC common mode rejection device (32), thus generating a difference signal. The difference signal from the common mode rejection device is demodulated. The demodulated signal is filtered to remove components of the AC carrier signal and its harmonics to retrieve a signal relating to the size of the object passing through the gate of the active cell. The output of the method and the device is similar to the results obtained with a DC detector but avoids the problem of electrolysis caused by the DC current in the active cell.