A narrow bandpass microstrip filter for high-speed fiber optic systems

Transmission systems employing passively retimed regenerators require a bandpass timing filter to extract the timing wave from the data stream. Typically, filters with Q exceeding 100 are necessary in long-haul fiber-optic systems. The fabrication of a microstrip filter with a loaded Q of over 500 and an insertion loss of 8 dB, is reported. The midband return loss in 4.8 dB. The filter is fabricated on a 1-in×1-in×1-in Ba₂Ti₉ O₂₀ substrate. This material has a dielectric constant of about 39 and can be formulated with temperature coefficients of resonant frequency from -2 to +6 p.p.m./°C. The fabricated device has a temperature coefficient of +2 p.p.m./°C. This combination of material properties facilitates high Q and small filter size and has the potential to compensate for temperature-induced electronic phase shifts between signal and clock recovery paths to the bit-decision circuit