Solid State Shortwave Receivers For Beginners - The Listeners Guide

layer, acts as the dielectric of the capacitor. The larger the reverse bias
that is applied to the diode, the deeper the depletion layer becomes. This,
in effect, increases the distance between the two plates of the capacitor,
and causes a reduction in the capacity of the diode.
The circuit shown in Fig.14 illustrates how a varicap. diode can be
used as the basis of a simple bandspread control. The fixed capacitor
provides D.C. blocking and prevents the voltage fed to the diode from
being short circuited through the tuning coil. The tuning voltage set at
the slider of the potentiometer must be fed to the varicap. diode via a
high value resistor, so that the capacitance of the diode is shunted by a
very high value of resistance. Since only minute leakage currents flow
in the resistor, a significant voltage is not dropped across it despite its
high value.
Another simple method of adding bandspread is shown in Fig.15.
The operation of this is very straightforward. The variable resistor
virtually cuts the capacitor out of circuit when it is set for maximum
resistance. When it is at minimum resistance the capacitor is shunted
across the tuning capacitor. Intermediate settings of the variable resistor
give intermediate levels of effective capacitance across the tuning capacitor,
and the potentiometer thus acts as a simple bandspread control.
While this method is much cheaper than the previous methods, it
does have the disadvantage that it will only provide a relatively small
tuning range. It cannot be used in quite the same way as the other two
circuits, and it is best to use the main tuning control to roughly tune to
the desired station, and then use the bandspread control for the fine
tuning.
37