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US 5,515,197 AGranted
Coherent optical multichannel receiver
AssigneeU.S. Philips Corporation
InventorsPieter W. Hooijmans, Markus T. Tomesen, Petrus P. G. Mols
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AssigneeU.S. Philips Corporation
InventorsPieter W. Hooijmans, Markus T. Tomesen, Petrus P. G. Mols
Estimating the term…
A coherent optical multichannel receiver includes a frequency down-conversion stage (10) having a laser local oscillator (12) for producing an IF signal which is applied to two frequency detection paths (26,28), the outputs from which are subtracted in a subtractor (30) to provide a data signal. AFC signal producing means (18,34) are also connected to the subtractor (30). In order to avoid a false lock occurring when the receiver is out-of-lock, detection means are provided for detecting an out-of-lock situation. The detection means is coupled either to a bandpass filter (36 or 38) to alter a property of the bandpass filter when the receiver is out-of-lock so that the subtractor (30) provides a positive output which is used to offset the frequency detection characteristic so that it has a single positive zero crossing or to an offset voltage input V.sub.ofs in the AFC loop. The offset is removed automatically when the receiver returns to a lock situation.
A method of tuning a coherent multichannel receiver having lock and out-of-lock modes wherein an error signal is produced to effect tuning of the receiver to a desired frequency, said method comprising the steps of:
A coherent optical multichannel receiver having lock and out-of-lock modes and wherein an error signal is produced to effect tuning of the receiver to a desired frequency, said receiver comprising:
A coherent optical multichannel receiver comprising a frequency down-converter including a local oscillator comprising a laser for frequency down-converting a FSK signal to a respective one of two IF signals, first and second signal detection paths coupled to the output of the frequency down-converter, subtracting means having inputs connected to outputs of the respective detection paths, each signal detection path including a bandpass filter and a square-law detector, an output of the subtracting means being connected to data demodulating means and to an automatic frequency control means for controlling the frequency of the local oscillator, characterised in that the automatic frequency control means includes means to detect the absence of an IF signal, means responsive to the detection of the absence of an IF signal for altering a property of one of the bandpass filters in the signal detection paths so that when the receiver is out-of-lock a non-zero output is produced by the subtracting means, which non-zero output is used to offset positively the AFC characteristic so that it has one zero crossing, said means also being responsive to the presence of an IF signal for removing the alteration made to the property of the one of the bandpass filters.
A frequency detection circuit for use in a coherent optical multichannel receiver, wherein means are provided which are responsive to the detection of an out-of-lock situation for automatically offsetting the frequency detection characteristic of the receiver so that it has only a single zero crossing in a desired direction and for automatically removing the offset when said out-of-lock situation is not present, wherein the frequency detection circuit comprises a dual path circuit and said means is adapted in response to detecting the out-of-lock situation to vary a property of one of the paths so that its response to noise is different and in that means are provided which are responsive to the difference for offsetting the frequency detection characteristic of the receiver, wherein each frequency detection path comprises a bandpass filter and a square law detector, in that when in lock the bandpass filters have substantially the same gains and pass bandwidths but are centered on different frequencies, and in that at least one of the bandpass filters includes means responsive to the occurrence of an out-of-lock situation for altering a property of the bandpass filter so that there is a difference in the amplitude of the noise signal produced by the respective square law detectors.
Dependencies are inferred from the claim text.