DESIGN OF A LOW-PHASE NOISE VCO FOR AN ANALOG CELLULAR PORTABLE RADIOAPPLICATION

A voltage-controlled oscillator (VCO) with a small power consumption a nd an excellent signal-to-noise (C/N) ratio was developed for a portab le telephone. In the domestic analog portable cellular portable teleph one, C/N is required to be more than 107 dBc/Hz at an off-carrier freq uently of 12.5 kHz. To realize this value, the noise behavior was stud ied for a 900-MHz oscillator using a small-signal, high-frequency sili con transistor. First, the relationship between the transistor noise a nd the C/N, the operating voltage and current dependence of the C/N an d the relationship between the collector breakdown voltage and the osc illator output were studied experimentally. A transistor was selected which is most suitable for a low-noise VCO at a low operating current of 3 to 7 mA. Next, by using this transistor, the relationship between the unloaded Q of the resonant circuit and the C/N was studied. It wa s found that Q of 60 is needed to obtain a C/N with a 3-dB margin. On the other hand, with a view to miniaturizing the resonator, a dielectr ic coaxial resonator with a material having a relative permittivity of 90 was studied experimentally. A Q of 200 was realized with a resonat or having a cross-sectional size of 1.8 x 1.8 mm(2). By combining it w ith a varactor diode, a Q of more than 60 was attained. Based on these results, a low-noise VCO was realized which has dimensions of 10 x 10 x 3 mm and is operated with a power supply voltage of 3 V and a curre nt consumption of 10 mA.