Patent ID: 7978800

Claim:
In a system that asserts a false synchronization signal at times when data is not present, a fiber-optic transponder that produces a lock signal only when data is present in a serial data signal, the transponder comprising: a controller chip that includes a phase locked loop adapted to operate in a hunting mode and a locked mode, wherein the phase locked loop is further adapted to assert a synchronization signal in the hunting mode when a hunting frequency passes through a data signal frequency and wherein the phase locked loop is further adapted to keep the synchronization signal asserted as long as the phase locked loop is locked onto the serial data signal; a timing circuit adapted to measure a period of time that the synchronization signal is asserted using a capacitor and resistor network and to produce a lock signal if the synchronization signal is asserted for at least a specified period of time, the timing circuit including a comparator that compares a first reference signal with a signal from the capacitor and resistor network used to measure the period of time that the synchronization signal is asserted, the comparator outputting the lock signal based on the comparison; and a demultiplexer arranged to receive a clock signal from the phase locked loop, the demultiplexer being configured to convert the serial data signal to a parallel data signal based on the clock signal.