GaAs Varactor Tuners for UHF TV

Recently, varactor tuners have become very popular in TV tuner, but the noise figure and selectivity of present day´s varactor tuners are inferior to that of mechanical tuners. Series resistance, Rs, of Si tuning diodes manufactured is around 0.8¿in mean value at 470 MHz and at diode capacitance of 9 pF. The value of 0.8¿is so high that low resistance diodes, Rs < 0.8¿, for instance, have been selected specifically for UHF tuners. Recently, low resistance diodes with Rs = 0.68¿have been fabricated by ion implantation technique. 1) Even if these diodes are employed, the Q value of tuning circuit is still much lower than that of mechanical tuner in UHF region. Such high resistance diodes lead to a higher noise figure and lower gain in tuner performance.