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 0.8Ω in mean value at 470 MHz and at diode capacitance of 9 pF. The value of 0.8Ωis so high that low resistance diodes, Rₛ < 0.8Ω for instance, have been selected specifically for UHF tuners. Recently, low resistance diodes with Rₛ = 0.68Ω 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.