Abstract: An apparatus and electronic circuit for the measurement of fluid levels in containers. The circuit and apparatus constitute an instrument which measures fluid levels by optical means. Only light signals are used near the fluid itself. The instrument thus provides an intrinsically safe method of measuring fluid levels in the presence of explosive or flammable fluids, vapours or gasses, or in containers which must be electrically isolated.

Abstract: A method for determining the highest, common bi-directional transmission rate between two stations of an ISDN communications link includes the steps of a) selecting a candidate transmission rate from a range of candidate transmission rates, b) transmitting a test signal at the candidate transmission rate through an attenuator along the link from one of the stations to the other of the stations for a first predetermined time interval, c) determining whether a return signal transmitted by the other station over the link and at the candidate transmission rate has been correctly received at the one station, d) repeating steps a), b) and c) until predetermined criteria have been satisfied; and e) storing the highest candidate transmission rate for which a return signal at that rate was correctly received within the first time interval; and f) removing the attenuator for final verification of operation.

Abstract: A circuit used in conjunction with RF modulating a laser diode and controlling the bias point is presented. The novel function is the amplitude and level shifting modulation of the drive signal at two different low (non-RF) frequencies. The light output is then detected to ascertain the relative amounts of the two different frequencies. The amplitude modulated signal appearing in the light output rises as there is increased distortion due to clipping on the threshold side or knee of the light current curve of the diode, and the amount of the level shifting modulation frequency signal drops with increased amounts of clipping. The two signals are used to derive a feedback signal to control the amplitude of the drive signal to maintain the relative detected amounts of the two signals appearing in the light output at a given ratio.