Varactor Tuners Eliminated an RF Amplifier

There is an urgent demand for low noise varactor tuners for use in TV receivers to meet the requirements of FCC rules. The RF amplifier has been frequently eliminated in mechanical tuners now in use, resulting in good cross-modulation characteristics and low cost. However, the elimination of the RF amplifier has not been attained in varactor tuners, because of high series resistance of varactor diodes. The series resistance of Si diodes at 470 MHz is around 0.7¿,and is much higher than that of mechanical capacitor, 0.15¿. To reduce the noise figure of varactor tuners, tuning diodes with low resistance, i. e. below 0.2¿must be developed. However, few reports have been published on low series resistance diodes for UHF use.