Low noise phase locked VCO at 2.5 GHz for optical transmission networks using fifth harmonic STW delay line

A hybrid VCO at 2488.32 MHz, which is phase locked to a crystal oscillator operating at 75.4 MHz, is described. As the frequency controlling element of the VCO, acoustic surface transverse wave (STW) delay lines operating at the fifth harmonic are used. In order to reduce the delay line electromagnetic feedthrough, a three-transducer type STW filter design is combined with a microstrip rat-race coupler. The filters were designed using advanced analysis methods and fabricated on rotated Y-cut quartz substrates with a standard photolithographic process. The delay lines were characterized by a center frequency of 2.49 GHz, an untuned insertion loss of 22 dB, an unloaded Q value of 1280, and a group delay time of 158 ns. The signal of the VCO is mixed with the local (crystal) oscillator signal multiplied by 32, thus providing an IF-signal of 75.4 MHz feeding the phase locked loop which has a bandwidth of 300 kHz. A single-sideband phase noise to carrier ratio of -112 dBc/Hz at 1 kHz offset has been attained. The present work arose from a requirement for a low noise frequency source for direct generation of the SONET (Synchronous Optical Network) bit frequency of 2488.32 MHz.