To implement a fPLL, the delta-sigma modulator is turned on. This allows the feedback M divider to take on fractional values, which allows precision frequency synthesis. When used as a standard M/N PLL, both M and N values are integers and the delta-sigma modulator is disabled.
Replacement of Voltage-Controlled Crystal Oscillators
Optical transport network (OTN) multiplexing transponder (muxponder) applications generally require expensive VCXOs that synthesize the various client frequencies from information embedded in the aggregate data stream. The fPLLs are designed to replace these VCXOs, resulting in a cost-effective, highly integrated solution. As shown in Figure 2, the client frequency information in the aggregate data stream is used to control the delta-sigma modulators in the fPLLs, allowing for precise synthesis of the required client line rate frequencies.