VARACTOR DIODES --AN ALTERNATIVE TO VARIABLE CAPACITORS On the
topic of Varactors, they are virtually all 'good' BUT they are
not the same as a variable capacitor. You cannot substitute them
directly and achieve equivalent performance, but you can nearly
always incorporate a varactor into a circuit and achieve good
performance if you take all the effects into account. These
different effects include: - temp coefficient - junction
conductance - non-linearities with respect to instantaneous
voltage, that is the sum of both the tuning voltage and the RF
voltage at any time but especially the peak - V.tune impeadance
Others have already addressed the different temperature
coefficient, so I'll leave well enough alone. All solid state
juntions suffer from one significant difference from a 'real
capacitor'. IF the RF voltage is large enough the diode junction
will conduct during that portion of the cycle that exceeds the
diode 'on' voltage. This introduction of non-linear effects
includes changing the RF impeadance