Design of a low‐phas noise VCO for an analog cellular portable radio application

Abstract

AbstractA voltage‐controlled oscillator (VCO) with a small power consumption and an excellent signal‐to‐noise (C/N) ratio was developed for a portable telephone. In the domestic analog portable cellular portable telephone, C/N is reguired to be more than 107 dBc/Hz at an offcarrier frequency of 12.5 kHz. to realize this value, the noise behavior was studied for a 900‐MHz oscillator using a small‐signal, high‐frequency silicon transistor.First, the relationship between the transistor noise and the C/N, the operating voltage and current dependence of the C/N and the relationship between the collector breakdown voltage and the oscillator 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 was 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 dielectric coaxial resonator with a material having a relative permittivity of 90 was studied experimentally. A Q of 200 was realized with a resonator having a cross‐sectional size of 1.8 × 1.8 mm2. By combining it with 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 × 10 × 3 mm and is operated with a power supply voltage of 3 V and a current consumption of 10 mA.