Measuring system for passband of communications optical-fibre - has signal sequence generator with output converted to optical signal supplying photodetectors feeding comparator fed with reference

摘要

A photodiode receives the output from an optical fibre link. The signal is applied to a low noise preamp to drive a filter network where only one filter is selected to transmit a signal free from perturbations. A summer controls a gain controlled amplifier to amplify the fundamental frequency and avoid the parasitic frequencies. A processing circuit receives the filter output voltage and provides a display with the transmission frequency. The processing circuit controls a local oscillator driving a mixer for the input signal. A discriminator provides automatic frequency control for the local oscillator. The mixer output is filtered and demodulated and applied to a log. amplifier (64) to provide an attenuation display scaled in dB's, when the amplifier output is compared with a reference level.