At BFO (Black Forest Observatory) we synchronize seismic data acquisition systems to a 1 PPS (pulse per second) signal generated by GPS disciplined clocks (Meinberg GPS 166 and GPS 167). While the clocks are operated in the laboratory building, the 1 PPS signal must be distributed to the instruments in the mine. We use optical fibres (FO) to transmit the signal over distances of several hundred meters. The circuits described below are used to convert the TTL level 1 PPS signal to an FO signal. And they are used to serve mutiple FOs from one single input signal. A distributor (sec. 2.1) is installed in the electronics vault in the mine. This circuit receives the 1 PPS signal through one FO input and feeds six FO outputs. Finally the optical signal is converted to TTL, RS 422, or RS 232 near the data acquisition system (secs. 2.2 and 2.3)
