The University of British Columbia

Lightwave CATV transmission based on intensity modulation by a multichannel CATV signal is simple, but requires very linear components and high received powers. Alternatively, the multichannel signal can be FM-modulated onto one RF carrier, and transmitted with conventional intensity modulation. However, optoelectronic modulators reported to date are complex and bulky, and an electronic approach is needed. We describe a wideband FM modulator with record linearity over a wide tuning range, and phase noise equal to the best optoelectronic modulator results. An emitter-coupled multivibrator (ECM) core coupled to a linear transadmittance amplifier drives fully-differential limiter and output driver circuits. With an ECM very wide linear tuning range can be achieved if appropriately-designed collector loads are used. The circuit was fabricated in Nortel's NT-25 silicon bipolar process; the high-frequency layout is fully differential and physically symmetric. The ECM core operates at currents around 1 mA, optimized in conjunction with transistor sizing, and a -4.5 V power supply is used. The linearized transadmittance amplifier is a single-ended multitransistor circuit with passive feedback and single-pole stabilization. We report results for two versions: in mod1a the ECM current is set by a current mirror, while mod2a includes the 50 Omega;-matched linearized amplifier, which consumes 150 mW total