SEARCH Articles Figures Tables A 100-kHz sine-wave oscillator, with stable frequency and amplitude controls, was used to pulse the drive electrode. Each sensing electrode was connected to a current-to-voltage converter preamplifier. The preamplifier outputs were bandpass filtered at 100 kHz 5 Hz and amplitude-demodulated. The demodulated signals were amplified and DC-coupled to a first-order low-pass filter to give density signals. [Publishers Imprint]